True actual size
Calibrate with any bank or ID card, which all share the same 85.60 by 53.98 mm size, and the ruler matches real centimeters and inches.
True to size after one quick calibration
Online Ruler Actual-size ruler in cm, mm and inches
This free online ruler shows real centimeters, millimeters and inches on your screen. Calibrate once with any bank card, then measure objects with draggable handles, in horizontal, vertical or fullscreen mode.
- Actual size
- Card calibration
- Measuring handles
- Fullscreen and vertical
Actual-size ruler
Not calibrated yet, so sizes are approximate.
5.00 cm
50.0 mm1.97 inabout 1 31/32 in
Drag the blue handles, or tap anywhere on the ruler to move the nearest one. With a handle focused, use the arrow keys (hold Shift for bigger steps). Keep browser zoom at 100%.
Calibrate your screen
Every screen has a different pixel density. Calibrate once and the ruler will be true to size on this device. Your calibration is saved in this browser.
Hold any standard bank, credit, debit or ID card flat against the screen, lined up with the top-left corner of the outline. Adjust until the outline matches the card exactly (85.60 by 53.98 mm).
Enter your screen's diagonal size in inches, as listed in its specifications. Your browser reports -.
If you already know your screen's CSS pixels per inch, enter it here. Most desktop browsers assume 96, which is rarely exact.
Why it's better
Why this online ruler is better
Many online rulers guess your screen size and show the wrong length. This one is calibrated to your actual screen and adds proper measuring tools.
cm, mm and inches together
Show metric, imperial or both at once: millimeters on the top edge and inches in sixteenths on the bottom edge.
Measuring handles
Drag two handles to measure anything between them, with the length shown in cm, mm, decimal inches and the nearest fraction.
Vertical and fullscreen
Turn the ruler vertical for tall objects, or go fullscreen to use every millimeter of your display.
Three ways to calibrate
Use a card for the best accuracy, your screen's diagonal size for speed, or type in a known pixels-per-inch value.
Remembers your screen
Your calibration is saved in this browser, and you're warned if the page zoom changes and the ruler needs checking.
How it compares with typical online rulers
| Feature | This online ruler | Typical ruler sites |
|---|---|---|
| Calibration | Card, screen size or PPI | Often a single guess |
| cm and inches at once | Yes, dual-edge | Sometimes |
| Measuring handles with readout | Yes, with fractions | Rarely |
| Vertical ruler | Yes | Sometimes |
| Fullscreen mode | Yes | Rarely |
| Zoom change warning | Yes | Rarely |
| Keyboard accessible | Yes | Rarely |
How to use
How to measure with the online ruler
Calibrate once, then measure as often as you like on this device.
Tip: press Ctrl + 0 (Cmd + 0 on Mac) to reset browser zoom to 100% before calibrating and measuring.
Calibrate with a card
Press Calibrate, hold a bank or ID card against the screen and adjust the outline until it matches the card. Press Save.
Choose your units
Pick cm and mm, inches, or both. Switch to vertical for tall items or fullscreen for extra length.
Place the object
Hold the object against the screen with one end at the 0 mark on the left, or on top in vertical mode.
Drag the handles
Move the blue handles to each edge of the object. The length updates instantly in every unit.
Read the result
Read the size in cm, mm, decimal inches or the nearest sixteenth of an inch.
Accuracy
Why an online ruler needs calibrating
Websites measure in CSS pixels, but the physical size of a pixel depends on your screen's size and resolution. A 13-inch laptop and a 27-inch monitor can show the same number of pixels at very different sizes.
Bank, credit and ID cards are made to the ISO/IEC 7810 ID-1 standard of 85.60 by 53.98 mm, so they are a perfect reference that almost everyone has in their wallet.
- Use a real card rather than a paper receipt or a card with a sleeve.
- Look straight at the screen to avoid parallax when lining up edges.
- Keep browser zoom at 100% and recalibrate after changing display scaling.
