Online Ruler for CM, MM and Inches

Measure small objects on your screen with a clean online ruler in centimeters, millimeters, inches and pixels. Calibrate once with a card or screen diagonal for a closer real-size scale.

Calibration is stored only in this browser.
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Drag across the ruler to measure a span.
⌨ Keyboard shortcuts

C cm · I inch · P px · R rotate · F fullscreen · G guides · X crosshair · A angle · D dark · 4 four edges

Calibrate1Ruler

Match a standard bank card width (85.60 mm) to the guide below.

85.60 mm

How to use the online ruler

Choose cm/mm, inches or pixels, place the object against the ruler edge, then drag across the scale to read a distance. Use Rotate when a vertical edge is easier to work with, Fullscreen when you need more room, and Popup when you want the ruler by itself.

1. Start with the correct unit

The metric scale marks every millimeter and labels centimeters. The inch scale is divided into sixteenths. Pixel mode measures CSS-pixel distance and is useful for screenshots, layouts and interface checks.

2. Calibrate for real physical size

A browser can report screen pixels but cannot reliably determine the physical width of every display. For a true-size centimeter or inch scale, open Calibrate and match the guide to a standard bank card that is 85.60 mm wide. If you know the physical diagonal of the active screen, you can use that method instead.

3. Keep the display scale unchanged

After calibration, keep browser zoom at 100% when possible. If you move the window to another monitor, change system scaling or alter browser zoom, recalibrate before relying on physical-size readings.

Works on modern phones, tablets, laptops and monitors

1Ruler uses the dimensions and pixel ratio reported by the browser and redraws the scale when the viewport changes. It does not pretend that two screens with the same resolution have the same physical size; a 1920×1080 panel can exist at very different diagonals, which is why calibration remains the reliable reference.

  • Apple: iPhone, iPad, MacBook, iMac and external displays in current Safari or Chromium browsers.
  • Android: Samsung Galaxy, Google Pixel and other modern phones and tablets.
  • Windows: Surface devices, laptops, desktop monitors and touch displays.
  • Browsers: current Chrome, Safari, Firefox, Edge and Opera releases with JavaScript enabled.

Practical measuring tips

  • Rest small objects against the same screen edge as the zero mark instead of measuring at an angle.
  • Use the bank-card method when you can; it gives the ruler a known physical reference on the exact display in front of you.
  • Recalibrate after switching monitors or changing display scaling.
  • Use pixel mode when the job is purely digital; physical calibration is not required for CSS-pixel distances.
  • For legal, medical, engineering or manufacturing work, confirm the result with an appropriate physical instrument.

Why 1Ruler is different from a static ruler image

A ruler image has one fixed pixel size, so it can appear too large on one screen and too small on another. 1Ruler draws the scale in the browser, stores your calibration locally and can redraw horizontally or vertically without stretching a bitmap. That makes the tool useful both for quick real-world checks and for on-screen measurements.

Questions people ask before measuring

How accurate is an online ruler after calibration?

After calibration, the scale is tied to a physical reference on your current display. Accuracy still depends on how precisely you match the card or screen size and whether browser zoom or display scaling changes afterward. For critical measurements, verify with a physical measuring tool.

Why does 1Ruler need calibration?

Browsers know pixels but do not reliably know the physical width of every screen. Calibration establishes how many CSS pixels on the current display correspond to one millimeter or one inch.

What size card should I use for calibration?

Use a standard bank or ID-1 card that is 85.60 mm wide. Place the card flat against the screen and adjust the on-screen guide until the widths match.

Can I use 1Ruler on a phone or tablet?

Yes. The ruler is responsive and supports touch. For physical-size measurements, calibrate on the exact screen you are using and avoid pinch-zoom after calibration.

Does browser zoom affect the ruler?

Yes. Zoom and display scaling can change the relationship between CSS pixels and physical size. Keep zoom at 100% when possible, and recalibrate if the scale changes.

What is the difference between cm/mm, inches and pixels?

Centimeters, millimeters and inches require a physical-size calibration. Pixel mode measures CSS-pixel distance on the screen and does not use the physical ruler scale.

Is my calibration uploaded or shared?

No. The calibration value is stored locally in your browser so the ruler can remember the current display. The measuring tool does not need an account.

When should I use a physical ruler instead?

Use a certified or appropriate physical instrument for legal, medical, engineering, manufacturing or safety-critical measurements. 1Ruler is intended for everyday screen-based measurement and quick checks.