Online ruler at exact, actual size
Exact Ruler turns your screen into a real ruler in centimeters, millimeters, inches and pixels. Calibrate it once with any bank card and every mark on the screen matches the real world.
How to measure in three steps
- Calibrate. Click the status badge in the toolbar and match the outline to a bank card. It takes about ten seconds and is saved for your next visit.
- Pick a ruler edge and a unit. Turn on the top, bottom, left or right ruler, then choose cm, in or px. You can show all four rulers at once.
- Measure. Hold the object flat against the screen with one end on the 0 mark and read where the other end falls. For a precise reading, click the ruler at each end to drop guides; the distance between them appears automatically.
Why screen rulers are often the wrong size
Browsers measure everything in CSS pixels, and the CSS standard fixes one inch at 96 CSS pixels. Real screens don’t follow that rule. A laptop with display scaling, a phone and a large monitor all put a different number of CSS pixels into a physical inch, so a ruler that trusts the 96-pixel inch can be off by 10 to 40 percent.
Calibration measures that number for your screen. Once it is known, a centimeter on the screen is a centimeter on your desk.
Which calibration method to use
Bank card (most accurate)
Credit cards, debit cards, ID cards and most membership cards share the ISO/IEC 7810 ID-1 size of 85.60 × 53.98 mm. Matching a real card measures your actual screen, so it works on any device, including external monitors and unusual laptops. Look straight at the screen while matching the edges and use the + and − buttons for fine steps.
Your device
On recent iPhones, iPads and MacBooks, Exact Ruler usually recognizes the model and sets the size automatically. You can also pick your model from the list. Some models report identical screen settings to the browser, so check the detected name.
Screen size
If you know the exact diagonal of your screen, enter it and the ruler works out the rest from your current resolution. Marketing sizes are often rounded, so look up the exact figure.
Check your calibration
Measure an object with a standardized size. If the reading is within a millimeter, your ruler is accurate.
| Object | Size |
|---|---|
| Bank, ID or loyalty card | 85.60 × 53.98 mm (3.370 × 2.125 in) |
| US penny | 19.05 mm (0.750 in) across |
| US nickel | 21.21 mm across |
| US quarter | 24.26 mm across |
| 1 euro coin | 23.25 mm across |
| 2 euro coin | 25.75 mm across |
| SD card | 32 × 24 mm |
| microSD card | 15 × 11 mm |
| A4 paper | 210 × 297 mm |
| US Letter paper | 8.5 × 11 in (215.9 × 279.4 mm) |
Reading the ruler
Centimeters and millimeters
Numbered marks are whole centimeters, the medium mark is half a centimeter, and each small mark is one millimeter. An object that ends four small marks after the 7 is 7.4 cm, or 74 mm.
Inches
Numbered marks are whole inches. The marks get shorter as they divide the inch into halves, quarters, eighths and sixteenths. The live readout and the guides show inches as fractions, such as 2 3/8 in, so you don’t have to count.
| Fraction | Decimal | Millimeters |
|---|---|---|
| 1/16 in | 0.0625 | 1.59 mm |
| 1/8 in | 0.125 | 3.18 mm |
| 1/4 in | 0.25 | 6.35 mm |
| 3/8 in | 0.375 | 9.53 mm |
| 1/2 in | 0.5 | 12.70 mm |
| 5/8 in | 0.625 | 15.88 mm |
| 3/4 in | 0.75 | 19.05 mm |
| 1 in | 1.0 | 25.40 mm |
Pixels
Pixel mode measures in CSS pixels, the unit used in CSS, Figma and most design tools. It doesn’t depend on calibration, so it’s handy for checking spacing and element sizes on a web page.
Tools that make measuring easier
- Guides: click or drag from any ruler to place a red guide line. Each guide shows its position, and the gap between neighboring guides is labeled.
- Measuring box: drag and resize a box over an object to read its width and height at once.
- Show a length: enter a size like 5 cm or 2 inches and see exactly how big it is, useful for checking a size before you buy something online.
- Actual-size grid: a faint background grid in real centimeters or quarter inches.
Tips for accurate results
- Keep browser zoom at 100%. Press Ctrl+0, or Cmd+0 on a Mac, to reset it.
- Look straight at the screen to avoid parallax, especially with thick objects.
- Start at the 0 mark rather than the edge of the screen frame.
- Rest objects gently on the glass and never press on the display.
- Calibrate again after moving to another monitor or changing display scaling.
Common questions
Is this online ruler accurate?
After calibrating with a bank card, readings are typically within about 1 mm over 10 cm. Without calibration the ruler uses an estimate for your type of device, which can be noticeably off, so calibrate before measuring anything that matters.
Can I use my phone as a ruler?
Yes. Open the page on your phone, calibrate with a card held upright, and use the left or right ruler for the longest measuring length. Most phones give you around 14 cm along the long side.
How do I see 1 inch or 1 cm at actual size?
Calibrate first, then use Show a length in the toolbar and enter 1 inch or 1 cm. The bar on screen is exactly that long.
Why does the ruler look too big or too small?
Usually the page is zoomed, the ruler hasn’t been calibrated, or the window moved to a different screen. Reset zoom to 100% and calibrate again with a card.
Is my calibration saved?
Yes, it is stored in your browser on this device only. Clearing site data resets it.
Is Exact Ruler free?
Yes. It runs entirely in your browser with no download or account.