Calibrate an Online Ruler With a Bank Card: Sources and Limits

The calibration, step by step

Under each step, the line in monospace type is the exact text the ruler shows at that point, taken from the ruler's own code.

1. Open the ruler and see the assumption named

With nothing stored, the ruler draws itself from the CSS specification's fixed inch of 96 pixels, whatever your panel is, and its state line says so.

UNCALIBRATED: assumed 96 CSS ppi
2. Find a clean, undamaged card

Pick a flat ID-1 card with smooth edges: a bank card, ID card or driving license. For the whole match the card stays just above the display. Do not let it touch the glass, and do not press or slide it there. A bowed card or a damaged edge does not give a straight reference, and the page cannot detect either.

Reference: bank card, ID card or driving license
3. Align it just above the outline and drag until the edges agree

Look square-on, with the card still just above the display, because the printed error band leaves out any extra gap or tilt. Each step of the slider moves the outline by at most 0.35% of the object you are matching; on a card at a typical density that is one pixel, or 0.31%. The slider's travel depends on the edge you pick, because the plausible range belongs to the reference: 129 to 1369 px for the long edge and 81 to 863 px for the short one. Match the long edge if you can. At 85.60 mm it is 1.59 times the length of the 53.98 mm short edge, so the same 0.7 mm of hand slop is a smaller share of it and the printed band is narrower.

On a phone in portrait, hold the card upright just above the display and match it down the screen. The long edge does not fit across: an iPhone 15 has about 64 mm of display width against the card's 85.60 mm. Upright, the reference is still the full 85.60 mm and the band is still ±1.2%. Use the short edge only when neither way up fits, because its band is wider at ±1.7%.

324 px wide · steps of 1 px
4. Press the button and read your error band

The readout turns green; only a completed match does that on this site. The figure after the comma is your error band: the object's own dimensional spread plus your alignment, added as bounds. Every measurement you take afterwards carries it.

CALIBRATED: bank card, ID card or driving license long edge, ±1.2%
5. Come back, and let it re-check itself

The page stores device pixels per millimeter. Page zoom divides back out of that figure on Chrome, Edge and Firefox, and so does an operating-system display-scaling change on Windows and on GNOME, so the calibration survives both and the readout reports the correction. WebKit does not report page zoom through the pixel ratio, and a macOS scaled resolution puts the factor into the reported pixel count instead; the page catches each as a change and offers a new match. Three things move the readout out of green into a neutral state that names what happened: a display that reports something new, a pinch-zoom still in force, and six months without a new match.

CALIBRATION MAY BE STALE: your display changed, re-check

The reference objects that are offered

Three things must be on record before you can select an object: its dimensional tolerance, read out of a published standard; at least two independent sources; and a verification date. The ruler checks this itself when it loads: a row switched on without that evidence moves to the withheld list with the missing item named, so editing a flag cannot add a row to this table. The table and the source notes below are generated from that data file.

For a bank card, the tolerance comes from the ID-1 row of ISO/IEC 7810:2019, Figure 1, under the heading for personalized and returned cards: 85.37 to 85.90 mm across and 53.82 to 54.18 mm tall. That row governs a card in a wallet: the standard's definition of an unused card excludes anything that has been through a personalization operation, and every bank card, ID card and license has been printed, embossed or encoded. The limits sit unevenly around the nominal size, so the figure quoted is the larger of the two excursions, because a ± sign reads as a bound.

For a sheet of A4, the tolerance comes from clause 7.1 of ISO 216:2007. The ±1.5 mm figure often quoted for A4 applies only to dimensions of 150 mm and under; both A4 edges are larger than that, so both fall in the ±2 mm band. That 2 mm is 0.673% of the 297 mm edge and 0.952% of the 210 mm edge, so each edge has its own recorded figure. Use the 297 mm edge: at ±1.0% it has the tightest band on this page.

