Guide · Reference objects · Checked 2026-08-09
Credit card size, and the other objects you can measure with
A credit card is 85.60 × 53.98 mm. In other units that is 8.560 × 5.398 cm, or 3.370 × 2.125 inches, and it is the same in Tokyo as in Toledo because a standard says so rather than because a habit settled there. That is what makes the card in your wallet the most useful measuring reference most people own — and this page is the rest of the drawer, with the size of each object, how far each one is permitted to stray from it, and which of them are good enough to calibrate a screen from.
The numbers in the table below are rendered straight out of the same data file the ruler calibrates from, so the page and the tool cannot drift apart. Every row also says whether this site will calibrate from that object, and if not, why not.
How big is a credit card?
85.60 mm long by 53.98 mm high, with rounded corners. Debit cards, credit cards, national ID cards, most driving licences, hotel key cards and the majority of gift and loyalty cards are all the same rectangle: the ID-1 format of ISO/IEC 7810, the international standard for identification cards. The dimensions are given to hundredths of a millimetre because cash machines, card readers and passport gates all have to accept a card cut on another continent a decade earlier.
Converted, since the standard is metric and half the world is not:
- Millimetres: 85.60 × 53.98 mm
- Centimetres: 8.560 × 5.398 cm — near enough 8.6 × 5.4 cm for anything you would do with a tape measure
- Inches: 3.370 × 2.125 in
- In fractions: 2 1⁄8 in is 2.125 in, so the short edge is a fraction you can find on a tape measure to within two ten-thousandths of an inch. The long edge is not: 3 3⁄8 in is 3.375 in, five thousandths of an inch over.
The conversions are exact arithmetic — an inch is defined as 25.4 mm, with no rounding anywhere in it — so nothing is lost going between the columns. What is not exact is the object, and that is the next section.
How much a card may actually vary
85.60 mm is the nominal size. The standard also publishes the envelope a card has to stay inside, and the row that governs a card in a wallet is the one for personalized and returned cards in Figure 1 of ISO/IEC 7810:2019 — because the standard's own definition of an unused card excludes anything that has been through a personalization step, and every bank card, ID card and licence has been printed, embossed or encoded. That row is 85.37 to 85.90 mm across and 53.82 to 54.18 mm high.
The envelope is not symmetric about the nominal size, so the honest figure is the larger excursion rather than the average of the two: 0.30 mm over 85.60 mm is 0.351% on the long edge, and 0.20 mm over 53.98 mm is 0.371% on the short one. Those percentages are small, and they are not the biggest term in a screen calibration. Lining two edges up by eye is taken here as 0.7 mm across the pair, which is 0.818% of the long edge — a little over twice the object's own contribution. The card is not the limiting factor, but it is a real term in the sum and not a rounding error, which is why the ruler prints ±1.2% beside a long-edge match and ±1.7% beside a short-edge one rather than a single tidy number.
Two things sit outside those percentages and no band covers them. A card that has spent two years bowing to a back pocket presents a foreshortened width, and nothing on a screen can see that it is bent. And the standard's limits are defined at 23 °C ± 3 °C and 40–60% relative humidity, so a card measured on a car dashboard in August is outside the conditions its own envelope assumes.
Every object this site carries, in millimetres, centimetres and inches
Sixteen objects, of which two are offered as references. The last two columns are the part worth reading: publishing a dimension and offering to calibrate from it are different acts, and an object can have a perfectly good published size and still be a bad thing to hand a beginner. Where a row is not offered, the reason recorded against it is printed in full rather than summarised.
