How it works
01
We try to recognize your device automatically. Check it in a second by holding a credit card against the blue card mark on the ruler, or match a card or enter your screen size.
02
Show the ruler on the top, right, bottom or left edge, or on several at once to make an L-square for width and height together.
03
Place the object against the ruler’s zero and read where it ends. Drag a guide line out of the ruler to mark the far edge, or fit the measuring box around it for width and height.
About the tool
Real Size Ruler is a free online ruler that shows real, actual size on the screen you already have. Most ruler images on the web are drawn in pixels, and a pixel is a different physical size on every phone, tablet and monitor, so a “1 inch” mark on one screen can be half an inch on another. This online ruler tool fixes that by calibrating to your display first. Once it’s calibrated, every tick is drawn to scale, so you get an accurate online ruler in inches, centimeters and millimeters without downloading anything.
Switch to inches and you get a 12 inch online ruler with markings down to 1/16 of an inch, just like the ruler in your desk drawer. Whole inches are numbered, and the half, quarter, eighth and sixteenth lines get shorter step by step, so fractions are easy to read. On most laptops and desktop monitors the full 12 inches fit across the screen in fullscreen. On a phone, turn it to landscape for the longest online ruler inches actual size reading your screen allows, and measure longer items in sections.
Prefer metric? Press 1 for the online ruler in cm. Numbered lines mark each centimeter, the medium line marks 5 mm, and every short line is 1 mm. That makes it a handy mm online ruler for screws, beads, jewelry, buttons and other small parts. Because one centimeter is exactly ten millimeters, the online ruler in mm and the online ruler cm scale are the same ruler: you read both at once.
An online ruler to scale has to know your screen’s pixel density. You can calibrate in one of three ways. Auto-Detect matches your resolution and pixel ratio against a database of phones, tablets and laptops. Screen Diagonal calculates the density from the size printed in your device’s specs. Credit Card lets you lay any bank card, an A4 or Letter sheet, or a physical ruler on the screen and drag a line to its far edge. Card calibration is typically accurate to about 1 mm over 10 cm, which is what turns a picture of a ruler into a real online ruler you can trust. Your calibration is saved in your browser, so the actual size online ruler is ready the next time you open it.
Some people search for an online ruler camera, hoping their phone’s camera can measure objects from a distance. Camera and augmented-reality apps estimate size from an image, and their results shift with angle, lighting and distance. This tool works differently: you place the object directly on the screen and compare it with a calibrated scale, the same way you would with a physical ruler. For small, flat items that fit on your display, that is far more dependable than an estimate.
You can show a ruler on any edge of the screen, or on two adjacent edges to make an L-square that gives width and height in one placement. Drag guide lines out of a ruler and the gap between them is labeled automatically. A resizable measuring box reads out width × height, and a true-size grid in 1 cm or 1 inch squares helps you size prints, stickers and cut-outs. A pixel scale is there for designers and developers who work in CSS pixels.
Set your browser zoom to 100%, look straight down at the screen to avoid parallax, and line the object up with the 0 mark rather than the edge of the screen. Then check the result against the blue credit-card mark on the ruler. If a card lines up with it, your online ruler actual size reading is correct. Real Size Ruler is free, needs no sign-up and runs in any modern browser on iPhone, Android, iPad, Windows and Mac.
Calibration
A “1 cm” line on an uncalibrated website can be anywhere from 6 mm to 15 mm long, depending on the screen. Calibration tells the ruler how many pixels make a real millimeter on yours.
Reads your screen’s resolution and pixel ratio and matches it against our screen database: iPhone, iPad, Galaxy, Pixel, MacBook, Surface and more. When several devices share a screen, you pick yours.
Enter your screen’s diagonal in inches or centimeters. It’s printed in the device’s specs or on the monitor’s label. We calculate the pixel density from your resolution.
Lay a credit card (85.60 × 53.98 mm), a sheet of A4 or Letter paper, or a real ruler on the screen with one edge on the 0 line, and drag a single line to the other edge. The longest side that fits is used for the best precision.
