Free Online Ruler — Measure in Inches, cm, mm & px at Actual Size
Accurate Ruler is a free, browser-based measurement tool that turns any screen — desktop, laptop, tablet, or phone — into a precision ruler. Measure in centimetres, millimetres, inches, or pixels at true physical size. No download, no sign-up, no ads. Calibrate once and every return visit is already accurate.
Accurate Measurements — Calibrated to Your Screen
Most online rulers default to 96 PPI — the outdated web standard — but modern screens vary from 82 PPI on a budget 27-inch monitor to 460 PPI on the latest iPhone. Without calibration, every measurement is a guess. Accurate Ruler solves this with four precise calibration methods:
- Auto-detectAutomatically identifies your device and applies the factory PPI. Works instantly for most iPhones, iPads, MacBooks, and popular Android devices.
- Device pickerBrowse 400+ models by brand and select your exact device for pinpoint accuracy. Saved to your browser — calibrate once, accurate forever.
- Screen diagonalEnter your screen's diagonal in inches. The tool calculates the precise PPI from your native resolution.
- Credit card calibrationHold a credit card, debit card, or ID card flat against your screen and drag the slider until the rectangle matches. No hardware required.
Your calibration is saved to localStorage — so every return visit uses your exact PPI automatically.
Measure in Inches, Centimetres, Millimetres, or Pixels
Switch instantly between four measurement units using the toolbar or keyboard shortcuts (1–4):
cmkey 1Centimetres
The international metric standard. Ideal for everyday tasks, science, and engineering.
inkey 2Inches
US measurements, craft projects, and American standard sizes. The 12-inch ruler spans a full foot.
mmkey 3Millimetres
Fine precision tasks — PCB layout, jewellery sizing, component measurement. Every mm has its own tick mark.
pxkey 4Pixels
Screen pixel measurements for UI design and digital layout work. Exact pixel counts from your calibrated PPI.
Tick marks automatically scale to your calibrated PPI so 1 cm on screen equals exactly 1 physical centimetre — making this a true online ruler to scale. Pixel mode is unique to Accurate Ruler — most free online rulers stop at three units.
Place Rulers on Any Screen Edge
Unlike a physical ruler, Accurate Ruler can appear on all four edges simultaneously — creating a full-frame measuring environment:
- →Top rulerA horizontal ruler along the top of the canvas
- →Bottom rulerA horizontal ruler along the bottom edge
- →Left rulerA vertical ruler on the left side
- →Right rulerA vertical ruler on the right side
Guide Lines with Undo/Redo, Crosshair & Fullscreen Mode
Accurate Ruler goes beyond tick marks with a suite of precision features — including several not found on any other free online ruler:
Guide lines + Undo/RedoClick the canvas to drop a reference line. Unlike other rulers, Accurate Ruler supports full Undo (Ctrl+Z) and Redo (Ctrl+Shift+Z) for guides — and a Clear button to remove all at once. Drop lines freely; mistakes cost nothing.
Crosshair modeA live crosshair follows your cursor showing real-time X and Y coordinates in your chosen unit. Toggle with C or the toolbar button.
Fullscreen modePress F to hide all browser chrome and expand the ruler to edge-to-edge. The most accurate view for physical objects placed on screen.
Dark and light themeToggle with D. Your preference is saved between sessions. Guide line colours adapt automatically for visibility in both themes.
Who Uses Accurate Ruler?
Accurate Ruler is used daily by anyone who needs fast, reliable measurements without reaching for a physical ruler:
- →Graphic and web designers checking element sizes and spacing
- →Architects and engineers verifying scale drawings and blueprints
- →Students measuring printed diagrams, maps, and worksheets
- →Tailors and crafters measuring fabric, templates, and materials
- →Photographers comparing filter thread sizes and print proportions
- →DIY enthusiasts getting quick measurements on the go
- →UI developers measuring pixel distances in mockups
- →PCB designers verifying component and trace dimensions
Frequently Asked Questions
Can I use my phone as a ruler?+
Yes. Open Accurate Ruler in your phone's browser and activate any ruler edge from the toolbar. For accurate results, tap Calibrate and choose your device from the picker (or use the credit card method). Once calibrated, your phone screen becomes a ruler accurate to within a fraction of a millimetre.
Is there a free ruler online?+
Yes — Accurate Ruler is completely free, works in any modern browser, and requires no download or sign-up. It measures in cm, mm, inches, and pixels at actual size, with calibration tools to match measurements precisely to your screen's pixel density.
How do I measure 1 inch online?+
Switch to inches (press 2 or click "in" in the toolbar). After calibrating your screen, the "1" mark on the ruler represents exactly one inch. A US quarter coin (~0.96 in wide) is a handy physical check.
How do I measure in cm online?+
Press 1 or click "cm" in the toolbar, activate a ruler edge, then place your object against it. After calibration, every 10 small ticks equal 1 cm — just like a physical ruler.
Can I measure in millimetres online?+
Yes — Accurate Ruler supports millimetre mode (press 3 for "mm"), which is missing from many other free online rulers. Every individual tick mark represents 1 mm, with longer marks at each centimetre boundary.
Can I undo a guide line?+
Yes — press Ctrl+Z to undo the last guide. Ctrl+Shift+Z redoes it. You can also clear all guides at once with the Clear button in the toolbar or by pressing Esc. No other free online ruler offers guide undo/redo.
How accurate is an online ruler?+
After calibration, Accurate Ruler is accurate to within a fraction of a millimetre for objects placed directly on screen. The default 96 PPI assumption is only accurate for specific monitors — always calibrate for precision work.
Can a smartphone measure objects?+
Yes, in two ways. AR apps like Apple Measure estimate distances using depth sensors. Browser tools like Accurate Ruler turn the screen itself into a ruler — more accurate for small objects (up to ~15 cm) placed directly against the screen.
How do I read an online ruler?+
In cm mode: numbered marks are centimetres; the smallest ticks are millimetres. In inch mode: numbered marks are full inches; smaller ticks divide each inch into halves, quarters, and eighths. Align the "0" end with your object and read where the other end falls.
How do I view a ruler in Word Online?+
In Microsoft Word Online, click the View tab in the ribbon and toggle Ruler on. This shows the document margin ruler — it's not a physical measurement tool. To measure real-world objects, use Accurate Ruler in a separate tab.
How do I show a ruler in PowerPoint Online?+
In PowerPoint Online, go to View and check the Ruler checkbox. This shows the slide-dimension ruler for positioning objects — not physical inches. For screen-accurate physical measurements, use Accurate Ruler.