Real Size Ruler

To show a real inch on screen, a page has to know how many pixels your display packs into an inch. This ruler works it out from your screen’s diagonal size and resolution — or from a coin or card you hold against the glass — then draws a true-size ruler in centimetres and inches. You can also click two points to measure anything shown on the screen.

Why screens need calibrating

A screen’s pixel density — pixels per inch (PPI) — varies enormously: a 24-inch 1080p monitor has about 92 PPI, a 15.6-inch 4K laptop about 282, and phones often 400 or more. On top of that, operating systems scale everything so text stays readable. Web pages work in “CSS pixels”, which are nominally 96 per inch but rarely match the real size. Calibrating tells the ruler how many CSS pixels make a real millimetre on your screen.

PPI formula

For a 15.6-inch laptop at 1920 × 1080: √(1920² + 1080²) ≈ 2,203 pixels, ÷ 15.6 ≈ 141 PPI. At 125% scaling, that is about 113 CSS pixels per inch, or 4.44 per millimetre.

Two ways to calibrate

1. Screen size and resolution: enter the diagonal from your device’s specifications; the page pre-fills the resolution and scaling from your browser. Best for laptops and monitors where you know the exact size.
2. Coin or card: hold a bank card, quarter, euro or pound coin against the screen and drag the slider until the dashed outline matches. Works on any device, including phones.
3. Check the result by measuring a real object against the ruler.
4. Your calibration is remembered in this browser.

Measuring on screen

Click two points on the ruler strip to measure the distance between them in centimetres and inches — useful for checking the real size of an image, a design mock-up at 100% zoom, or a document shown at actual size. Keep the browser zoom at 100% while measuring; zooming changes the size of everything on the page.

Common screens and their pixel density

Worked example: checking a print size

You’ve designed a 3.5 × 2 inch business card and want to see it at real size before printing. Calibrate the ruler, set the design app to 100% zoom, and measure the card on screen by clicking its two edges on the ruler strip. If it measures 3.50 inches, what you see is the size it will print — useful for judging whether text is large enough to read.

Browser zoom and display scaling

Two settings change on-screen sizes: operating-system display scaling (for example 125% or 150% on Windows, or “Retina” scaling on Macs) and browser zoom. The calculator accounts for display scaling in its PPI method; browser zoom should stay at 100% while you measure. If you calibrate with a coin or card, both are automatically included — which is why that method is the most reliable for quick checks.

Uses for an on-screen ruler

• Checking the size of a ring, bead, screw or coin against the screen when you don’t have a ruler.
• Previewing print designs at real size.
• Measuring small parts in product photos shown at a known scale.
• Teaching children centimetres and inches with a large, clear scale.
• Checking the size of text for readability.

Screen size myths

• A screen’s advertised size is the diagonal of the visible picture, not the width.
• Two monitors with the same resolution but different sizes have different pixel densities.
• Phones report scaled “device pixels” to web pages, so resolution alone doesn’t tell you the real size.
• Printed pages don’t care about screen PPI — print at 100% to get real size on paper.

Printing a real-size ruler

• Print the ruler page from the pack at 100% (Actual size) — never “fit to page”.
• Check it against a card (85.6 mm) or a real ruler.
• If it’s off, your printer is scaling; adjust the printer’s scale setting.
• Print on card stock or laminate it for everyday use.

Printable pack

The optional pack includes printable real-size rulers in centimetres and inches, a calibration check page, a paper measuring tape and a coin and card size reference. The on-screen ruler is free.

AI-assisted content

This page was drafted with AI assistance and reviewed by Kedop. Accuracy depends on the values you enter and your calibration.