# Screen Ruler Online

> Last updated: 2026-05-12

> Screen Ruler Online (https://screenruler.online) is a free, browser-based toolkit with nine tools: an accurate screen ruler (0.5mm precision), an online protractor (0°–360°), a random spinner wheel, a device specs database covering 69+ phones, tablets, laptops, and monitors, a dead pixel test that cycles full-screen solid colors to find defective pixels, an aspect ratio calculator with eight common presets and shareable URLs, a pixel-to-millimeter/inch converter at any DPI, a printable ruler PDF generator with credit-card calibration, and a screen size calculator that maps diagonal + aspect ratio to physical width × height plus THX/SMPTE/4K viewing-distance recommendations. It supports 50+ device profiles and works in 20 languages. No download required.

## What It Does

Screen Ruler Online provides free measurement and utility tools that work directly in your browser:

1. **Screen Ruler** — displays a real-size ruler on your phone, tablet, or computer screen. After a 10-second calibration with a credit card or coin, measurements are accurate within 0.5mm. Supports centimeters, millimeters, and inches.
2. **Online Protractor** — measure angles from 0° to 360° by dragging two rays. Features angle snapping (1°/5°/15°/45°) and 0.1° precision.
3. **Random Spinner** — a customizable wheel spinner (also called picker wheel or wheel of names) for making random decisions. Add your own options and spin to pick a winner.
4. **Device Specs Database** — verified screen specifications for 69+ devices across 11 brands. Browse iPhone, Samsung Galaxy, Google Pixel, iPad, MacBook and more with PPI, resolution, refresh rate, and display technology. Filter by brand, OS, release year, device type, or screen size.
5. **Dead Pixel Test** — a free full-screen color cycler to spot dead, stuck, and hot pixels on any LCD or OLED screen. Cycles nine solid colors (white, black, red, green, blue, cyan, magenta, yellow, gray) plus a Stuck Pixel Fixer mode that rapidly flashes primary colors to revive frozen subpixels. Keyboard shortcuts (Space/arrows/R/G/B) for fast navigation.
6. **Aspect Ratio Calculator** — convert pixel dimensions to a simplified ratio (e.g. 1920 × 1080 → 16:9) or reverse-calculate a missing width or height for any preset. Includes eight common ratios (16:9, 4:3, 1:1, 9:16, 21:9, 3:2, 2:3, 2.39:1 cinema) plus a Social Media Sizes panel with one-click presets for YouTube, TikTok, Instagram (Square / Reels), Twitter / X, LinkedIn, Pinterest, and Facebook. Shareable URLs preserve exact configurations.
7. **Pixel Converter** — convert pixels to millimeters, centimeters, and inches at any DPI. Five DPI presets (96 web, 72 Mac legacy, 150 print draft, 300 print high quality, 600 photo print) plus custom DPI input. Useful for translating Figma/Photoshop pixel exports into print-ready physical dimensions, or sizing CSS values from physical mockup measurements.
8. **Printable Ruler PDF** — generate an actual-size ruler PDF that prints at exact physical scale on A4, US Letter, or US Legal paper. Length 5 cm to ~315 mm, units in cm-only / mm-only / inch (with 1/8 or 1/16 divisions) / dual cm + inch. Built-in credit-card calibration loop: print, measure the calibration outline against a real ISO 7810 card, enter the measured value, and regenerate a scale-corrected PDF.
9. **Screen Size Calculator** — bidirectional sizing for any TV/monitor scenario. From a diagonal + aspect ratio, returns physical width, height, and area; or from a sofa-to-screen distance, returns the recommended diagonal range. Visualizes the THX (cinema-grade), SMPTE (recommended max), and 4K-immersive viewing-distance standards on a draggable bar with adjustable wall distance. Sub-page `/screen-size-calculator/tv-size-for-room` answers "what TV size for my X-meter living room?" directly.

## Key Facts

- Accuracy: within 0.5mm after calibration
- Calibration objects: credit card (ISO 7810 ID-1, 85.60 × 53.98 mm), coins from 6 countries (US quarter 24.26mm, 1 Euro 23.25mm, UK pound 22.5mm, India 2 rupee, Indonesia 1000 rupiah, Turkey 1 lira), A4 paper (297mm x 210mm)
- Device database: 50+ devices with pre-configured PPI (iPhone, Samsung Galaxy, iPad, Xiaomi, OPPO, Vivo, Realme, OnePlus)
- Languages: 20 (English, Chinese, Spanish, French, German, Japanese, Korean, Hindi, Indonesian, Vietnamese, Turkish, Thai, Malay, Croatian, Czech, Romanian, Serbian, Slovenian, Filipino, Chinese Traditional)
- Calibration persistence: saved in localStorage, valid for 30 days
- Works on: any device with a modern web browser — Chrome, Safari, Firefox, Edge

## How Calibration Works

Every screen has a different pixel density (PPI). A pixel on a 5.8-inch phone is physically much smaller than a pixel on a 27-inch monitor. Screen Ruler solves this by having users place a known-size object (like a credit card) on the screen and adjusting a slider until the on-screen outline matches the physical object. This tells the ruler the exact physical size of each pixel, enabling accurate conversion from pixels to real-world units.

