Free Online Ruler

An accurate, actual-size online ruler in centimeters, millimeters and inches. Calibrate once, measure anything that fits on your screen — free forever.

Place your object against the corner. Drag anywhere to measure a distance.

Calibrate your ruler

We read your screen resolution and pixel density and match it against a database of popular phones, tablets and laptops.

Why this online ruler is different

3 calibration methods

Auto-detect your device, enter your screen size, or match a credit card for real, to-scale measurements.

Rulers on any edge

Show the ruler on the top, bottom, left or right edge — or all four at once.

cm, mm and inches

Metric with millimeter ticks, or inches divided down to 1/16".

Drag to measure

Drag anywhere to measure a straight line or diagonal. Hold Shift to lock it straight.

Full-screen mode

Turn your whole display into a 12 inch online ruler (or longer on big monitors).

Private and free

Runs entirely in your browser. No sign-up, no download, nothing uploaded.

An accurate online ruler that shows actual size

Need to measure something but can't find a ruler? Free Forever Ruler is a free online ruler that turns the screen you're already holding into a real, actual-size measuring tool. Whether you need an online ruler in cm, an online ruler in mm, or an online ruler in inches, it works instantly in your browser on an iPhone, Android phone, iPad, laptop or desktop monitor — no app to download and no account to create.

Most websites that call themselves an "online ruler" simply draw lines on the page and hope your screen matches. Screens don't work that way: a centimeter drawn on a 6-inch phone and the same centimeter drawn on a 27-inch monitor are very different physical sizes. That's why our tool is built around calibration. Once calibrated, the ruler is drawn to scale for your exact display, so one centimeter on screen is one real centimeter on the object in your hand.

Three ways to calibrate your ruler

We give you three calibration methods so you can always get an accurate online ruler, whatever device you're on:

  1. Auto-detect device. We read your screen resolution and pixel density and match it against a built-in database of popular iPhones, iPads, Pixels, Galaxy phones and MacBooks. On a supported device you get an actual size online ruler with one tap.
  2. Screen diagonal. Enter your screen's size in inches (for example 6.1", 13.3", 15.6" or 27") and we calculate the exact pixels per inch. This works well on Windows laptops and desktop monitors.
  3. Credit card calibration. Every standard credit, debit or ID card is exactly 85.60 × 53.98 mm. Hold one against the screen and resize the outline until it matches. It's the most precise way to make an online ruler to scale on any display.

Your calibration is saved privately in your own browser, so the next time you open the site your real online ruler is ready to use immediately.

Measure in centimeters, millimeters or inches

Switch units with one tap. The metric ruler shows every millimeter with a medium mark at 5 mm and a numbered line each centimeter — perfect when you need an mm online ruler for screws, jewelry, rings, beads, or electronic parts. The imperial ruler shows whole inches divided into halves, quarters, eighths and sixteenths, just like the physical 12 inch online ruler on your desk. Use full-screen mode on a large monitor for a true online ruler 12 inch length or more.

Place the ruler on any edge of your screen

Unlike most tools that lock you to a single horizontal bar, Free Forever Ruler lets you choose where the ruler sits: top, bottom, left, right, or any combination. Measure width and height at the same time by placing your object in the corner where two rulers meet. You can also drag a line anywhere on the measuring area to read a straight or diagonal distance, with the horizontal and vertical components shown live.

How to use this online ruler tool

  1. Set your browser zoom to 100% (press Ctrl+0 or ⌘+0).
  2. Click Calibrate and pick auto-detect, screen size, or the credit card method.
  3. Choose cm, mm or inches and select which edges show a ruler.
  4. Place your object at zero, in the corner, and read the measurement at its far end.
  5. Need more length? Tap Full screen.

Tips for the most accurate results

Keep the object flat against the glass and look straight down at it to avoid parallax. Don't use pinch-to-zoom while measuring, because zooming changes the scale. If you change your display resolution or move the browser to a different monitor, calibrate again. For anything larger than your screen, measure in sections and add them up.

Free forever, private by design

Free Forever Ruler is, as the name promises, free forever. The whole tool runs locally in your browser: nothing you measure is uploaded, and there's no login. It loads fast, works offline once opened, and supports light and dark mode. Bookmark it and you'll always have an accurate, real online ruler in your pocket — whether you're checking a ring size, a phone case, a screw, a photo print, or the size of a package before you ship it.

Frequently asked questions

Can I use my phone as a ruler?

Yes. Open Free Forever Ruler on your phone, tap Calibrate and choose Auto-detect (or the credit card method for best precision). The ruler then shows true, actual-size centimeters, millimeters and inches along the edge of your screen, so you can lay small objects directly on the phone and measure them.

Is there a ruler online?

Yes — Free Forever Ruler is a free online ruler that works in any modern browser on phones, tablets, laptops and desktop monitors. There is nothing to download or install, no sign-up, and it stays free forever.

How to identify 1 inch?

One inch equals 2.54 centimeters (25.4 millimeters). On our inch ruler, every long numbered line marks one full inch, and it is divided into 16 smaller marks of 1/16 inch each. Without a ruler, the top joint of an adult thumb is roughly one inch wide.

How to measure cm online?

Select the cm unit, calibrate once, then place your object against the corner where the rulers meet. Read the number at the object's far end. Each numbered line is one centimeter and each small line is one millimeter. You can also drag across the screen to measure any distance.

Can a smart phone measure?

Yes. A smartphone can measure small objects on its screen with an online ruler like this one, and many phones also include augmented-reality measuring apps that use the camera for larger distances.

Can I use my camera as a ruler?

Camera-based AR apps (like Apple Measure or Google Measure) can estimate the length of larger objects. For small objects, an on-screen ruler is more precise because it measures true physical size directly on the display once calibrated.

Can we measure online?

Yes. With a calibrated online ruler you can measure real objects up to the size of your screen. On a 15.6" laptop that is about 34 cm (13.5 inches) horizontally; on a large monitor it can exceed 12 inches.

How to read a ruler online?

Find the zero at the corner. Long numbered lines are whole units (centimeters or inches). On the cm ruler, the medium line is 5 mm and short lines are 1 mm. On the inch ruler, lines get shorter from 1/2, 1/4, 1/8 down to 1/16 inch. Count from the last number to the end of your object.

How to view the ruler in Word Online?

In Word for the web, go to the View tab and check Ruler (available in Print Layout view). If you do not see it, switch from Reading View to Editing mode first.

How to add a ruler in Word Online?

Open your document in Word for the web, select View on the ribbon, and turn on the Ruler checkbox. The horizontal ruler appears above the page so you can set margins, tabs and indents.

How to show the ruler in PowerPoint Online?

In PowerPoint for the web, open the View tab and select Ruler. Rulers appear on the top and left of the slide to help you align objects.

How to use an online ruler?

Calibrate first (auto-detect, screen size, or credit card), choose cm, mm or inches, pick which screen edges show a ruler, then place your object at zero and read the measurement. Use Full screen for the longest possible ruler.

How do I make sure the online ruler is actual size?

Keep your browser zoom at 100% and calibrate once. The credit card method is the most accurate: every standard bank card is exactly 85.60 × 53.98 mm, so matching the outline sets the true scale for your exact screen.