Free Online Ruler, True to Actual Size

Yes. Real Online Ruler is a free online ruler that measures in inches, centimeters, and millimeters. It runs in your browser with no app or account. Because screens differ in size, calibrate it once so its markings match real-world lengths.

To use this online ruler, select Calibrate Your Screen and finish one of the 3 methods. Choose your units and turn on a ruler edge. Then hold a small object flat against the screen, line up one end with the zero mark, and read the other end. You can also drag along the ruler to place markers and see the distance between them.

Longer lines mark larger units. On an inch scale, the longest lines are whole inches, followed by shorter lines for 1/2, 1/4, 1/8, and 1/16 of an inch. On a metric scale, each numbered line is 1 cm and each small tick is 1 mm, so 10 ticks make 1 cm. Find the last full number before the end of your object, then count the smaller lines after it. On very small or low-resolution screens, the finest ticks may not appear.

Calibrate your screen, then choose cm as the unit. Place your object against the screen with one end at the zero mark, and read the number at the other end. Each small tick is 1 mm, so a reading of 4 cm plus 3 ticks is 4.3 cm. For a length measured between two points, drag along the ruler and read the result in the readout.

One inch equals exactly 2.54 cm, or 25.4 mm. On an inch ruler, it is the distance between the zero mark and the line numbered 1, or between any two numbered lines in a row. For a rough check without a ruler, a US quarter is about 0.96 inches across, and the top joint of an adult thumb is often close to 1 inch.

Yes. Open this online ruler in your phone’s browser and place small objects against the screen. Rulers respond to touch, and many phones and tablets calibrate automatically. A phone screen is short, so rotate it to landscape to measure longer objects.

Yes, in two main ways. An on-screen ruler like this one measures objects small enough to fit on the display. A camera-based augmented reality app, such as Apple’s Measure app on iPhone, estimates the size of larger objects and spaces. On-screen rulers depend on calibration, while camera apps give estimates that vary with lighting and how steady you hold the phone.

Yes, with an augmented reality measuring app that uses your phone’s camera, though its results are estimates. This site works differently and never uses your camera. You place the object against your calibrated screen and read the ruler, so there are no permissions to grant and no images to upload.

Yes. A calibrated online ruler can measure real objects, as long as they fit on your screen. For anything longer than your display, mark where the ruler ends, slide the object along, and add the readings. For tight tolerances, use calipers or a physical ruler instead.

Word for the web includes a ruler, so there is nothing to install. Open your document, select the View tab, and turn on Ruler. Select it again to hide the ruler. If you need a vertical ruler, Word’s desktop app has a setting for it under File > Options > Advanced.

Go to the View tab in Word for the web and turn on Ruler. It then appears above your document, where it shows margins and indents. If you can’t find the option, check that the document is open for editing rather than in Reading view. This ruler is for page layout and doesn’t measure real objects, so use a calibrated online ruler for that.

Open the View tab and look for a Ruler option. In PowerPoint’s desktop app, it is in the Show group. Microsoft’s ruler instructions cover the desktop app and don’t list PowerPoint for the web, so if the option is missing in your browser, open the presentation in the desktop app and turn on Ruler there.

A browser knows how many pixels your screen has, but not how large it physically is. The same 1920 × 1080 grid can be a 13 inch laptop or a 32 inch monitor. Calibration tells the ruler how big a pixel really is on your display.

It depends on the method. After credit card calibration, accuracy is limited by how closely you can match the outline, and the dialog shows how many millimeters one pixel covers on your screen. Auto-detect and diagonal rely on the listed screen size, which manufacturers often round. To check any result, hold a card against the ruler: it should read 85.6 mm (8.56 cm or 3 3/8 in) along its long side.

After you calibrate, the ruler corrects for zoom changes in browsers that report them, such as Chrome, Edge, and Firefox. Safari doesn’t report page zoom, so keep it at 100% there. Set zoom to 100% before you use Auto-Detect or Screen Diagonal. On phones, pinch-zooming changes the scale, and the site warns you when it detects this.

Yes. The site never uses your camera, and it never asks for card details. You only compare the card’s size with an outline, so keep the numbers facing away from anyone nearby. Any card of the same size works, such as a library card or most driver’s licenses.

Your ruler settings and calibration are saved in your browser’s local storage, on this device only. Clear your site data to remove them, or use Reset Calibration for the current screen.