Features
Every ruler carries a blue mark at a credit card’s length and width, so you can check the calibration with the card in your wallet.
Your unit and ruler edges are kept in the page address, so a bookmark or a shared link opens the same ruler.
Turn on two adjacent edges and both rulers start from the same inside corner, so one placement gives you width and height.
Drag lines out of any ruler. Each shows its position, and the gap between neighboring guides is labeled automatically.
A resizable box that reads out width × height. It opens at credit-card size, so it doubles as a calibration check.
A 1 cm or 1 inch grid drawn at true size across the screen. Handy for sizing prints, stickers and cut-outs.
If browser zoom changes after you calibrate, the ruler rescales itself to stay at real size in Chrome, Edge and Firefox.
No account, no tracking cookies. Your calibration stays in your browser and never leaves your device.
Check accuracy
After calibrating, hold a credit card against the blue card mark on the ruler, or measure one of these everyday objects. If the reading is off by more than a millimeter, recalibrate with the credit-card method.
| Object | Metric | Imperial |
|---|---|---|
| Credit, debit or ID card | 85.60 × 53.98 mm | 3.370 × 2.125 in |
| US quarter | 24.26 mm Ø | 0.955 in Ø |
| US penny | 19.05 mm Ø | 0.750 in Ø |
| 1 euro coin | 23.25 mm Ø | 0.915 in Ø |
| 2 euro coin | 25.75 mm Ø | 1.014 in Ø |
| US dollar bill | 155.96 × 66.29 mm | 6.14 × 2.61 in |
| SD card | 32 × 24 mm | 1.26 × 0.94 in |
| microSD card | 15 × 11 mm | 0.59 × 0.43 in |
| A4 paper | 210 × 297 mm | 8.27 × 11.69 in |
| US Letter paper | 215.9 × 279.4 mm | 8.5 × 11 in |
Reading a ruler
Numbered lines are whole inches. Lines get shorter from ½ to ¼, ⅛ and finally 1/16 of an inch. 1 inch = 2.54 cm exactly.
Numbered lines are centimeters. The medium line between them is 5 mm, and each short line is 1 mm. 10 mm = 1 cm.
The px scale counts CSS pixels (the unit designers and developers use) with labels every 100 px. It doesn’t depend on calibration.
| Fraction (in) | Decimal (in) | Millimeters |
|---|---|---|
| 1/16″ | 0.0625 | 1.59 mm |
| 1/8″ | 0.125 | 3.18 mm |
| 3/16″ | 0.1875 | 4.76 mm |
| 1/4″ | 0.25 | 6.35 mm |
| 5/16″ | 0.3125 | 7.94 mm |
| 3/8″ | 0.375 | 9.53 mm |
| 1/2″ | 0.5 | 12.70 mm |
| 5/8″ | 0.625 | 15.88 mm |
| 3/4″ | 0.75 | 19.05 mm |
| 7/8″ | 0.875 | 22.23 mm |
| 1″ | 1.0 | 25.40 mm |
Keyboard
FAQ
Yes, once it’s calibrated. Screens have different pixel densities, so no website can show real size without knowing your screen. Auto-Detect handles most phones, tablets and Macs instantly; you can confirm it with a credit card against the blue card mark on the ruler.
Hold any credit, debit or ID card against the top ruler with its corner at 0. Its long side (85.6 mm) should end on the blue card mark; on a phone, use the short side (54 mm) instead. If it’s off, open Calibrate → Credit Card.
Open the ruler, check the status says your phone was detected (or match a credit card), and choose a unit. Lay the object on the screen with one end at the 0 mark and read the other end. If it’s away from the ruler, drag a guide line to it. Rotate the phone to landscape for a longer ruler.
Usually because it isn’t calibrated yet, browser zoom isn’t at 100%, or the calibration was made on a different monitor. Press Ctrl/⌘ + 0 to reset zoom, then recalibrate with the credit-card method.
