Free Online Ruler (Actual Size)
A real-size ruler for your phone, tablet or computer. Calibrate once with a card or coin, then measure in centimeters, millimeters, inches or pixels. No download and no sign-up.
How to use the online ruler
- Calibrate once using the button above (about 20 seconds). The badge turns green when the ruler is true to size.
- Place your object against the screen with one end on the 0 line.
- Tap or drag to where the object ends. Marker B snaps there and the readout shows the length in cm, mm, inches with the nearest 1/16 fraction, and pixels.
- Need more room? Switch to the vertical ruler (↕), go full screen, or use Measure longer to add up sections.
Why most online rulers show the wrong size
A web page measures in CSS pixels, and a CSS pixel is a different physical size on every screen. A 15.6″ laptop shows about 96 pixels per inch, while a phone can show 150 or more. Without calibration, an online ruler can be off by 30% or more. That is why this ruler asks you to match a real object once. It then stores the exact pixels-per-millimeter for your screen, and if you change browser zoom later, it adjusts so you don't have to calibrate again.
Calibrate for true size
- Set zoom to 100%. Press Ctrl + 0 (⌘ + 0 on Mac). After calibrating, the ruler corrects itself automatically if you zoom.
- Press Calibrate. Hold a credit card, debit card, ID or gift card flat against the screen and resize the on-screen card until it matches. No card? Use a coin, a dollar bill, a sheet of A4 or Letter paper, your screen size, or pick your device from the list.
- Save. Your calibration is stored on this device only, so next time the ruler opens at true size.
Calibration options compared:
| Method | Accuracy | Best for |
|---|---|---|
| Credit card or ID card | Best (±0.2 mm) | Any screen |
| Coin (US, €, £, CA, AU) | Very good | Phones and small screens |
| Dollar bill, A4 or Letter paper | Very good | Large monitors |
| Enter screen diagonal | Good | When you know your screen size |
| Pick your device | Good | iPhone, iPad, Pixel, Galaxy, MacBook and common monitors |
Ruler features
cm, mm, inches and pixels
Metric with millimeter ticks, inches to 1/16, both scales at once like a real ruler, or CSS pixels for designers.
Two-point measuring
Drag markers A and B anywhere to measure a gap or a part of an object, not just from zero.
Measure longer than the screen
Add sections together for objects bigger than your display.
Vertical & full screen
Turn the ruler to use the long side of a phone, or fill the whole display.
Guides & crosshair
Double-click to drop guide lines; turn on the crosshair for precise alignment.
Dark mode, offline, shortcuts
Works offline after the first visit. Keys: 1 cm · 2 in · 3 both · 4 px · R rotate · C crosshair · F full screen · K calibrate · L measure longer · Esc reset.
How to read a ruler
Centimeters and millimeters
Each numbered line is one centimeter. The ten small spaces between them are millimeters, and the medium line in the middle marks 5 mm. A reading of 4 cm plus 3 small marks is 4.3 cm, or 43 mm.
Inches and fractions
Each numbered line is one inch. The marks get shorter as the fractions get smaller: ½″ is the longest, then ¼″ and ¾″, then eighths, then sixteenths. Count the sixteenths past the last whole inch and simplify; 2 inches plus 6 sixteenths is 2 ⅜″.
| Fraction | Decimal inch | Millimeters |
|---|---|---|
| 1/16″ | 0.0625 | 1.59 mm |
| 1/8″ | 0.1250 | 3.17 mm |
| 3/16″ | 0.1875 | 4.76 mm |
| 1/4″ | 0.2500 | 6.35 mm |
| 5/16″ | 0.3125 | 7.94 mm |
| 3/8″ | 0.3750 | 9.52 mm |
| 7/16″ | 0.4375 | 11.11 mm |
| 1/2″ | 0.5000 | 12.70 mm |
| 9/16″ | 0.5625 | 14.29 mm |
| 5/8″ | 0.6250 | 15.88 mm |
| 11/16″ | 0.6875 | 17.46 mm |
| 3/4″ | 0.7500 | 19.05 mm |
| 13/16″ | 0.8125 | 20.64 mm |
| 7/8″ | 0.8750 | 22.22 mm |
| 15/16″ | 0.9375 | 23.81 mm |
| 1″ | 1.0000 | 25.40 mm |
Centimeters to inches converter
1 inch = 2.54 cm exactly, and 1 cm = 0.3937 inches. See the full cm to inches chart.