The sheet is only ever a second opinion. Ask the tool to calibrate from paper and it refuses by name and tells you to match a card first. Paper bows away from a vertical screen, and that bias is systematic, unlike tolerance and hand slop, so it falls outside the ±1.0%. What the sheet adds is length, and the cross-check section below shows how it is used.

Reference and edge	Nominal	Object tolerance	Alignment slop	Error band	Verified on
A4 sheet, long edge	297.00 mm	0.673%	0.236%	±1.0%	2026-08-09
A4 sheet, short edge	210.00 mm	0.952%	0.333%	±1.3%	2026-08-09
bank card, ID card or driving license, long edge	85.60 mm	0.351%	0.818%	±1.2%	2026-08-09
bank card, ID card or driving license, short edge	53.98 mm	0.371%	1.297%	±1.7%	2026-08-09

A4 sheet, 210 × 297 mm (ISO 216 trimmed size)

Offered · verified 2026-08-09 · 3 sources · tier crosscheck

Where the tolerance comes from. ISO 216:2007(E), clause 7 "Tolerances", 7.1 "Permissible tolerances", quoted: "Unless closer tolerances are specified at the time of ordering, the permissible tolerances on the dimensions are as follows: a) for dimensions <= 150 mm: upper limit +1,5 mm / lower limit -1,5 mm; b) for dimensions > 150 mm but <= 600 mm: upper limit +2 mm / lower limit -2 mm; c) for dimensions > 600 mm: upper limit +3 mm / lower limit -3 mm." A4's nominal 210 × 297 mm is Table 1 of the same standard. Both A4 edges fall in band (b), because 210 mm and 297 mm are each greater than 150 mm and not greater than 600 mm; band (a) governs A5's 148 mm edge and smaller. The envelope is symmetric, so the bound is the excursion: long edge 2/297 = 0.67340% -> 0.673%; short edge 2/210 = 0.95238% -> 0.952%. Clause 7.1 calls this the range outside which a sheet cannot be regarded as being of a given size, and says a process tolerance "is likely to be more stringent", so the figure bounds the object from above, which is the only safe direction. Clause 7.2 conditions the measurement on the ISO 187 standard atmosphere.

Limits of the band. The ±2 mm is not a manufacturing tolerance, as clause 7.1 itself says: a sheet outside it stops counting as A4. A real ream is almost certainly tighter, by an amount that cannot be known, because mill tolerances are agreed between buyer and supplier and are not published. So the band bounds what ISO 216 permits and says nothing about the particular sheet in your hand. Two further effects fall outside it. Clause 7.2 measures under a controlled atmosphere, and paper is hygroscopic: in an ordinary room a sheet changes size with the humidity, more across the grain than along it, by a further tenth to half a percent, and the figure leaves that out. And a sheet held against a vertical screen bows away from the glass, which is why paper is offered here as a second opinion and never as the calibration itself.

Bank/payment card, ID card, driving license (ISO/IEC 7810 ID-1)

Offered · verified 2026-08-09 · 5 sources · tier primary

Where the tolerance comes from. ISO/IEC 7810:2019(E), Figure 1 "Card size dimensions", block headed "Personalized card and returned card requirements", row ID-1: dimension a (width) max 85,90 / min 85,37; dimension b (height) max 54,18 / min 53,82 mm. Clause 5.2 requires the whole outline to fall between two concentric rectangles at those limits; clause 5.1 conditions them at 23 C ± 3 C and 40-60 % RH. The envelope is asymmetric about the nominal, so the bound is the larger excursion. Long edge: 85,90 - 85,60 = +0.30 mm against 85,60 - 85,37 = -0.23 mm, so 0.30/85.60 = 0.3505% -> 0.351%. Short edge: 54,18 - 53,98 = +0.20 mm against 53,98 - 53,82 = -0.16 mm, so 0.20/53.98 = 0.3705% -> 0.371%.

