Régua Digital
Units
Edge

Click or drag on the ruler to drop markers. Drag a line or its label to reposition it; double-click to remove it.

Guide & FAQ ↓

Online ruler

A real ruler, on your screen

A free actual size ruler: calibrate once, then measure real objects in centimeters, millimeters, inches, or pixels — on phone, tablet, or desktop.

How to use

  1. 1 · Calibrate — Open Calibrate and use auto-detection, a credit card, or the screen diagonal.
  2. 2 · Pick an edge — Snap the ruler to the top, bottom, left, or right and rest the object against the real screen bezel.
  3. 3 · Measure and mark — Switch cm, mm, inches, and pixels with keys 1, 2, 3, and 4. Click the ruler to drop lines with exact readouts; Esc clears everything.

Keyboard shortcuts

  • Switch units1234
  • Move the ruler edge↑↓←→
  • Dark modeD
  • Full screenF
  • Clear markersEsc

Online ruler · Guide

A real online ruler at actual size

Régua Digital is a free online ruler that turns your phone, tablet, or computer screen into a genuine measuring instrument. No app to download, no sign-up, no fees: open the page, calibrate once, and use this actual size ruler to measure small objects, check printouts, compare coins and screws, or verify layout margins. The scale sits flush against the screen edge — exactly where the object rests — and you can snap it to any of the four edges (top, bottom, left, or right), switching between centimeters, millimeters, inches, and pixels with one tap or with keys 1, 2, 3, and 4.

How to use the online ruler

Using this real online ruler takes less than a minute. First, calibrate the screen: open the Calibrate panel and use auto-detect, the credit-card method (Credit card tab), or the screen diagonal (Screen diagonal tab) — any of them tunes the PPI until 1 cm on screen equals exactly 1 physical centimeter. Then pick the ruler's edge (top, bottom, left, or right), rest the object against it, and read the size straight off the scale, just like a physical ruler. To use the screen ruler in cm, mm, inches, and pixels, simply switch units in the floating control island. To record a measurement, click or drag on the ruler: every marker line shows its exact readout, and the summary reports the span between the first and last mark. The Esc key clears all markers at once.

How to calibrate with a credit card

The most reliable way to calibrate any display is the card method: every standard credit, debit, or ID card measures exactly 85.60 × 53.98 mm (3.37 × 2.13 in). In the Card tab, an outline appears on screen — hold your physical card against it and resize with the slider or the − and + buttons until the edges line up perfectly. On apply, the page computes your display's true PPI and saves the calibration in the browser, so your online ruler stays accurate on future visits. Any standard-size card works, and the − and + buttons give fine, single-PPI steps. If the screen is too narrow for the card at full size, the outline rotates so you can match the card's short edge; if it still does not fit, use the Screen diagonal tab.

Measuring on phone, tablet, and desktop

On a phone, the virtual ruler shines for small objects: auto-detect recognizes common handsets (iPhone 14, 15, and 16, Galaxy S23 and S24) and you measure with the phone held sideways to use the full width. On a tablet, such as 10.9″ to 12.9″ iPads, there is room for larger templates, drawings, and craft pieces — iPads are identified automatically, with nothing to pick from a list. On desktop and laptop screens (MacBook Air and Pro, 24″ and 27″ monitors), maximize the browser window, rest the object on the top or side edge, and press F for fullscreen to measure edge to edge. Everywhere, dark mode (D key) follows your system and the ruler moves between all four edges — horizontal or vertical — without losing calibration.

FAQ

Online ruler questions, answered

Is this digital ruler accurate?

Yes — once calibrated, this online ruler shows true physical dimensions. Every screen has a different pixel density (PPI), so the tool auto-detects your device on load, and you can fine-tune it with a credit card or your screen diagonal. After that, 1 cm on this actual size ruler equals 1 real centimeter, and the px unit is exact by definition.

How do I calibrate with a credit card?

Open Calibrate — the panel has four tabs: Auto-Detect, Device, Screen diagonal, and Card / Physical Ruler. On the Card tab, hold a standard credit, debit, or ID card (ISO/IEC 7810 ID-1: 85.60 × 53.98 mm) against the dashed outline and drag the high-precision slider, or use the − and + buttons in 0.1 steps with live redraw, until the edges match exactly (a 10 cm reference line is shown too), then press Apply calibration. The raw decimal PPI is saved to localStorage and the canvas redraws in real time.

Can I measure on mobile?

Absolutely. This real online ruler works on phone, tablet, and desktop with no app to install. Calibration runs by itself the moment the page opens: the heuristics engine matches your handset in its device database (iPhone 14/15/16 Pro Max and standard at 460 PPI, Galaxy S22/S23/S24 Ultra at 500 PPI, Galaxy S22/S23/S24 at 425 PPI, Redmi Note 11T 5G at 399 PPI, iPad Pro at 264 PPI — each converted to its exact CSS density), resolves a Retina MacBook to 227 ÷ devicePixelRatio and a 1920 × 1080 laptop to a 15.6″ panel at 142 PPI, and green-badges the result Auto-Calibrated. Only a screen it cannot recognize gets a red Needs Calibration badge with a pulsing Calibrate button, where you can still pick your exact model on the Device tab or fine-tune with the Card tab or the Diagonal tab. Move the ruler to any of the four edges — top, bottom, left, or right — hold the phone sideways for maximum width, and rest the object against the real bezel to read sizes down to 0.1 mm.

How do I switch between cm, mm, inches, and pixels?

Use the Units buttons in the floating control island, or press 1 for centimeters, 2 for millimeters, 3 for inches, and 4 for pixels. The scale, marker readouts, and span summary update instantly, so the same screen ruler covers cm, mm, inches, and px without recalibrating.

Can I move the ruler to the top, bottom, or side of the screen?

Yes. The ruler locks to one edge at a time — Top, Bottom, Left, or Right — using the Edge buttons or the arrow keys. It sits flush against the real boundary of the display, so you can rest the object right on the bezel and read the measurement along the scale without any gap.

Do I need to install anything or create an account?

No. Régua Digital is a free web app that runs entirely in your browser — no download, no sign-up, no fees. Your calibration is stored locally on your device, and you can measure, drop marker lines on any edge, and use fullscreen mode right away.