Can I use my phone as a ruler? How long can it measure?

Yes. Open this page on the phone and calibrate. The ruler can only be as long as the screen. A 6.1 inch phone has a screen about 14 cm tall, so it measures small objects. Turn the phone sideways to use the longer edge.

Can I use my phone camera as a ruler?

Not here. A camera ruler needs a reference object of known size in the same photo, which is a different tool. This ruler measures flat objects that you hold against the screen.

Is the ruler true to size on a Mac, Windows PC, Chromebook and iPad?

It can be, on all of them, after you calibrate. The screens differ in pixel density and scaling, so the page cannot know the real size without your help. A saved calibration is kept per screen, so a laptop and an external monitor each keep their own.

How accurate is an online ruler?

It depends on the calibration. Before you calibrate, the ruler is a guess based on your screen type, and the size can be well off. After you match a credit card to the outline, the result depends on how closely you line up the edges. Zoom in, use the 0.1 mm buttons, and run the check step with a coin.

How do I calibrate an online ruler with a credit card?

Press Calibrate at the top right of the page (on a phone, above the ruler), then choose the Card tab. Hold a card flat against the screen and line up one edge with the outline. Drag the grip on the right edge until the other edge matches. Use the 0.1 mm buttons for fine changes, then press "My card matches". Then check the result with a coin.

Is it free? Do I need to install an app?

It is free and runs in the browser. There is nothing to install and no account to make.

Online Ruler for Phone: Measure on Your Screen

A ruler on your phone screen

This page puts the online ruler on your phone. The ruler runs down the left edge of the screen, along its long side, so a phone held upright gives the most length. The unit switch is above the ruler. Use cm and inches together, or pick one. If you need a ruler across the top as well, turn it on in Options.

Calibrate first. A phone is a small, dense screen, and an uncalibrated ruler is only a guess. Hold a credit card against the screen and match it. On a phone the card is shown on its short side, because the long side is wider than the screen.

How long can a phone ruler be?

It can be as long as the screen. A 6.1 inch phone with a 19.5:9 screen is about 14 cm tall. Larger phones reach about 15 to 16 cm. A tablet gives more. Turn the phone sideways to use the long edge for a horizontal ruler.

For anything longer, use a real ruler or a tape measure. A phone ruler is for small flat objects: a coin, a key, a screw, a bead, a stamp or a label.

How to calibrate on a phone

Open the page and press Calibrate at the top right.
Hold a credit card flat on the screen. The outline shows the short side of the card, 53.98 mm, because the long side is wider than most phones.
Drag the grip on the right of the outline until the card edge matches it. Use the 0.1 mm buttons to fine-tune.
Press "My card matches". The tool then shows a coin outline. Lay a coin on it to check.

Keep your phone still and look straight at the screen. A phone on a table works better than one in your hand.

Measuring with the vertical ruler

On a phone held upright, the ruler runs down the left edge. That is the long side of the screen, so it gives the most length. Place the object next to it with one end at zero.

Press Add guide. A horizontal line appears. Drag it to the other end of the object. The readout shows the length in the units you chose. If you want to mark both ends, pull a second guide out of the ruler. The readout then shows the gap between the two guides.

The controls hide when you open the calibration, so the whole screen is free for the card.

What fits on a phone screen

A phone screen is small, so pick the right objects. These fit on a 6.1 inch phone, which has a screen about 14 cm tall and 6.5 cm wide.

Coins, keys, SIM cards and memory cards.
Screws, bolts, nails and small tools.
Beads, buttons, earrings and charms.
Stamps, tickets and business cards. A business card is often 3.5 by 2 inches in the US, so it fits across the phone.

A US dollar bill is 6.14 inches long, which is about 15.6 cm. It will not fit on most phones, so use a tablet or a computer.

Troubleshooting on a phone

The ruler looks too big or too small. Calibrate again. Check that pinch zoom is off.
The browser bars move. The address bar can hide and show as you scroll. This changes the screen height but not the scale.
The result changed after rotating. The scale stays the same in both directions. If it did not, calibrate again and tell us.
The screen goes dark. The page asks the phone to stay awake, but some browsers do not allow it. Raise the screen timeout in the phone settings.

Tips for iPhone and Android

Keep the screen on. Where the browser allows it, the page asks the phone to stay awake while you measure.
Do not pinch zoom. Pinch zoom changes the page size, so the ruler is no longer true. The tool warns you when it is on.
Keep the object flat. A thick screen protector or a case lip can lift an object off the glass.
Full screen. On iPhone, Safari may not offer full screen for a web page. Add the page to your Home Screen to hide the browser bars.
Check with a coin after you calibrate. The check step does this for you.

Camera rulers and other options

Some apps use the camera to measure. That needs a reference object of known size in the photo, and it is a different tool. This ruler does not use the camera. It measures flat objects that you place on the screen.

If you use a computer, the actual size ruler page explains calibration in detail. See also the cm ruler and the inch ruler.