## Pages

- Homepage (Screen Ruler tool): https://screenruler.online/
- All Tools: https://screenruler.online/tools
- Online Protractor: https://screenruler.online/protractor
- Random Spinner: https://screenruler.online/spinner
- Device Specs Database: https://screenruler.online/device-specs
- Per-device specs pages: https://screenruler.online/device-specs/iphone-17-pro (and 68+ other devices)
- Dead Pixel Test: https://screenruler.online/dead-pixel-test
- Aspect Ratio Calculator: https://screenruler.online/aspect-ratio-calculator
- Pixel Converter: https://screenruler.online/pixel-converter
- Printable Ruler PDF: https://screenruler.online/printable-ruler
- Screen Size Calculator: https://screenruler.online/screen-size-calculator
- TV Size for Room (sub-page): https://screenruler.online/screen-size-calculator/tv-size-for-room
- FAQ: https://screenruler.online/faq
- Changelog: https://screenruler.online/changelog
- How to Use Guide: https://screenruler.online/guide
- About: https://screenruler.online/about
- Device ruler index: https://screenruler.online/ruler
- Device-specific rulers: https://screenruler.online/ruler/iphone-15-pro-max (and 50+ other devices)
- Length-specific rulers: https://screenruler.online/length/10cm (1cm to 30cm, 1inch to 12inch)
- Calibration object pages: https://screenruler.online/compare/credit-card
- Blog: https://screenruler.online/blog

## Blog Articles

1. [Best Online Ruler Tools Compared (2026)](https://screenruler.online/blog/best-online-ruler-tools) — Tested 6 online rulers on 3 devices. Most failed on phones and laptops.
2. [Online Ruler Accuracy: What Affects Precision](https://screenruler.online/blog/online-ruler-accuracy) — Deep dive into PPI, calibration, and display technology.
3. [Standard Credit Card Size (ISO 7810)](https://screenruler.online/blog/credit-card-size) — Dimensions, tolerances, and why credit cards make ideal calibration objects.
4. [Coin Sizes by Country](https://screenruler.online/blog/coin-sizes-by-country) — Diameter and thickness of coins from 20+ countries.
5. [How to Measure Without a Ruler](https://screenruler.online/blog/how-to-measure-without-a-ruler) — Common household objects with known dimensions.
6. [What Is Screen PPI and Why It Matters](https://screenruler.online/blog/what-is-screen-ppi) — Technical explanation of pixels per inch.
7. [How to Measure Your Phone Screen Size](https://screenruler.online/blog/measure-phone-screen-size) — Step-by-step guide for any smartphone.
8. [How to Measure Ring Size at Home](https://screenruler.online/blog/measure-ring-size-at-home) — Using an online ruler for jewelry sizing.
9. [A4 vs Letter Paper Size](https://screenruler.online/blog/a4-vs-letter-paper-size) — Comparison with exact dimensions.
10. [Can You Print a Ruler?](https://screenruler.online/blog/print-a-ruler) — Pros, cons, and accuracy considerations.

## Frequently Asked Questions

Q: How accurate is an online screen ruler?
A: After calibration with a credit card or known reference object, Screen Ruler achieves accuracy within 0.5mm on most devices. Precision depends on screen resolution and calibration quality. Higher PPI displays like Retina screens provide better accuracy.

Q: Does the ruler work on mobile phones and tablets?
A: Yes. It works on any device with a web browser — iPhones, Android phones, iPads, and Android tablets. Touch controls are fully supported.

Q: Do I need to download an app?
A: No. This is a web-based ruler that runs directly in your browser. You can add it to your home screen on mobile for quick access.

Q: Why do I need to calibrate?
A: Every screen has a different pixel density (PPI). A 14-inch laptop and a 27-inch monitor may have the same resolution but very different physical pixel sizes. Calibration tells the ruler your screen's exact scale.

Q: What objects can I use for calibration?
A: A standard credit card (85.60 × 53.98 mm, ISO 7810 ID-1) is most reliable. You can also use coins (US quarter, Euro, UK pound, and more) or A4 paper (297mm x 210mm).

Q: Does calibration persist between visits?
A: Yes. Settings are saved in localStorage and remain valid for 30 days on the same device and browser.

## Competitor Comparison Summary

In testing across 3 devices (desktop 92 PPI, MacBook 227 PPI, iPhone 460 PPI), most online rulers showed 40-380% error on non-desktop screens because they assume a fixed 96 PPI. Screen Ruler is the only tool that provides physical object calibration, achieving <1% error on all device types. See the full comparison: https://screenruler.online/blog/best-online-ruler-tools

## Full Content

For complete product documentation, see: https://screenruler.online/llms-full.txt