| Object | Millimetres | Centimetres | Inches | Offered here? | Why not, in the words recorded with the row |
|---|---|---|---|---|---|
| bank card, ID card or driving licence | 85.60 × 53.98 mm | 8.560 × 5.398 cm | 3.370 × 2.125 in | Offered as the primary reference — ±1.2% on the long edge, ±1.7% on the short edge · verified 2026-08-09 | It is offered. |
| older-style national ID card (105 × 74 mm) | 105.00 × 74.00 mm | 10.500 × 7.400 cm | 4.134 × 2.913 in | Verified and withheld — would have printed ±1.0% on the long edge, ±1.4% on the short edge | Verified, deliberately not offered. ID-1 and ID-2 are both "a card from my wallet", so a mis-pick calibrates 22.7% wrong with every on-screen signal identical — an unbounded identification error dressed in a ±1.0% band. Legacy-only in issue, and the commonest ID-2 object is a visa sticker in a passport, which cannot be laid flat against a screen. |
| mini-SIM plug-in card (25 × 15 mm) | 25.00 × 15.00 mm | 2.500 × 1.500 cm | 0.984 × 0.591 in | Verified and withheld — would have printed ±3.6% on the long edge, ±5.8% on the short edge | Verified, deliberately not offered. The band this object earns is ±3.6% on the long edge and ±5.8% on the short — not a calibration. The object a user holds is almost certainly a nano-SIM (12.3 × 8.8 mm, an ETSI form factor with no published ISO tolerance), so a mis-pick is 103% wrong. And the mandated corner bevel means one edge is not a clean straight edge to butt against. |
| A4 sheet | 297.00 × 210.00 mm | 29.700 × 21.000 cm | 11.693 × 8.268 in | Offered as the second reference only — ±1.0% on the long edge, ±1.3% on the short edge · verified 2026-08-09 | It is offered. |
| US Letter sheet | 279.40 × 215.90 mm | 27.940 × 21.590 cm | 11.000 × 8.500 in | Carried, not offered — no verified tolerance | Tolerance unverified, and the honest finding is that 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 that does not govern retail paper and has no independent corroboration. A4 is offered instead, and it is a better cross-check anyway at 297 mm. |
| US banknote (any denomination) | 155.96 × 66.29 mm | 15.596 × 6.629 cm | 6.140 × 2.610 in | Carried, not offered — no verified tolerance | Tolerance unverified. The spike entered 0.10% as a placeholder because §6.4 says "tight; worn notes bias" and gives no number; the addendum blocked the row on exactly this. A small assumed figure UNDERSTATES the band, which §4.1 forbids, so the number is not carried forward at all. Needs a BEP or Federal Reserve dimensional specification, and a second source, before it can ship. |
| 50 euro note | 140.00 × 77.00 mm | 14.000 × 7.700 cm | 5.512 × 3.031 in | Carried, not offered — no verified tolerance | Tolerance unverified — needs an ECB technical specification plus a second source. |
| 20 euro note | 133.00 × 72.00 mm | 13.300 × 7.200 cm | 5.236 × 2.835 in | Carried, not offered — no verified tolerance | Tolerance unverified — needs an ECB technical specification plus a second source. |
| 10 euro note | 127.00 × 67.00 mm | 12.700 × 6.700 cm | 5.000 × 2.638 in | Carried, not offered — no verified tolerance | Tolerance unverified — needs an ECB technical specification plus a second source. |
| 5 euro note | 120.00 × 62.00 mm | 12.000 × 6.200 cm | 4.724 × 2.441 in | Carried, not offered — no verified tolerance | Tolerance unverified — needs an ECB technical specification plus a second source. Euro notes differ by denomination, so there is one row per denomination and never a family row. |
| 2 euro coin | 25.75 mm diameter | 2.575 cm | 1.014 in | Carried, not offered — no verified tolerance | Tolerance unverified — needs an EU coin specification plus a second source. |
| US quarter | 24.26 mm diameter | 2.426 cm | 0.955 in | Carried, not offered — no verified tolerance | Tolerance unverified — needs a US Mint specification plus a second source. Also a last-resort reference by design: at 24.26 mm the alignment slop alone is 2.9%. |
| 1 euro coin | 23.25 mm diameter | 2.325 cm | 0.915 in | Carried, not offered — no verified tolerance | Tolerance unverified — needs an EU coin specification plus a second source. |
| US nickel | 21.21 mm diameter | 2.121 cm | 0.835 in | Carried, not offered — no verified tolerance | Tolerance unverified — needs a US Mint specification plus a second source. |
| US penny | 19.05 mm diameter | 1.905 cm | 0.750 in | Carried, not offered — no verified tolerance | Tolerance unverified — needs a US Mint specification plus a second source. |
| US dime | 17.91 mm diameter | 1.791 cm | 0.705 in | Carried, not offered — no verified tolerance | Tolerance unverified — needs a US Mint specification plus a second source. |
Two patterns are worth naming. The euro notes get one row per denomination because they genuinely differ — a €5 is 120 × 62 mm and a €50 is 140 × 77 — so a single "euro note" row would be a wrong number for five of the six. And US Letter is carried but unverified for an unusual reason: no size-defining standard states a tolerance for it. ASME Y14.1 fixes the 8.5 × 11 inch sheet and says nothing about how far a real sheet may stray, and the cut-size office paper specification that once did was withdrawn in 2014 without replacement. A4 is offered instead, and at 297 mm on the long edge it is the better check anyway.