Check your calibration with everyday objects
After calibrating, measure one of these to confirm the ruler is accurate:
| Everyday object | Metric | Inches |
|---|---|---|
| Credit / debit / ID card | 85.60 × 53.98 mm | 3.370 × 2.125 in |
| US quarter (diameter) | 24.26 mm | 0.955 in |
| US penny (diameter) | 19.05 mm | 0.750 in |
| €1 coin (diameter) | 23.25 mm | 0.915 in |
| UK £1 coin (diameter) | 23.43 mm | 0.922 in |
| US dollar bill | 155.96 × 66.29 mm | 6.14 × 2.61 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 |
| Standard paper clip | ≈ 33 mm | ≈ 1.3 in |
| AA battery (length) | 50.5 mm | 1.99 in |
What you can measure with an online ruler
- Rings and jewelry: use the ring size finder for US, UK, EU and Japanese sizes.
- Screws, bolts, nails and drill bits: length and thread diameter in mm or fractions of an inch.
- Photos, prints and documents: check print sizes like 4×6 in or 10×15 cm.
- Beads, buttons, fabric and craft parts for sewing and crafts.
- Small parts and packages when you don't have a tape measure nearby.
- Design work: switch to pixels to measure UI elements on screen.
More free measuring tools
Printable ruler
Make a custom paper ruler: inches (1/16, 1/8 or tenths), cm/mm, 6 in to 1 m, Letter or A4.
Print a ruler →Ring size finder
Measure a ring on screen and get US, UK, EU and Japanese sizes, or print a ring sizer strip.
Find ring size →Online protractor
Measure any angle with draggable arms. Works on phones, tablets and screenshots.
Measure angles →Printable tape measure
A 150 cm or 60 inch tape you cut, tape together and use for body measurements.
Print a tape →Graph paper
1 mm, 5 mm, 1 cm, ¼ in and ⅛ in grids, plus dot paper, at exact size.
Print graph paper →What is my screen size?
Find your screen's real diagonal in inches, resolution, pixel density and aspect ratio.
Check screen →Know the limits
A calibrated screen ruler is accurate to about half a millimeter, which is great for everyday measuring. For engineering, medical or legal measurements use a certified physical instrument. Don't press hard on your screen, and don't measure hot, sharp or wet objects on it.
Frequently asked questions
Is this online ruler actual size?
Yes, once calibrated. Screens have different pixel densities, so no website can know the physical size of a pixel on its own. Calibrate once with a credit card, coin, paper or your screen size and the ruler shows true centimeters and inches. The calibration is saved on your device.
How do I calibrate the ruler without a credit card?
Use any object of known size: a US quarter or penny, a €1 or £1 coin, a dollar bill, the short edge of A4 or Letter paper, or a real ruler. You can also type your screen's diagonal size or pick your phone, tablet, laptop or monitor from the device list.
How can I measure something longer than my screen?
Use Measure longer. Mark where the object reaches with marker B, press the button, slide the object so that mark sits on 0, and keep going. The ruler adds each section to a running total.
Can I use my phone as a ruler?
Yes. Open this page on your phone, calibrate with a card or coin, and switch to the vertical ruler for the longest scale. Phone screens are small, so use Measure longer for anything over about 12 cm (5 inches).
How do I read inch fractions on a ruler?
Each inch is split into 16 parts. The longest mark between whole inches is 1/2, the next longest are 1/4 and 3/4, then the 1/8 marks, and the shortest are 1/16 marks. The readout shows the nearest 1/16 fraction for you, such as 2 5/16″.
Why does the ruler look too big or too small?
It is not calibrated yet, browser zoom is not at 100%, or your operating system uses display scaling. Press Calibrate and match a real object; the ruler compensates for scaling and for later zoom changes automatically.
Can I print a ruler instead?
Yes. The printable ruler page builds an exact-size ruler for Letter or A4 paper in inches, centimeters or both. Print at 100% (Actual size), then check the test square on the page.
Is the online ruler free?
Yes. Every tool on OnlineRulerFree.com is free, needs no download or sign-up, and works offline once loaded.