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Free Screen Ruler

Measure real sizes on your screen in cm, mm and inches with a draggable, rotatable ruler, a drag-to-measure mode and a protractor, calibrated with a bank card or your screen size.

Not calibrated yet. Until you press "Calibrate", lengths use a standard guess of 96 pixels per inch, which is often wrong by 10 percent or more.

Calibrate your screen

Pages cannot tell how big your screen really is, so match something of known size once. The result is saved in this browser.

Hold a bank card, driving licence or ID card (the standard 85.60 x 53.98 mm size) flat against the screen. Drag the round handle, or use the slider and the "βˆ’1 px" and "+1 px" buttons, until the box is exactly as wide as the card. If the long side is wider than your screen, choose the short side.

Hold your card against the screen and match this box to it

1 cm is 37.9 pixels on this screen.

Turn on "Measure distance", then drag across what you want to measure.

Accuracy: this is as good as your calibration. One pixel of error when matching the card is about 0.26 mm on a typical screen, and any scale error grows with the length you measure. The screen-size method assumes square pixels and the advertised diagonal, so expect a few percent of error. Use it for quick measurements, not where precision matters.

Ruler keys: arrow keys move it by 1 pixel (10 with Shift), [ and ] turn it by 1 degree (15 with Shift). Dragging the round handle snaps to horizontal and vertical; hold Shift to snap to 15 degree steps.

πŸ”’ Your calibration is saved in this browser only and never leaves your device. Nothing goes in the page address.

How It Works

The page draws the ruler with SVG at a size worked out from your calibration, so a centimetre on screen is as long as the number of pixels you measured. Dragging and measuring use pointer events, and the calibration is stored in this browser.

  1. 1
    Calibrate once
    Press "Calibrate" and match the box to a bank card (85.60 x 53.98 mm), or type your screen diagonal. Press "Save calibration".
  2. 2
    Place the ruler
    Drag the ruler where you need it and drag its round handle to turn it. Choose "cm / mm" or "Inches" and a "Ruler length".
  3. 3
    Measure a distance
    Turn on "Measure distance" and drag across the screen. The length is shown in millimetres, centimetres and inches.
  4. 4
    Read an angle
    Turn on "Protractor", move it over a corner and read the angle against its scale. The line you measure also shows its angle to the horizontal.

Your calibration stays in this browser. It is never put in the page address and never uploaded.

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Frequently Asked Questions