Online Ruler: an actual-size cm, mm, and inch ruler on your screen
Any standard bank, credit, debit, or ID card works: the ISO/IEC 7810 ID-1 format is exactly 85.60 mm wide everywhere in the world. Hold the card flat against the screen, match the width, then apply.
A ruler that earns the words actual size
Most online rulers draw their markings assuming every screen packs 96 pixels into an inch, which has not been true of real displays for a long time: a 24-inch office monitor sits near 92 PPI while a modern phone exceeds 460. This ruler is honest about that. It starts from a sensible estimate, tells you it is an estimate, and gives you four ways to calibrate in under a minute, after which the centimetres on your screen are real centimetres. Drag the orange markers to measure between two points, switch on the corner ruler to measure in two axes, use the measure box for on-screen objects, and print a paper ruler when you need to take the measurement with you. Everything runs in your browser; nothing you do here leaves your device.
Is an online ruler actual size?
Only after calibration. A web page cannot reliably ask your screen how large its pixels physically are, so any uncalibrated on-screen ruler is an approximation that may be off by 20 percent or more depending on the display. Calibrating against an object of known size fixes this completely: a bank card is ideal because the ISO/IEC 7810 ID-1 standard makes every card exactly 85.60 mm wide. Match the on-screen outline to your card once, and the ruler is then rendered at your screen's true pixels-per-inch.
The section below shows how widely real displays vary, computed from their specifications.
How to calibrate the online ruler in 30 seconds
Set browser zoom to 100%
Press Ctrl+0 (Cmd+0 on a Mac) to reset zoom. Any zoom level other than 100% scales the ruler along with the page and breaks the calibration.
Open Calibrate and pick a method
The bank card method is the most reliable: every card follows the ISO/IEC 7810 standard at exactly 85.60 mm wide. On a large monitor, a sheet of A4 or Letter paper is equally accurate and easier to hold.
Match the outline to the real object
Hold the card or paper flat against the screen and drag the slider until the outline's width matches the object exactly. Take your time at the edges; a millimetre of mismatch on the card is roughly a 1 percent error in everything you measure.
Apply, and it sticks
Press Apply. The calibration is saved on this device, so the ruler is correct every time you return, and the toolbar shows your screen's measured PPI. Recalibrate only if you change displays or display scaling.
Measure
Read lengths directly off the scales, drag the two orange markers to measure between points with readouts in cm, decimal inches, tape fractions, and pixels, or switch on the measure box to size an object held against the screen.
Why every screen needs its own calibration
Pixel density is the number of pixels a display packs into one physical inch, and it varies enormously across common hardware. The chart shows PPI computed from each display's published specifications using the formula PPI = √(width² + height²) ÷ diagonal. A ruler drawn for one of these screens is wrong on all the others, which is why a fixed 96 PPI assumption cannot deliver actual size.
Computed values; manufacturers may quote marginally different figures based on exact panel dimensions. The point stands regardless: real displays span a five-fold range of pixel density, so actual size requires per-device calibration.
Reference objects: things with standardized sizes
Calibration and sanity checks both rely on objects whose dimensions are fixed by standards. These are the reliable ones most people have within reach.
| Object | Width | Height | Why it is reliable |
|---|---|---|---|
| Bank / credit / debit card | 85.60 mm | 53.98 mm | ISO/IEC 7810 ID-1: identical worldwide, rigid, easy to hold to a screen |
| A4 paper | 210 mm | 297 mm | ISO 216 standard used in most of the world |
| US Letter paper | 215.9 mm (8.5 in) | 279.4 mm (11 in) | ANSI standard size in the US and Canada |
| US quarter | 24.26 mm dia. | – | US Mint coin specification |
| US one-dollar bill | 156 mm (6.14 in) | 66 mm (2.61 in) | All US notes share one size, per the Bureau of Engraving and Printing |
Centimetres, inches, and tape fractions
One inch is exactly 25.4 mm, fixed by international agreement in 1959, so every conversion below is exact arithmetic, not approximation. The fractions table is the tape-measure ladder the ruler's fraction readout uses.
