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.

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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 digital ruler · Guide

Online digital ruler — actual size digital ruler on any screen

Régua Digital is a free Online digital ruler that turns your phone, tablet, or computer screen into a genuine measuring instrument. This digital ruler on phone needs no app to download, no sign-up, no fees: open the page, calibrate once, and use this actual size digital ruler to measure small objects, check printouts, compare coins and screws, or verify layout margins. As a true real scale ruler, 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 this digital ruler on phone

Using this Online digital ruler takes less than a minute. First, calibrate the screen: open the Calibrate panel and use auto-detect, the credit-card method (Card tab), or the screen diagonal (Diagonal tab) — any of them tunes the PPI until 1 cm on screen equals exactly 1 physical centimeter, giving you a true real scale ruler. 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 this actual size digital 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 your real scale ruler 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 of this Online digital ruler, 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 actual size digital ruler stays accurate on future visits, whether as a digital ruler on phone, tablet, or desktop. Any standard-size card works; if the screen is too narrow for the card at full size, use the Screen diagonal tab.

Actual size digital ruler for phone, tablet, and desktop

As a digital ruler on 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 — a pocket real scale ruler. On a tablet, such as 10.9″ to 12.9″ iPads, there is room for larger templates and craft pieces. On desktop and laptop screens, maximize the browser window and press F for fullscreen. Everywhere, this Online digital ruler moves between all four edges without losing calibration.

FAQ

Digital ruler on phone: 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 by picking your model from the built-in device database (dozens of phones, tablets, laptops and monitors, searched by name, alias or common misspelling), with a credit card, or with 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 scale 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. On phones and small tablets a one-time hint appears above the control bar — “Exact scale: Select your device model for 100% precision” — that opens the Device tab so you can search your model directly; you can dismiss it and it stays hidden. Calibration otherwise 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.

How does the actual size online digital ruler work?

The actual size online digital ruler matches the on-screen scale to your display's pixel density (PPI). When the page opens, the tool auto-detects your model, or you calibrate with a standard card or your screen diagonal; after that, 1 cm shown on the digital ruler equals exactly 1 physical centimeter, with nothing to install.

How to use the ruler on mobile (iPhone & Android) accurately?

To use the ruler on mobile accurately, tap the exact-measurement hint and pick your iPhone or Android model on the Device tab — the factory PPI is corrected for this screen's pixel ratio instantly. Then hold the phone sideways for maximum width, rest the object against the real screen edge, and read the size in cm or mm down to 0.1 mm.

How to measure a ring size with a digital ruler?

To measure a ring size with a digital ruler, align the inner edge of the ring on the screen and read the millimeters from one side of the band to the other. That inside diameter in mm gives you the ring size — compare it with a jeweler's size chart to confirm the number before ordering.

How to measure your foot size for shoes using a screen ruler?

To measure your foot size for shoes using a screen ruler, stand on a sheet of paper, mark your heel and the tip of your longest toe, then hold the screen against the marks to read the centimeters between them. Note the foot length in cm and check the manufacturer's chart to pick the right shoe size.

How many millimeters are in a 30 cm ruler?

There are exactly 300 mm in a 30 cm ruler, because every centimeter equals 10 millimeters. On this digital ruler, just switch to the mm unit (key 2) to read all 300 millimeters straight off the scale.

How to measure cm and mm on screen?

To measure cm and mm on screen, use the Units buttons in the floating panel, or press 1 for centimeters and 2 for millimeters. The scale redraws instantly, so you can read cm with decimals or whole mm without recalibrating.

How to measure things without a physical ruler?

To measure things without a physical ruler, use the calibrated digital ruler in your browser: open this page on your phone, tablet, or computer, calibrate once with your device model or a standard card, and rest the object against the screen edge. You get free cm, mm, and inch readings with no app to download.