Measurement · card-calibrated
Online Ruler
An online ruler in centimeters and inches that reads true physical size once you calibrate it against a bank card. Drag anywhere on the rule to place the marker and it reports millimeters, centimeters, decimal inches and fractional inches to the nearest sixteenth. It starts on the CSS default of 96 pixels per inch, which is close on some screens and badly wrong on others — the thirty-second calibration below fixes that permanently.
Millimeters
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Centimeters
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Inches (decimal)
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Inches (tape)
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Click or drag anywhere on the rule to place the measuring marker. Arrow keys nudge it a millimeter at a time; hold shift for a centimeter.
The ruler above assumes 96 CSS pixels per inch. That assumption is wrong on most phones and on many high-DPI laptops. Calibrate against a bank card and it becomes correct for this screen.
Printed from HumanBenchTests. Set your printer to 100% scale — “fit to page” will resize the rule and make it wrong.
Why an uncalibrated screen ruler is wrong
The CSS specification defines a reference pixel as 1/96 of an inch at a nominal viewing distance. If that were the whole story, drawing a line 96 pixels long would produce exactly one inch on every display and no calibration would be needed.
It is not the whole story. Browsers apply a device pixel ratio, phones apply their own scaling so that text is legible at arm’s length, operating systems offer display scaling at 125%, 150% and 200%, and users zoom. Each of those multiplies the physical size of a CSS pixel. The result is that an uncalibrated on-screen ruler can be out by 40% — enough to make it worse than useless, because it looks authoritative while being wrong.
Calibrating divides the measured card width in pixels by 85.60 to get pixels per millimeter for your specific display. Everything else — tick spacing, inch marks, the marker readout — is derived from that single number.
How to calibrate
- 01
Find any bank card
A credit card, debit card, driving license or store card. They are all ISO/IEC 7810 ID-1, so any of them works.
- 02
Hold it flat against the screen
On the dashed outline shown during calibration. Flat, not angled, and look straight down at it.
- 03
Drag until the outline matches
Adjust the slider until the outline is exactly the size of the card, then use the fine buttons for the last fraction of a millimeter.
- 04
Save it once
The calibration is stored on this device and reused every time you come back. Recalibrate if you move to a different screen or change your display scaling.
Decimal inches to sixteenths
The marker readout gives inches in tape-measure form because that is how imperial measurements are actually used. Conversion rounds to the nearest sixteenth — the finest division on a standard tape — and then reduces the fraction, so 8/16 reads as 1/2 and 16/16 rolls over into the next whole inch rather than reading “0 16/16”.
| Decimal inches | Millimeters | Tape-measure form |
|---|---|---|
| 0.1250 | 3.17 | 1/8" |
| 0.5000 | 12.70 | 1/2" |
| 1.0625 | 26.99 | 1 1/16" |
| 2.3750 | 60.32 | 2 3/8" |
| 3.4375 | 87.31 | 3 7/16" |
| 5.9375 | 150.81 | 5 15/16" |
| 11.9600 | 303.78 | 11 15/16" |
The last row is the interesting one: 11.96 inches is 191.36/16, which rounds to 191/16 — not a clean number, so it reads 11 15/16 rather than being quietly rounded to a foot. Conversions that round to the nearest whole unit before converting are the most common source of error in imperial arithmetic.
What this is useful for
- Measuring something small when there is no rule to hand — a screw thread, a photograph, a watch strap width, a phone case cutout.
- Checking a print size before printing — hold the sheet against the screen and compare, or measure the object and compare against the intended output.
- Ring and band sizing — although a printed sizer is more reliable for circumference, since a screen rule cannot wrap.
- Verifying that a display is scaled as you expect — the calibration readout tells you how far your screen is from the 96 px-per-inch assumption, which is a useful thing to know when designing anything for print.
Measuring the screen rather than with it? scrolltest.io covers device characteristics — refresh rate, touch sampling, scroll behavior. And if you have arrived here from a color or vision task, the color blind test is the other instrument here that depends on your display behaving predictably.
Online ruler questions
Is an online ruler accurate?
Only if it is calibrated. A CSS pixel is defined as 1/96 inch at a reference viewing distance, but browsers on phones and high-DPI laptops apply their own scaling on top, so an uncalibrated on-screen ruler can be 40% wrong. Calibrate against a bank card once and this one is accurate to well under a millimeter for that screen.
Why calibrate with a credit card?
Because its size is legislated. Every credit card, debit card, driving license and most ID cards conform to ISO/IEC 7810 ID-1: 85.60 mm by 53.98 mm, to a tolerance of a few hundredths of a millimeter. It is the most precise physical reference object most people have in their pocket.
Does the calibration stick?
Yes. It is stored in your browser's localStorage on this device, so it survives reloads and returns. It is per-browser and per-device — calibrate again on your phone. Nothing is uploaded and there is no account.
Can I use this ruler in inches?
Yes, both edges are live at once. The bottom edge is graduated in inches down to sixteenths, and the readout gives inches as a decimal and in tape-measure fractional form — 3.4375 inches shows as 3 7/16 inch, reduced properly.
Can I print this ruler?
You can, but set the printer to 100% scale rather than 'fit to page', which resizes everything and makes the rule wrong. A printed ruler is calibrated by the printer, not by the screen calibration above, so check it against a card afterwards.
How precise is a screen ruler really?
After calibration, precision is limited by pixel pitch. A typical laptop has roughly 4 pixels per millimeter, so single-pixel placement is about a quarter of a millimeter — comparable to reading a plastic rule by eye. The bigger real-world error is parallax: look straight down at the screen, not across it.
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- check your typing speedA 25-word sprint that gives a usable speed reading in under a minute, plus the four things that distort it.WPM · CPM · accuracy
- average typing speed by percentileThe full distribution table — where 40 WPM actually sits, and what the 25th, 50th, 75th and 95th percentiles are.your percentile
Testing the hardware rather than the person? device tests — scroll, touch, refresh rate and input latency lives on a sister site. And if you want to see through someone else’s eyes rather than screen your own, use the simulate how an image looks to a protan, deutan or tritan viewer.