| Metric | Imperial |
|---|---|
| 1 cm | 0.394 in |
| 2.54 cm | 1 in (exact) |
| 5 cm | 1.97 in |
| 10 cm | 3.94 in |
| 15 cm | 5.91 in |
| 30 cm | 11.81 in |
| 30.48 cm | 1 ft (exact) |
| Fraction | Decimal | Millimetres |
|---|---|---|
| 1/16 in | 0.0625 | 1.5875 mm |
| 1/8 in | 0.125 | 3.175 mm |
| 1/4 in | 0.25 | 6.35 mm |
| 1/2 in | 0.5 | 12.7 mm |
| 3/4 in | 0.75 | 19.05 mm |
| 1 in | 1.0 | 25.4 mm |
What most online rulers are missing
| Capability | Typical online ruler | This ruler |
|---|---|---|
| Honest about uncalibrated accuracy | Claims actual size at a 96 PPI guess | Says it is an estimate until you calibrate |
| Calibration methods | One, if any | Card, A4/Letter paper, screen size, manual PPI |
| Calibration persists | Re-calibrate every visit | Saved on your device, plus a calibrated share link |
| Inch fractions like a tape measure | Decimal only | Readout to the nearest 1/16, reduced properly |
| Measure between two points | Read the scale yourself | Two draggable markers with live multi-unit readout |
| Two-axis measurement | Horizontal only | Corner ruler and a resizable measure box |
| Pixel ruler mode | Rare | Built in, for design and screenshot work |
| Printable paper ruler | No | Print-exact ruler using physical print units |
| Works offline, no ads or signup | Ad-loaded | Single self-contained page, nothing leaves your device |
An on-screen ruler for shopping, making, and checking
Will it fit? Check before you order
Product listings give dimensions; your desk gives doubt. A calibrated on-screen ruler turns "9.7 inches" into a length you can see at true size, useful for phones, frames, watch faces, and anything where a centimetre changes the decision. For rings and jewellery, measure an existing ring's inner diameter against the screen with the measure box.
Quick measurements without the toolbox
When the tape measure has wandered off, a calibrated screen handles seam allowances, button diameters, hem depths, and pattern checks, with the fraction readout speaking the same 1/16-inch language as sewing patterns and woodworking plans. For anything longer than your screen, print the paper ruler.
Verify sizes before printing
Check that a label, photo, or document element will print at the size you intend by measuring it on screen at 100% zoom with a calibrated ruler, and confirm the result afterwards against the printable ruler. Catching a wrongly scaled PDF before it hits 50 sheets of label stock pays for the 30-second calibration many times over.
A ruler in every browser
For classrooms and homework, an on-screen ruler means every student has the same instrument, and the calibration step itself is a genuinely good lesson: why screens differ, what pixel density means, and why measurement always begins with checking your instrument.
Six on-screen measuring mistakes
Trusting an uncalibrated ruler
Without calibration, an on-screen ruler is a guess that can be off by a fifth or more, in either direction, depending on your display.
Calibrate once against a bank card. It takes 30 seconds and is saved for next time.Browser zoom not at 100%
Zoom scales the whole page, ruler included, so a calibration done at 110% is wrong at 100% and vice versa.
Press Ctrl+0 (Cmd+0 on Mac) before calibrating and before measuring.Changing display scaling afterwards
Switching OS display scaling, moving the window to another monitor, or docking a laptop changes effective pixel density and silently invalidates the calibration.
Recalibrate after any display change; the saved value belongs to one screen at one scaling.Measuring thick or curved objects
Holding a mug or a wrist to the glass adds parallax: the object sits ahead of the pixels, so your eye's angle shifts the reading.
Measure flat objects against the screen, view straight-on, and use string-plus-ruler for circumferences.Mixing up 0.5 in and 5/8 in
Decimal inches and tape fractions sit close enough to confuse, and a 1/8-inch slip ruins a cut or a hem.
Use the fraction readout, which rounds to the nearest 1/16 and reduces it, and double-check against the decimal.Measuring from a photo of the screen
Photographing a ruler on one screen to measure on another stacks two unknown scales and a lens distortion on top of each other.
Measure on the device showing the ruler, or print the paper ruler and measure in the real world.Screen measurement terms, defined
- PPI (pixels per inch)
- The number of pixels a display fits into one physical inch. The single number that connects on-screen pixels to real-world length, and the value calibration measures.
- Pixel density
- The general term for how tightly packed a display's pixels are, usually quoted as PPI. Higher density means smaller pixels and sharper images.
- CSS pixel
- The unit web pages are laid out in. On high-density screens one CSS pixel spans several hardware pixels, which is one reason a page cannot simply assume its pixels have a fixed physical size.
- Device pixel ratio
- How many hardware pixels correspond to one CSS pixel, commonly 1 on basic monitors, 2 or more on phones and high-density laptop screens.
- Calibration
- Measuring your screen's true pixel density by matching an on-screen shape to a real object of standardized size, such as a bank card.
- ISO/IEC 7810 ID-1
- The international standard defining the size of bank and ID cards: 85.60 by 53.98 mm with rounded corners, which is what makes any card a reliable calibration object.
- Display scaling
- An operating system setting that renders everything larger or smaller than native resolution. Changing it changes effective pixel density, so the ruler must be recalibrated.
- Actual size
- Rendered so that one on-screen centimetre is one physical centimetre. Achievable on screen only after per-device calibration, and in print by using physical units.