Free Online Ruler · Actual Size
Measure anything on your screen in cm, inches or pixels
This free online ruler turns your monitor, laptop or phone into a precision measuring tool at true physical size. Calibrate once with auto-detect, your screen diagonal, or a real credit card — then read accurate measurements directly on screen. No downloads, no sign-up, 100% private.
Rulers appear automatically Measures cm, mm, inches & px Works on phones & desktops
Rulers on: top, left, right, bottom
Rulers are showing along all four edges of your window — top, bottom, left and right. Place your card or object against any edge and read the tick marks.
1 · Choose a screen edge
2 · Choose a unit
How to Use the Ruler
- 1
Calibrate for actual size. Most screens are detected automatically on load. For fine-tuning, click Calibrate and pick auto-detect, your device model, your diagonal, or align a physical credit card. This sets the PPI so 1 cm on screen = 1 physical cm.
- 2
Measure. Rulers already appear along all four edges — top, bottom, left and right. Click an edge button to show or hide it, then place your object against any ruler and read the ticks.
- 3
Switch units any time. Use the buttons or the 1 2 3 keys for cm, inches or px.
- 4
Drop guides. Click anywhere on a ruler to place a dashed reference line, or toggle Guides to drop lines anywhere. Press Esc to clear them all.
Keyboard shortcuts
- Switch units: cm / inches / px 1 / 2 / 3
- Toggle crosshair C
- Toggle guide mode G
- Fullscreen F
- Toggle this guide H
- Clear guide lines Esc
Calibrate your screen
Matches your screen against a built-in database of iPhones, iPads, MacBooks, laptops and common monitors using your exact pixel resolution, then applies the factory PPI.
Don’t see your screen detected automatically? Pick your exact model below — phone, tablet, laptop or monitor — for pinpoint accuracy.
Find it in your device specs. Common sizes: 11″, 13.3″, 14″, 15.6″, 24″, 27″.
Hold a real credit, debit or ID card (85.60 × 53.98 mm) against the dashed rectangle on your screen, then drag the slider until it fits exactly. Works with any card you have on hand — no other tool needed.
Use your keyboard arrow keys for pixel-perfect precision.
Step by step
How to use the online ruler
Using a screen ruler takes about ten seconds. Here is exactly what to do the first time you visit.
Calibrate for actual size
Click Calibrate and let auto-detect find your device, enter your screen diagonal, or hold a credit card against the card guide and drag the slider until it fits. Your calibration is saved for next time.
Place a ruler on an edge
Click Top, Bottom, Left or Right in the toolbar. The ruler appears along that edge of your window — use several edges at once to measure width and height together.
Measure in your unit
Choose cm, inches or px (keys 1/2/3). Align your object to the screen edge and read the ticks. Drop guide lines with a click and read exact distances.
The science
Why calibration matters
Almost every online ruler you find assumes a fixed 96 DPI — the pixel density of old 1990s monitors. Modern screens are wildly different. A budget 27-inch monitor squeezes 1920×1080 pixels into a panel of about 82 PPI, while a flagship phone packs 460 PPI into a display smaller than your hand. An inch drawn at 96 DPI on a phone would be almost five times too big.
That is why this tool measures your screen before it draws a single tick. PPI (pixels per inch) tells the ruler how many pixels make up one physical inch on your display. Once calibrated, one centimetre on screen equals exactly one physical centimetre — making this a true actual size online ruler.
Calibration also compensates for operating-system display scaling. Windows at 150% and macOS with display scaling both change the relationship between pixels and physical inches; the credit-card and diagonal methods measure the real panel, so scaling cannot break your measurements.
How PPI varies by device
| Device | Pixels per inch |
|---|---|
| iPhone 16 Pro Max | 460 PPI |
| Galaxy S24 Ultra | 505 PPI |
| MacBook Pro 14" | 254 PPI |
| MacBook Air 13.6" | 224 PPI |
| Common laptop 15.6" | 141 PPI |
| Monitor 24" 1080p | 92 PPI |
| Monitor 27" 4K | 163 PPI |
| Ultrawide 34" | 110 PPI |
A 96 DPI ruler is off by up to 5× on a modern phone and up to 15% even on many desktop monitors. Calibrate once and this tool stays accurate forever.
Everything included
A precision tool, not just a ruler
Four edge rulers
Place a ruler along the top, bottom, left or right of your window — or all four at once for a full measuring frame.