In Chrome, Edge and Firefox the ruler detects zoom changes after calibration and corrects for them. Safari doesn’t report zoom to websites, so keep Safari at 100% (⌘ + 0). Pinch-zoom on phones also breaks the scale, so zoom back out before measuring.
Device detection and credit-card calibration are typically within 1%: about 1 mm over 10 cm. Screen-diagonal calibration depends on how exact the advertised size is, usually within 2%.
Use fullscreen (F) for the most length, and two edges at once for an L-square. For longer items, mark where the ruler ends, slide the object along and add the measurements together.
Yes, in your browser’s local storage on this device. It’s never sent to us. Clearing site data or using a private window resets it.
Yes. Switch to inches and the ruler shows whole inches with 1/16-inch markings. Most laptop and desktop screens fit the full 12 inches across in fullscreen (F). On a phone, turn it to landscape and measure longer items in sections.
Yes, this page is one. It’s a free online ruler in inches, centimeters and millimeters that shows actual size once it knows your screen. Auto-Detect calibrates most phones, tablets and Macs instantly, and the credit-card method works on any other screen. There’s nothing to download or sign up for.
Yes. A calibrated online ruler draws every mark at real size, so you can measure anything small enough to lay on your screen, such as screws, coins, jewelry, buttons or a printed photo. Place the object at the 0 mark and read the other end, or use guide lines and the measuring box for items that don’t sit against the ruler.
Open the ruler and check it’s calibrated: the status shows your detected device, or you can match a credit card in Calibrate. Choose cm, mm or inches. Lay the object flat on the screen with one end at 0 and read the mark at the other end. Drag a guide line out of the ruler to mark an edge, and press F for fullscreen when you need more length.
Read it like a physical ruler. In inches, the numbered lines are whole inches and the lines between them get shorter for 1/2, 1/4, 1/8 and 1/16 of an inch. In centimeters, each numbered line is 1 cm, the medium line is 5 mm and every short line is 1 mm. Count from 0 to the last full mark, then add the smaller marks past it. The fraction chart above converts inch fractions to decimals and millimeters.
Calibrate the ruler, then press 1 (or pick cm) to switch to centimeters. Put one end of the object on the 0 mark and read where the other end falls: numbered lines are whole centimeters and each small line is 1 mm, so 4 cm and 7 small lines is 4.7 cm. For an object away from the ruler, drag a guide line to its edge and read the label.
One inch is exactly 2.54 cm (25.4 mm). On this ruler, switch to inches and the distance from 0 to the line numbered 1 is one real inch once it’s calibrated. Without a ruler, a US quarter is just under an inch across (0.955 in), and the top joint of an adult thumb is roughly an inch long, though that varies from person to person.
Yes. Open this page on your phone and it usually recognizes the model and calibrates automatically. If it doesn’t, match a credit card in Calibrate. Lay the object on the screen at the 0 mark and read the far end. Turn the phone to landscape for a longer ruler, and measure anything longer in sections.
Yes, in two ways. A calibrated on-screen ruler like this one measures small objects placed directly on the display, accurate to about 1 mm over 10 cm. Augmented-reality measuring apps use the camera to estimate larger distances like furniture or rooms, but they are less precise and depend on lighting, angle and distance.
This ruler doesn’t use your camera. Camera and AR apps estimate size from an image, and the result shifts with angle and distance. Here you lay the object on the calibrated screen and read it directly, which is more reliable for small, flat items.
In Word for the web, open the document in Editing mode, go to the View tab and select Ruler. The ruler appears above the page so you can set margins, indents and tab stops. If you don’t see the option, switch to Editing mode (not Viewing) or open the file in the Word desktop app. Word’s ruler is for page layout, not real size on your screen; to measure physical objects, use a calibrated online ruler like this one.
In PowerPoint for the web, open the View tab and select Ruler to show rulers along the top and left of the slide; Guides on the same tab help you line up objects. If the option isn’t available in your version, choose Editing → Open in Desktop App and turn it on from the View tab there. Like Word’s, this ruler measures slide layout, not physical size.
Completely free, with no sign-up and nothing to install.
Free, private and accurate to the millimeter.