Measuring without a ruler, and what each object is actually good for
The useful thing about a reference object is not that it is precise. It is that it is long. Your hand-and-eye error when you line an edge up is roughly fixed at a fraction of a millimetre; what changes is how big a share of the object that fraction is. Take the same 0.7 mm of slop across four of the objects above and it becomes 0.236% of an A4 sheet's long edge, 0.449% of a dollar bill's, 0.818% of a card's long edge, and 2.9% of a US quarter's diameter. That is the whole ranking in one line: long and rigid beats short and precise, every time.
For an inch. The neatest fact in the table is that a US penny is 19.05 mm, and 19.05 mm is exactly three quarters of an inch — 0.750 in, with no rounding at all. Four pennies in a row are three inches. A quarter is 24.26 mm, which is 0.955 in, so it is 4.5% short of an inch and is the wrong object to guess with. The card's short edge at 2.125 in is the closest thing to a ruled length most people carry: two of them, butted end to end, are 4.250 in.
For centimetres and millimetres. A card is 8.560 cm by 5.398 cm, which makes it a decent stand-in for an 8.5 cm line and a poor one for a 5 cm line. A US banknote is 15.596 cm long whatever the denomination. Down at the millimetre end, a dime is 17.91 mm and a nickel 21.21 mm — close enough together that guessing which is which matters more than the numbers do.
What none of them is. These coin and banknote figures are nominal dimensions, not verified ones. This site has not read a mint or central-bank tolerance out of a source it can cite for any of them, so it publishes the size and refuses to calibrate from the object, and the table above says so on every row. Arithmetic on a nominal figure is exact; the object it describes is only as good as the specification nobody here has checked yet.
The references that are the wrong answer
Never body parts. "My thumb is about an inch", "this knuckle is two centimetres". Body dimensions vary between people and your memory of them drifts; they are fine for guessing whether a shelf will fit and useless for calibrating an instrument, because the error is unknown and unbounded and you will never find out it was there.
Not a soft tape against glass. Cloth tapes stretch, steel ones bow away from a vertical screen, and the hook on the end is built for catching a board edge rather than sitting flush against a flat pane.
And not an object you might have mixed up. This is the failure that has no band. An older-style national ID card is 105 × 74 mm and its own tolerance is better than a bank card's — it would print ±1.0% instead of ±1.2% — and it is still refused here, because a bank card and an older ID card are both "a card from my wallet". Pick the wrong one and the calibration is 105/85.6 = 22.7% wrong with every signal on the page identical: a green state, a tight band, a plausible pixel density. A tolerance error is bounded by the number printed next to it. An identification error is bounded by nothing, and a tight band printed over a 23% error is worse than no row at all. The same argument retires the mini-SIM row, where the object most people are holding is a nano-SIM that the standard does not define at all, and the mis-pick is 103% wrong.
What to do with a reference once you have one
On a screen, the point of a known length is that it converts pixels into millimetres. You lay the card on the glass, drag the ruler's outline until the edges agree, and the page divides the matched width by 85.60 to learn how many pixels your display puts in a millimetre — which is a number no browser can otherwise know, and the reason an uncalibrated on-screen ruler is typically a quarter to a third short. Why that is true by definition rather than by accident is its own guide.
Three habits close most of the remaining gap, and they are the same three whichever object you use. Match the card's physical edges rather than its printed artwork, since the standard fixes the blank and not the printing. Look straight on, because the card sits on the surface of the glass while the pixels glow a little beneath it, and an angled view shifts the apparent alignment the way a speedometer reads differently from the passenger seat. And prefer the longest edge that fits: on a phone in portrait an 85.60 mm edge will not go across a 64 mm screen, so stand the card upright and keep the long edge rather than dropping to the short one and its wider band.
The figures on this page were read out of the site's reference data on 2026-08-09, and each row carries the date its own dimensions were verified. Where that date is absent, so is the verification, and the row says so.