Limits of the band. The band is a conformance requirement: the size a card must still measure if it were returned for testing. It does not come from measuring worn cards. A card bowed past 85.90 mm can exist, and it is non-conforming, so the band describes conforming cards and makes no claim about every card in circulation. The limits are also defined only at 23 C ± 3 C and 40-60 % RH, so a card measured in a hot car or in dry winter air is outside the conditions the envelope itself assumes. Clause 5.2 sets a zone between two concentric rectangles that the whole outline must sit inside, and it gives no per-edge ± for a single measurement. That is why the band spans the full range between the endpoints, and why uneven limits are bounded by their larger excursion.

Verified, and still withheld

These two were measured against the same standard, and their numbers are sound. They are withheld because a reference can fail in a way no error band covers: it can be confused with a similar object. A tolerance error can be no larger than the figure printed beside it. A mix-up has no such limit, and it shows the same tight band as a correct pick.

ISO/IEC 7810 ID-000 (mini-SIM / 2FF plug-in card)

Verified 2026-08-09 · withheld · would print ±3.6% on the long edge, ±5.8% on the short edge

Why it is not offered. Its numbers are verified, but the band this object earns is ±3.6% on the long edge and ±5.8% on the short, too wide to count as a calibration. The SIM a user is holding is almost certainly a nano-SIM (12.3 × 8.8 mm, an ETSI form factor with no published ISO tolerance), so picking this row for it is 103% wrong. The bevel the standard mandates on one corner also leaves one edge without a clean straight line to match.

Where its tolerance comes from. ISO/IEC 7810:2019(E) Figure 1, "Personalized card and returned card requirements", row ID-000: a max 25,20 / min 24,80; b max 15,15 / min 14,84 mm. The bound is the larger excursion from nominal: long ±0.20 -> 0.20/25 = 0.800%; short +0.15 against -0.16 -> 0.16/15 = 1.0667% -> 1.067%. Clause 5.2 also mandates a 3 ± 0,1 mm 45 degree ± 2 degree bevel on one corner.

What is still missing. ETSI TS 102 221 would be the proper second numeric source for SIM geometry. It was not read, because etsi.org returned HTTP 403, so nothing here rests on it.

ISO/IEC 7810 ID-2 card (older-style national ID card, visa sticker)

Verified 2026-08-09 · withheld · would print ±1.0% on the long edge, ±1.4% on the short edge

Why it is not offered. Its tolerance is verified, but ID-1 and ID-2 are both "a card from my wallet", so picking the wrong one calibrates 22.7% wrong while every on-screen signal looks like a correct match. That mix-up has no bound, and it would still be shown under a ±1.0% band. ID-2 is also a legacy-only format, and the commonest ID-2 object is a visa sticker in a passport, which cannot be laid flat against a screen.

Where its tolerance comes from. ISO/IEC 7810:2019(E) Figure 1, "Personalized card and returned card requirements", row ID-2: a max 105,30 / min 104,80; b max 74,30 / min 73,70 mm. The clause 5.2 concentric-rectangles rule and the clause 5.1 environment are the same as for ID-1. The bound is the larger excursion from nominal: long +0.30 against -0.20 -> 0.30/105 = 0.2857% -> 0.286%; short ±0.30 -> 0.30/74 = 0.4054% -> 0.406%.

What is still missing. The availability claims (Germany 2010, Romania 2021, Romanian ID-2 valid for travel to 2031) rest on a single encyclopaedia source. They would need a government registry behind them before the site relies on that reasoning.

Carried, with sizes but no verified tolerance

The nominal sizes of these objects are not in dispute, and they are printed below. The tolerance column is empty because the data holds no tolerance for these rows. Several of them once had a working figure, and it was dropped, because a small assumed tolerance understates the band while looking exactly like a measured one. This site treats an understated band as a false statement.

