Actual-Size Online Ruler
Measure a small object in inches, centimeters or millimeters right on your screen, with no ruler at hand. Calibrate once with a credit card and the markings match real-world millimeters.
- Free
- Actual size
- Inches, cm and mm
- Protractor included
100% private — Everything is processed directly in your browser: your files, voice, and image are never sent to Chatzam's servers.
How to measure with the online ruler
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Calibrate your screen
Hold a credit card against the screen and adjust the rectangle until it's exactly the same size. The setting is saved for your next visits.
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Place the object at 0
Line up the start of the object with the left edge of the ruler. Full screen moves the ruler right up to the edge of the screen.
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Position the cursor
Tap or drag the red cursor to the end of the object: the length is shown in cm, mm and inches.
A real ruler, only handier
Exact scale
Calibrating with a credit card (85.60 mm wide) makes each marking match a real millimeter on your screen.
Two scales
Centimeters and millimeters on top, inches in 1/16 on the bottom, with bold lines and a number at every centimeter and every inch.
Measure longer than the screen
Carry the ruler forward from the cursor as many times as needed: the markings continue and the total length adds up.
Protractor
A 0 to 180° protractor with a double scale and a movable arm to read an angle to the nearest half degree.
Why an on-screen ruler needs calibrating
A browser doesn't know the physical size of your screen. By convention, it assumes 96 CSS pixels per inch, or about 3.78 pixels per millimeter, but phones, tablets and laptops actually display pixels of different sizes. Without calibration, a "4-inch" ruler could measure 3 or 5 inches depending on the device. Calibration fixes this using an object with a universal size: the credit card, which measures 85.60 × 53.98 mm (3.370 × 2.125 in) all over the world (ISO/IEC 7810 ID-1 standard). Debit cards, most US driver's licenses and many ID cards use the same format. Once adjusted, the scale is saved on this device.
How to measure accurately
Keep your browser zoom at 100%: if it changes, the ruler tries to compensate, but recalibrating is the safest option. Press the object flat against the glass and look at it straight on, because at an angle the thickness of the glass shifts the reading. On a phone, turn it sideways and go full screen: the ruler then starts exactly at the edge of the screen and gains a few centimeters. Accuracy depends on calibration: being off by 1 mm on the card gives an error of about 1.2% on any measurement, or 1.2 mm over 10 cm. To check, measure a US quarter: it's 24.26 mm (0.955 in) across.
Measuring in inches, and beyond the screen
The bottom scale is in inches, divided into sixteenths like a standard US ruler: 1 inch is exactly 25.4 mm, so 1/16 inch is about 1.6 mm. The reading shows the nearest fraction, for example 3 7/8 in, along with the decimal value. For an object longer than the ruler, place the cursor at the far right, make a small pencil mark on the object (or keep your finger on it), then press "Continue from cursor": the markings pick up from that value. Slide the device to line up the ruler's left edge with the mark, and keep going. The total length adds up with each step.
Frequently Asked Questions
How can I measure something without a ruler?
Calibrate this online ruler with a credit card, then hold the object against the screen with its start lined up on 0, and place the cursor at the end. Other handy references: a Letter sheet is 8.5 × 11 in, and a US quarter is 24.26 mm across.
Is the ruler really actual size?
Yes, once calibrated: the markings then match real millimeters and inches on your screen. Without calibration, the scale is only an estimate and can be noticeably off depending on the device.
I don't have a credit card. How can I calibrate?
Any card in the credit card format works: a debit card, a loyalty card, a driver's license or an ID card. Otherwise, enter your screen's diagonal in inches; it's less accurate, especially on a computer.
Do I need to recalibrate on each device?
Yes, every screen has its own pixel density. The setting is saved in the browser on that device, and it's also used by our Online Eye Test. Recalibrate if you change screens or zoom level.
How do I measure an angle?
Open the Protractor tab, place the vertex of the angle on the center point and one side on the baseline, then drag the red arm onto the other side. The angle is shown to the nearest half degree.