Online Ruler — Actual Size (cm, mm, inches)
A real-size ruler on your screen — calibrate it with any bank card, then measure.
Ruler
Calibrate for actual size
Do this once for each screen: it is saved in this browser and used automatically next time.
Hold a bank card — or any card of the standard 85.60 × 53.98 mm size — flat against the screen with its corner in the top-left corner of the outline. Then make the outline exactly as big as the card, so its edges just disappear under the card.
85.60 × 53.98 mm
Hold the ruler or tape against the screen with its zero at the left end of the bar, then make the bar exactly that long.
About the Online Ruler — Actual Size (cm, mm, inches)
A ruler you can hold things against: centimetres and millimetres on one edge, inches down to 1/16 on the other, horizontal or vertical, in the page or full screen. Two movable markers measure the distance between them, so you can size a screw, a coin, a SIM card or a print without a real ruler.
Screens differ, so a centimetre on a web page is a real centimetre only by chance: web pages assume 96 pixels per inch, but a phone packs far more of them into a real inch than a TV does. Calibrate once by matching an outline to a bank or ID card (85.60 × 53.98 mm), by entering your screen’s diagonal, or by matching any length you know. The calibration is saved for this screen in this browser.
How to use it
- Under Calibrate for actual size, choose Bank or ID card, hold a card flat against the screen with its corner in the top-left corner of the outline, and move the slider until the outline is exactly the size of the card. Press Save calibration.
- No card? Choose Screen size and enter the diagonal from your device’s specifications (for example 15.6 inches), or Known length to match a length on a real ruler or tape.
- Choose Horizontal or Vertical and the units. For the longest ruler, press Full screen.
- Hold the object against the screen, then drag marker A to one end and B to the other — or select a marker and move it with the arrow keys. The distance between the markers is shown in mm, cm and inches.
Examples
The card’s long side matches the outline at 323.5 px
1 cm = 37.8 px · 1 inch = 96.0 px
323.5 ÷ 85.60 mm = 3.779 px per mm; × 25.4 = 96.0 px per inch.
15.6-inch laptop showing 1920 × 1080 at 100 % scaling
141.2 px per inch
√(1920² + 1080²) = 2,202.9 px across the diagonal, and 2,202.9 ÷ 15.6 = 141.2.
Marker A at 10.0 mm, marker B at 52.5 mm
42.5 mm = 4.25 cm = 1.67 in (about 1 11/16 in)
Common uses
- Measuring a screw, a bead, a button or a SIM card when there is no ruler at hand.
- Checking that a print — a business card, a label or a passport-size photo — came out at the right size.
- Finding the real size of things on your own screen, such as text or a button.
- Showing children how centimetres and inches compare on one ruler.
Why a ruler on a screen needs calibrating
Web pages are laid out in CSS, which defines 1 inch as 96 pixels and 1 cm as 96 ÷ 2.54 ≈ 37.8 pixels. On screens these are reference pixels, and the CSS specification says that the physical units then “might not match their physical measurements”. A monitor, a laptop and a phone each show 96 of them per inch at a different real size, so an uncalibrated ruler is right only by chance — this page warns you until you calibrate.
Calibrating finds how many of those pixels fit in a real millimetre on your screen. Every tick is drawn from that one number: the 1 cm tick sits 10 × (pixels per mm) from zero and the 1 inch tick 25.4 × (pixels per mm), because an inch is exactly 25.4 mm.
Which calibration to use
- Bank or ID card — usually the most accurate. Bank cards (debit, credit and ATM cards) have the ID-1 size of 85.60 × 53.98 mm set by ISO/IEC 7810, and so do many ID cards and driving licences; if in doubt, use a bank card. Match the long side when it fits on the screen: the longer the side you match, the smaller the error. If the outline cannot grow as big as the card, the page does not save it: turn the outline upright, or use one of the other methods.
- Screen size — quick, and accurate when the diagonal you enter is exact. Advertised sizes can be rounded: a monitor sold as “24-inch” can have a 23.8-inch picture, which makes the ruler about 1 % off. Calibrate at 100 % page zoom, and on the screen the browser window is on.
- Known length — match any length you trust, such as 10 or 15 cm on a real ruler or tape measure. Use the longest length that fits with room to spare: a bar that has reached the edge of the screen cannot be saved, because the real length may be longer still.
Measuring accurately
- Rest the object lightly on the screen; do not press, as pressing can damage a display.
- Look straight at the end you are reading: the picture sits slightly behind the glass, so looking at an angle shifts it.
- Use the markers rather than the zero end: put A on one end of the object and B on the other, so it does not matter where the object starts.
- Readings are shown to 0.1 mm, but lining things up by eye on a screen is rarely better than about half a millimetre, and a calibration that is 1 pixel off on the card grows with the length you measure.
- Each marker step is one press of an arrow key: 0.5 mm, or 1/32 inch with inch units (Shift for 5 mm or 1/4 inch, Page Up / Page Down for 1 cm or 1 inch).
Sources
- ID-1 card size, 85.60 × 53.98 mm: ISO/IEC 7810:2019, Identification cards — Physical characteristics (iso.org).
- 1in = 2.54cm = 96px, and the note that physical units may not match on screens: W3C, CSS Values and Units Module Level 4, §6.2 (w3.org).
- 1 inch = 25.4 mm exactly: NIST, SI Units – Length (nist.gov).
Limitations
- The ruler is only as accurate as its calibration and your eye: expect about ±0.5 mm on short lengths, more on long ones.
- It is as long as the screen (or the window) — use full screen for the longest ruler. iPhones do not let web pages go full screen.
- If you zoom the page or pinch-zoom on a phone, return to normal zoom before measuring. Most browsers report page zoom so the ruler can follow it; check against your card again if unsure.
- The calibration belongs to one screen in one browser: another monitor, device or browser needs its own. Clearing the site data removes it.
- A browser knows a screen only by its size in pixels. The page warns when that size or the pixel ratio has changed since you calibrated, but two screens with the same resolution and scaling look identical to it — check against your card after moving the window from one to the other.
- The screen-size method relies on the screen size the browser reports; privacy settings that hide it (such as Firefox’s resist-fingerprinting) make it wrong, so use the card there.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
Is this ruler actual size?
Once it is calibrated, yes: lined up carefully, small objects measure to within about half a millimetre. Before that it uses the web’s nominal 96 pixels per inch, which matches few screens, and the page tells you so.
What can I calibrate with if I don’t have a card?
Enter your screen’s diagonal (from the device’s specifications or the box) under Screen size, or match a length you know — 10 cm on a school ruler, for example — under Known length.
What size is a credit or debit card?
85.60 × 53.98 mm (3.370 × 2.125 in), the ID-1 size of ISO/IEC 7810, with corners rounded to a radius of about 3.18 mm.
Do I need to calibrate every time?
No. The calibration is saved in this browser for this screen and used automatically next time. Calibrate again for another monitor or device.
How do I measure something longer than my screen?
Use Full screen for the longest ruler. For longer objects, mark where the ruler ends, move the object along and add the lengths — or use a tape measure.
Does the ruler work on a phone?
Yes. Calibrate with a card held against the phone (the outline turns upright when the card would not fit across), then use the vertical ruler for the longest length.