Object carried	Nominal	Verified tolerance	Why it is not offered
1 euro coin	23.25 mm	none recorded	Tolerance unverified. Verifying it needs an EU coin specification plus a second source.
2 euro coin	25.75 mm	none recorded	Tolerance unverified. Verifying it needs an EU coin specification plus a second source.
10 euro note	127.00 × 67.00 mm	none recorded	Tolerance unverified. Verifying it needs an ECB technical specification plus a second source.
20 euro note	133.00 × 72.00 mm	none recorded	Tolerance unverified. Verifying it needs an ECB technical specification plus a second source.
5 euro note	120.00 × 62.00 mm	none recorded	Tolerance unverified. Verifying it needs an ECB technical specification plus a second source. Euro notes differ in size by denomination, so each denomination has its own row, and no row covers them all.
50 euro note	140.00 × 77.00 mm	none recorded	Tolerance unverified. Verifying it needs an ECB technical specification plus a second source.
US Letter sheet	279.40 × 215.90 mm	none recorded	Tolerance unverified, because no size-defining standard states one. ASME Y14.1 defines the 8.5 × 11 in sheet and is silent on tolerance; ASTM D3460, the cut-size office paper specification, was withdrawn in 2014 with no replacement; and the only live clause found is a government procurement trim tolerance, which does not govern retail paper and has no independent corroboration. A4 is offered instead, and at 297 mm it is the better cross-check.
US dime	17.91 mm	none recorded	Tolerance unverified. Verifying it needs a US Mint specification plus a second source.
US nickel	21.21 mm	none recorded	Tolerance unverified. Verifying it needs a US Mint specification plus a second source.
US penny	19.05 mm	none recorded	Tolerance unverified. Verifying it needs a US Mint specification plus a second source.
US quarter	24.26 mm	none recorded	Tolerance unverified. Verifying it needs a US Mint specification plus a second source. The quarter is also a last-resort reference by design: at 24.26 mm the alignment slop alone is 2.9%.
US banknote (any denomination)	155.96 × 66.29 mm	none recorded	Tolerance unverified. An early prototype used 0.10% as a placeholder, because §6.4 of the spec says "tight; worn notes bias" and gives no number, and the addendum blocked the row over that placeholder. A small assumed figure understates the band, which §4.1 forbids, so the site's data does not carry the number at all. The row needs a BEP or Federal Reserve dimensional specification, and a second source, before it can be offered.

The cross-check, and what a disagreement means

The cross-check runs on the ruler, under the card controls: calibrate against a card, then match a sheet of A4 and press the second button. The page prints the difference between the two measurements as a percentage of their mean. Agreement within 1% earns the cross-checked state; a wider disagreement is printed as a number, and the card measurement stands. A second, independent object that agrees with the first is the only evidence on this site strong enough to earn the word verified in the readout. Two measurements of the same object are refused, because they cannot confirm each other.

A card at ±1.2% and an A4 long edge at ±1.0% can legitimately disagree by 2.1% with both readings correct and inside their own stated bands, against a gate set at 1%. So a disagreement above 1% means the second object did not confirm the first; it never means the first one was wrong. The threshold stays at a fixed 1% because anyone can check a fixed figure, and a threshold set separately for each pair of objects would have to be taken on trust.

The screen-size shortcut, and why it never earns a badge

If you know your screen's diagonal, the ruler can use it to move the outline to roughly where it belongs, so there is less dragging. The arithmetic multiplies the reported resolution by the pixel ratio, takes the hypotenuse and divides by the inches to get a pixel density. Marketed diagonals are usually within about half a percent of the truth, so the answer often lands within a percent or two of a card match.

It is not a calibration and the tool will not let it become one. The arithmetic assumes the pixel count your browser reports is the panel's native one. That is false under macOS scaled resolutions, where the framebuffer is not the panel; in Safari under page zoom, which changes the size of a CSS pixel without reporting it; and wherever a fingerprinting-resistance mode rewrites those values on purpose. Once a card has been matched, the page prints the two figures side by side as a sanity check that changes nothing, in this form: your card says 4.02 px/mm; the screen size you gave implies 3.96, which is 1.5% apart. Try it on a 13.3-inch screen, a 15.6-inch one or a 27-inch monitor.