True-size calibration
Auto-detect your device, enter your screen diagonal, or align a real credit card. One tap and 1 cm really equals 1 cm.
Guide lines
Click anywhere to drop dashed reference lines, then read exact distances in your chosen unit. Clear everything with Esc.
Live crosshair
Follow the cursor with a precision crosshair that reports live X/Y coordinates in cm, inches or pixels.
Fullscreen mode
Expand to edge-to-edge fullscreen so rulers cover your entire display for the most accurate measurements.
Private & free
Everything runs in your browser. No accounts, no ads, no tracking — your calibration stays on your device.
Handy reference
How big is a credit card, A4 paper or a SIM card?
Compare everyday objects against your calibrated ruler. Pick one you have nearby, line it up on the screen ruler and check the ticks — the perfect way to confirm your calibration is spot-on.
Try it with the ruler| Object | Millimetres | Inches |
|---|---|---|
| Credit card (CR80) | 85.6 × 53.98 mm | 3.37 × 2.13 in |
| A4 paper | 210 × 297 mm | 8.27 × 11.69 in |
| US Letter paper | 216 × 279 mm | 8.5 × 11 in |
| US $1 bill | 155.9 × 66.3 mm | 6.14 × 2.61 in |
| iPhone 16 Pro Max | 163 × 77.6 mm | 6.42 × 3.06 in |
| Standard SIM card | 25 × 15 mm | 0.98 × 0.59 in |
| AAA battery | 44.5 × 10.5 mm | 1.75 × 0.41 in |
| US quarter coin | 24.3 mm across | 0.96 in across |
FAQ
Online ruler questions, answered
Everything people ask about measuring with a screen, calibrating PPI, and using an online ruler in cm, inches or pixels.
How do I measure cm online?
Open this page, select cm in the unit toggle (or press 1), then activate a ruler edge with the Top, Bottom, Left or Right button. Calibrate your screen once for true physical size — auto-detect, screen diagonal or credit card. Place your object against the ruler edge: each numbered tick is a centimetre and the small ticks are millimetres.
Can I use my phone as a ruler?
Yes. Open this online ruler in your phone’s browser, tap Calibrate and choose auto-detect — the tool recognises hundreds of common phone panels (iPhone, Galaxy, Pixel and more) from their exact pixel resolution. Your phone screen then becomes an accurate ruler for small objects up to about 15 cm. For maximum accuracy, use the credit-card calibration method.
How do I identify 1 inch on screen?
Switch to inches (press 2). After calibration, every major tick labelled with a number is exactly one inch, and each inch is divided into halves, quarters, eighths and sixteenths. As a real-world check: a US quarter coin is about 0.96 inches (24.3 mm) across, and a standard credit card is 3.370 inches (85.60 mm) wide.
How accurate is an online ruler?
An online ruler is as accurate as your calibration. Many sites assume the old 96 DPI standard, which can be off by 20–30% on modern displays. After you calibrate with the diagonal or credit-card method, this tool is accurate to roughly a millimetre — plenty for measuring objects, cards, paper and small components.
What is PPI and why does calibration matter?
PPI (pixels per inch) describes how dense the pixels are on your screen. Budget 27-inch monitors run about 82 PPI while flagship phones reach 400–500 PPI, so “one inch” occupies wildly different pixel counts on different screens. Calibration measures your screen’s real PPI so the ruler draws true physical size instead of a guess.
Can I measure in pixels with an online ruler?
Yes. Press 3 to switch the ruler to px mode, which shows 10-pixel minor ticks and numbered 100-pixel major ticks. Pixel mode needs no calibration — it is purely a screen measurement — and is ideal for checking element widths, margins and spacing while designing websites or apps.
Do I need to install anything to use this ruler?
No. This is a browser-based screen ruler — there is nothing to download, no account to create and no sign-up. Everything runs in your browser. Your calibration is saved locally on your device, so your ruler is ready and accurate on every future visit.
Does this online ruler collect my data?
No. The tool runs 100% in your browser and no data ever leaves your device. There are no analytics, no cookies and no server-side logging. The only storage used is your browser’s localStorage, which remembers your calibration, unit and theme preferences.
Calibrate once. Measure forever.
Your calibration is saved on your device, so the ruler is ready and accurate every time you visit — completely free, with no ads and no account.
Start measuring now