A true actual-size ruler, calibrated to your screen
Most online rulers assume the old web standard of 96 pixels per inch. Real screens range from about 90 pixels per inch on a large office monitor to more than 450 on a modern phone, so an uncalibrated “inch” can be off by a third or more. This ruler measures how big a millimeter really is on your display and draws every tick from that.
You can calibrate in four ways:
- Auto-detect recognizes iPhones, iPads and MacBooks from their screen configuration and applies the factory pixel density.
- Device lets you pick your phone, tablet, laptop or monitor from a list of popular models and common screen sizes.
- Screen size calculates the pixel density from the diagonal you enter, for any screen.
- Object is the most precise: match an outline to a bank card, a coin, the short edge of an A4 or Letter sheet, or a physical ruler.
The result is stored in this browser only. If you change the browser zoom, the ruler rescales itself to stay at actual size. When you move to another screen, calibrate again.
Centimeters, millimeters, inches and pixels
Centimeters (cm) show a numbered tick for every centimeter, a medium tick at each half centimeter and a small tick for every millimeter, exactly like a school ruler.
Millimeters (mm) use the same ticks but number them in millimeters (10, 20, 30…), which is handy for technical drawings, sewing patterns and 3D printing.
Inches (in) divide each inch into halves, quarters, eighths and sixteenths. Measurements are shown as decimals and as the nearest fraction, such as 2.375 in (2 3/8″).
Pixels (px) show CSS pixels, so web designers can check sizes and spacing of elements on screen without any calibration.
Edge rulers, guides, measuring tape and crosshair
Turn on a ruler along any edge of the workspace, or all four at once for a full measuring frame. In fullscreen mode the rulers sit on the real edges of your display.
- Guide lines: click a horizontal ruler to drop a vertical guide and a vertical ruler to drop a horizontal one. Each guide shows its position, and the gap between neighboring guides is measured for you. Drag a guide to move it, or drag it back onto a ruler to delete it.
- Measuring tape: drag anywhere to measure a distance. You see the length, the width and height of the box it spans, and the angle. Ends snap to guides; hold Shift to lock to 0°, 45° or 90°.
- Crosshair: follows your pointer and shows its X and Y position in the current unit.
- Grid: lays a millimeter (or 1/8 inch) grid over the workspace, like graph paper.
- Keyboard control: add guides with V and H, nudge the selected guide with the arrow keys, and switch units with 1 to 4.
Inch fractions chart
Each inch on the ruler is split into 16 parts. The longer the tick, the bigger the fraction. Use this chart to convert a reading to decimal inches and millimeters.
| Fraction | Decimal inch | Millimeters |
|---|---|---|
| 1/16″ | 0.0625 | 1.588 |
| 1/8″ | 0.1250 | 3.175 |
| 3/16″ | 0.1875 | 4.762 |
| 1/4″ | 0.2500 | 6.350 |
| 5/16″ | 0.3125 | 7.938 |
| 3/8″ | 0.3750 | 9.525 |
| 7/16″ | 0.4375 | 11.112 |
| 1/2″ | 0.5000 | 12.700 |
| 9/16″ | 0.5625 | 14.288 |
| 5/8″ | 0.6250 | 15.875 |
| 11/16″ | 0.6875 | 17.463 |
| 3/4″ | 0.7500 | 19.050 |
| 13/16″ | 0.8125 | 20.638 |
| 7/8″ | 0.8750 | 22.225 |
| 15/16″ | 0.9375 | 23.813 |
| 1″ | 1.0000 | 25.400 |
How to read a ruler
Line up one end of the object with the 0 mark and look at where the other end falls.
On the metric scale, numbered ticks are centimeters and the small ticks are millimeters. If the object ends three small ticks after 4, it is 4.3 cm or 43 mm long.
On the inch scale, the longest unnumbered tick is 1/2 inch, then 1/4, 1/8 and the shortest is 1/16. Count sixteenths from the last whole inch and simplify: 6/16 is 3/8, so a reading of 2 and six small steps is 2 3/8 inches.
Objects of known size to check your ruler
After calibrating, measure one of these to confirm the ruler is accurate.
| Object | Size |
|---|---|
| Bank, credit or ID card (ISO/IEC 7810 ID-1) | 85.60 × 53.98 mm (3.370 × 2.125 in) |
| A4 sheet of paper | 210 × 297 mm (8.27 × 11.69 in) |
| US Letter sheet of paper | 8.5 × 11 in (215.9 × 279.4 mm) |
| US banknote | 155.96 × 66.29 mm (6.14 × 2.61 in) |
| US quarter (25¢) | 24.26 mm (0.955 in) across |
| US penny (1¢) | 19.05 mm (0.750 in) across |
| 1 euro coin | 23.25 mm across |
| Standard business card (US) | 3.5 × 2 in (88.9 × 50.8 mm) |
| Standard business card (EU) | 85 × 55 mm |
| SIM card (nano) | 12.3 × 8.8 mm |
Keyboard shortcuts
| Key | Action |
|---|---|
| 1 · 2 · 3 · 4 | Switch units: cm, mm, inches, pixels |
| T · B · L · R | Show or hide the top, bottom, left or right ruler |
| V · H | Add a vertical or horizontal guide |
| ← → ↑ ↓ | Move the selected guide by 1 pixel |
| Shift + arrows | Move the selected guide by 1 mm, 1/16 in or 10 px |
| Delete | Remove the selected guide or the measurement |
| Esc | Clear all guides and the measurement (in fullscreen: exit fullscreen) |
| G | Show or hide guides |
| C | Toggle the crosshair |
| N | Toggle the grid |
| F | Toggle fullscreen |
| K | Open calibration |
| D | Toggle dark mode |
| Shift while measuring | Snap to 0°, 45° or 90° |
| I | Open an image or scanned document to measure |
| M | Move or stop moving the open image |
| ? | Show these shortcuts |
Tips for the most accurate measurements
- Keep the browser zoom at 100% while calibrating. Press Ctrl + 0 (⌘ + 0 on a Mac).
- Calibrate with the longest object you have. A physical ruler or the edge of an A4 sheet beats a coin.
- Look straight at the screen, not from an angle, so the object and the ticks line up without parallax.
- Rest the object lightly. Pressing on an LCD screen can damage it and does not improve the reading.
- Use Fullscreen and maximize the window so the left and top rulers start at the real edge of the display.
- For prints, test your printer instead: on-screen calibration does not change how paper prints.
Who uses an online ruler?
- Students and teachers measuring worksheets, maps, diagrams and small objects in class.
- Designers and web developers checking element sizes, margins and spacing in pixels or millimeters.
- Crafters, sewists and makers sizing fabric swatches, beads, jewelry, screws and 3D-printed parts.
- Online shoppers and sellers checking the size of a ring, a button or a product photo against the real thing.
- Architects and engineers verifying scale drawings and printouts.
- Anyone without a ruler at hand who needs a quick, accurate measurement.
The ruler runs entirely in your browser. No sign-up, no ads in the tool, and nothing you measure leaves your device.