What is measured, how it is kept, and what it cannot tell you

What is measured. The band combines two quantities from two places. The object's dimensions and permitted spread come from a published standard and are recorded with their clause and their arithmetic, so you can check the derivation without fetching the document again. Your alignment is given a fixed bound of 0.7 mm across the two edges you line up. That one figure applies to every reference, so the bands of different references differ only through their lengths and tolerances. The band printed on a reading is those two bounds added.

How it is kept. A match is stored as device pixels per millimeter, with a snapshot of what the display was reporting at the time: pixel ratio, native pixel count, browser chrome height, layout scale and pinch scale. On the next visit the page compares those readings with the display as it is then. The stored figure already absorbs a pixel-ratio change, so the page corrects for it and reports the correction. A changed native pixel count cannot be corrected that way, so the readout leaves green and asks for another twenty seconds. A stored record that fails its own schema or bounds is discarded, and the page names the reason.

What it cannot tell you. The page cannot tell whether you picked up the right object. That is the reason for the withheld list above, and the reason only one primary reference is offered. It also cannot tell that you moved to a different monitor if the new one reports the same resolution. That swap goes undetected and can leave the ruler a third wrong, so the page offers a new calibration after about six months even when every signal still agrees. Finally, it cannot see your card bowing, a gap or tilt above the display, or a curved panel. None of those is inside the band, and widening the band to cover them would make the printed number mean something other than what it says.

Questions

Which card can I use without touching the screen?

Use a clean, flat, undamaged bank card, credit card, ID card or driving license with smooth edges, in ISO/IEC 7810 format ID-1, which is nominally 85.60 by 53.98 mm. Not every card with one of those names is ID-1. Hold it just above the display without touching it; do not press or slide it on the glass. Look square-on, because any gap or tilt is outside the printed error band. Do not use a card that is bowed or has a damaged edge.

Why are only two reference objects offered?

Because only a bank card and an A4 sheet meet the bar. A reference is offered only when its dimensional tolerance has been read out of a published standard and recorded with at least two independent sources and a date. The ID-2 card and the mini-SIM have tolerance records as well, but they are withheld because they can be confused with other card formats. Banknotes, US Letter sheets and coins are carried with their nominal dimensions and no tolerance; the tool refuses them by name and does not calibrate from a guessed tolerance.

An older-style ID card has a better tolerance. Why is it refused?

Because it is easy to mix up with a bank card. Its own band would be about 1.0 percent against a bank card's 1.2, so on tolerance alone it wins. Both are cards from your wallet, though, and picking the wrong one calibrates 22.7 percent wrong with every signal on the page identical and no second reference to catch it. The printed band covers tolerance, and no band covers picking the wrong object.

Where does the plus or minus 1.2 percent come from?

It is two bounds added together. Under the personalized and returned card row of ISO/IEC 7810:2019, the card may sit up to 0.30 mm above its nominal 85.60 mm long edge, which is 0.351 percent. Lining two edges up by eye is taken as 0.7 mm across the pair, which is 0.818 percent of 85.60 mm. The two are added, not combined in quadrature, because a plus-or-minus sign reads as a bound and a bound is what the page states.

How often does a calibration need redoing?

Only when the page asks, or when you change monitors. The page re-checks the display on every visit and on a timer, and asks you to match again when the display reports something new or after about six months without a new match. It cannot detect a swap to a different monitor of the same resolution, so if you have moved the same profile to another screen, calibrate again even though the page has not asked.

Go and measure something

Open the ruler as it comes, or start it with the choice already made: with the long edge horizontal, with the long edge upright (the one a phone in portrait needs), or on the short edge, whose band is wider by 0.5 of a percentage point. In every mode, hold the clean card just above the display without touching it. Each link sets its reference, edge and orientation in the address, and if the ruler cannot use one of the three it refuses it by name and does not fall back to a default it did not announce. The other instruments are in the navigation above.

Calibrating an on-screen ruler with a bank card

The calibration, step by step

1. Open the ruler and see the assumption named

2. Find a clean, undamaged card

3. Align it just above the outline and drag until the edges agree

4. Press the button and read your error band

5. Come back, and let it re-check itself