- Recalibrate on each device and each browser you use.
- For long objects, measure in sections and add the results.
- Remember that screens can't measure things thicker than a few centimeters accurately.
Reference
Sizes of common objects
Use these everyday objects to double-check your calibration or to compare sizes.
| Object | Metric | Inches |
|---|---|---|
| Bank, credit or ID card | 85.60 × 53.98 mm | 3.370 × 2.125 in |
| Passport photo (India, UK, EU) | 35 × 45 mm | 1.38 × 1.77 in |
| US business card | 88.9 × 50.8 mm | 3.5 × 2 in |
| A4 paper | 210 × 297 mm | 8.27 × 11.69 in |
| US Letter paper | 215.9 × 279.4 mm | 8.5 × 11 in |
| US quarter coin (diameter) | 24.26 mm | 0.955 in |
| 1 euro coin (diameter) | 23.25 mm | 0.915 in |
Need a passport photo at the right size? Use the passport size photo maker.
Reading a ruler
How to read inches and centimeters
On the metric edge, each numbered line is a centimeter, the medium lines are half centimeters and each small line is one millimeter. So 4 numbered lines plus 7 small ones is 4.7 cm, or 47 mm.
On the inch edge, each inch is split into 16 parts. The tallest lines mark half inches, then quarters, eighths and sixteenths. One inch equals exactly 25.4 mm.
| Inch fraction | Decimal | Millimeters |
|---|---|---|
| 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 |
| 1 in | 1.0 | 25.4 mm |
| 1 cm | 0.3937 | 10 mm |
Benefits
When an online ruler comes in handy
No ruler in the drawer? Your screen can measure small things accurately once it knows its own size.
Ring and jewellery sizes
Measure the inside diameter of a ring or the width of a bracelet before ordering online.
Online shopping
Check the size of screws, cables, buttons, cards and small parts against product specifications.
Homework and crafts
Measure drawings, paper cut-outs and sewing pieces, or teach children how to read a ruler.
Design and print checks
Check that printed business cards, photos and labels come out at the intended size.
Popular things to measure
- Ring size
- Screws and bolts
- Passport photos
- Business cards
- Phone cases
- Buttons and beads
- Paper and labels
- Pills and small parts
FAQ
Online ruler FAQ
Is this online ruler actual size?
Yes, once calibrated. Every screen has a different pixel density, so calibrate once with a bank card or your screen size and the ruler shows true centimeters and inches.
How do I calibrate the ruler with a credit card?
Press Calibrate, choose Bank or ID card, hold a standard card against the screen and adjust the slider until the outline matches the card exactly. All standard cards are 85.60 by 53.98 mm.
How accurate is an online ruler?
After card calibration it is usually accurate to within about a millimeter across the screen. Keep browser zoom at 100% and look straight at the screen while measuring.
Can I measure in both centimeters and inches?
Yes. Choose cm and mm, inches or Both to show metric on the top edge and inches in sixteenths on the bottom edge.
How do I measure something longer than my screen?
Mark where the ruler ends on the object, slide the object along and add the lengths together. Fullscreen and the vertical ruler also give you more length to work with.
How do I read a ruler in inches?
Each inch is split into 16 parts. The longest lines mark half inches, then quarters, eighths and sixteenths, so one inch plus five small marks is 1 5/16 inches.
Why isn't the ruler the right size on my phone?
Phones report scaled pixels, so the default size is only approximate. Calibrate with a card and the ruler will be accurate on that phone.
Does zooming the page affect the ruler?
Yes. Browser zoom changes the size of every pixel. Reset zoom to 100% with Ctrl + 0, or recalibrate after changing zoom. The ruler warns you when zoom has changed.
How many millimeters are in an inch?
Exactly 25.4 millimeters. One centimeter is about 0.3937 inches, and 10 centimeters is about 3.94 inches.
Measure it now
Calibrate in 20 seconds with any bank card and get a true-to-size ruler on your screen. Free and ready on any device.
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