Softonic

Online Inch Ruler

Step 1 / 3
Calibrate against a real object (US credit card is 3.37 in) if you need accuracy.

A 12-inch ruler drawn straight in your browser, numbered at every inch and marked down to an eighth of an inch. A screen cannot know its own physical size, so the ruler starts from the browser default of 96 DPI and you correct it: hold a card of known size against the display and drag the Screen DPI slider until the edges line up. Inches are the everyday unit in the United States; almost everywhere else people measure in centimetres, so if you are outside the US or UK the online cm ruler is probably the one you want.

How to calibrate the inch ruler

  1. 1

    Hold a known object against the screen

    A bank card is ideal because its long side is exactly 3.370 in (85.60 mm) on every card in the world. A US quarter (0.955 in) works too.

  2. 2

    Drag the Screen DPI slider

    Move it between 72 and 220 until the object lines up with its known length on the scale. That number tells the page how large a pixel really is on your display.

  3. 3

    Write your DPI number down

    The ordinary one-page ruler returns to 96 when reloaded. In the multi-step ruler, the safe DPI setting travels in the page URL to the next steps, without separate persistent storage.

  4. 4

    Measure

    Hold the object flat against the glass, line one edge up with 0 and read the markings. If 12 inches is wider than your window, scroll the ruler sideways.

Reference measurements you can trust

Object Real width
US quarter 0.955 in
US dime 0.705 in
US dollar bill, short side 2.61 in
US dollar bill, long side 6.14 in
Bank card, long side 3.370 in
Bank card, short side 2.125 in
US Letter paper, short side 8.5 in

What this ruler actually shows

  • 12 inches, numbered at every whole inch.
  • Fractions down to 1/8 inch: a whole inch, a half, a quarter and an eighth, each drawn at its own height.
  • No finer marks. This ruler stops at eighths; it does not draw 1/16, 1/32 or 1/64. For millimetre work use the online cm ruler instead.
  • No unit switch. For centimetres, use the online cm ruler (30 cm).
  • Nothing leaves your browser. The ruler is drawn locally, nothing is uploaded and no measurement is stored.

When inches are the right unit

  • US construction and joinery. Lumber, plywood and hardware are sold in fractional inches.
  • US pipe and plumbing sizes. Nominal pipe size is imperial across most of North America.
  • US paper sizes. Letter, Legal and Tabloid are all defined in inches.
  • Imported US goods and specs. Screen sizes, tyre and wheel dimensions and some hardware are quoted in inches almost everywhere.

Outside these cases, most of the world works in millimetres and centimetres. One inch is exactly 2.54 cm, so if a task is naturally metric the cm ruler will be quicker and less error-prone.

Honest limits

  • The calibration is only as good as your reference. Lining a card up by eye gets you to roughly 1 to 2 percent, fine for a quick check and no substitute for a real ruler when a fraction of an inch matters.
  • Keep browser zoom at 100 percent. Zooming rescales the CSS pixel and therefore the ruler.
  • Recalibrate after any display change. A different monitor, or a change to Windows or macOS display scaling, needs a new DPI number.
  • Measure flat against the glass. Holding an object away from the screen adds a parallax error.
  • Calibration is not kept in separate persistent storage. The ordinary one-page ruler returns to 96 on reload. In the multi-step ruler, the safe DPI setting is carried only in the page URL for its next steps.

Frequently Asked Questions

Because the browser assumes a nominal 96 pixels per inch, which is a convention rather than a measurement of your actual screen. Drag the Screen DPI slider until a known object, such as a bank card at 3.370 in, matches the scale, and the ruler becomes true for your display.

The ordinary one-page ruler resets the Screen DPI slider to 96 on reload. In the multi-step ruler, the safe DPI setting is included in the page URL so it follows the next steps. It is not a stored measurement or a separate persistent profile.

To one eighth of an inch. This ruler draws whole, half, quarter and eighth-inch marks, and no finer. If you need sixteenths or millimetres, use a physical ruler or the online cm ruler.

Not in this tool: it draws inches only. Use the online cm ruler, which shows 30 cm marked every millimetre, or the length converter for a number you already have. One inch is exactly 2.54 cm.

Usually not. Inches are standard in the United States and common in the UK, but most countries measure length in centimetres and millimetres. Reach for this ruler when a spec is genuinely imperial, such as US hardware or paper sizes, and use the cm ruler the rest of the time.

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