Online ruler
Measure real-world dimensions directly on your screen in centimeters, millimeters, inches, and pixels.
Calibrate your screen for accurate real-size measurements
Precise millimeter ticks with 10mm numbering
Drafting fractions down to 1/16th inch
Calibrate Display
Hold a standard debit card or driver's license (85.60 mm wide) flat against the screen outline. Adjust the calibration slider until the on-screen card matches your physical card.
Align Object to Origin
Place the item you want to measure (e.g. coin, screw, bolt, key, or photo) flat against your display. Align its left edge to the zero marking on the ruler.
Read Measurement
Drag the caliper handle to the end edge of your object or click on the ruler marking. Read the distance simultaneously in millimeters, centimeters, inches, and fractions.
The Physics and Mathematics of Screen Measurement
Why CSS Pixels Do Not Equal Physical Inches
Under the W3C CSS specifications, 1 CSS inch is formally defined as 96 CSS pixels (1 CSS pixel = 1/96th of a CSS inch). However, this definition was created in the 1990s for low-density CRT monitors.
Modern physical screens range from ~92 PPI on a 24-inch 1080p monitor to over 460 PPI on an iPhone OLED display. Because operating systems apply display scaling (125%, 150%, 200% Retina scaling), a web browser cannot reliably assume physical dimensions without direct calibration.
The ISO/IEC 7810 ID-1 Reference Standard
To eliminate guesswork, ANRuler uses the globally standardized credit card specification (ISO/IEC 7810 ID-1). Every standard payment card, modern driver's license, European national identity card, and transit card in circulation measures exactly:
By aligning this physical constant to your screen, the software computes your panel's true physical millimeter-to-pixel ratio.
Display & Measurement Utilities
Complementary tools to calculate panel density and convert between physical dimensions.
Display PPI & DPI Calculator
Calculates raw physical panel pixel density and translates it to browser CSS coordinate space.
Monitors have physical hardware pixels, but web browsers execute layout in CSS pixels. On high-density screens (Retina, 4K, smartphones), your operating system scales the display by a factor called devicePixelRatio (current browser DPR: —).
Precision Length Converter
Instant conversion across metric, imperial, and physical screen pixels (at 96.0 PPI).
Centimeters vs. Millimeters vs. Inches: Choosing the Right Scale
Centimeters (CM)
Standard everyday metric unit. 1 cm equals 10 mm or approximately 0.3937 inches. Best for general product measurements, schoolwork, and medium-sized household objects.
Millimeters (MM)
High-precision engineering and drafting unit. 1 mm equals 0.1 cm. Ideal for hardware, jewelry, small screw threads, bolts, electronic components, and 3D printing tolerances.
Inches (IN)
Imperial standard unit. 1 inch is defined as exactly 25.4 millimeters. ANRuler displays both fractional divisions (1/2, 1/4, 1/8, 1/16) and decimal values for machining and carpentry.
Frequently Asked Questions About Screen Measurement
Technical explanations on screen calibration, pixel density, and real-world accuracy.
What is an online ruler?▼
An online ruler is an interactive digital measuring tool displayed directly in your web browser. When calibrated to your device's physical screen pixel density (PPI), it renders real-world 1:1 scale markings in centimeters, millimeters, inches, and pixels, allowing you to measure physical objects directly against your screen.
How accurate is an online ruler?▼
Unlike simple graphics that guess screen size, ANRuler relies on physical screen calibration. Once calibrated using a standard credit card (85.60 mm wide) or a known physical object, accuracy matches real-world drafting scales to within sub-millimeter tolerances on unscaled displays. Uncalibrated digital rulers, by comparison, often differ by 15% to 40% due to varying hardware pixel densities.
Why does an online ruler need calibration?▼
Web browsers and operating systems do not know the physical diagonal measurements of your monitor or smartphone screen. A 1080p resolution on a 5.8-inch smartphone screen packs over 400 pixels per inch, while the same resolution on a 27-inch desktop monitor has only ~81 pixels per inch. Calibration establishes the exact ratio of CSS pixels to physical millimeters for your individual screen.
How do I calibrate my screen with a credit or debit card?▼
All standard credit cards, bank cards, driver's licenses, and transit cards conform to the international ISO/IEC 7810 ID-1 standard: exactly 85.60 mm wide by 53.98 mm high. Click 'Calibrate', place your physical card flat on the on-screen outline, and adjust the slider until the outline matches your card's exact dimensions. Once saved, your scale is permanently stored locally in your browser.
Does browser zoom affect ruler accuracy?▼
Yes. If your browser zoom is set to 110%, 125%, or 90%, CSS pixels are scaled relative to your display, which changes physical dimensions. For best results, ensure your browser zoom is at 100% (press Ctrl+0 on Windows/Linux or Cmd+0 on macOS) and calibrate your screen.
Can I measure centimeters and millimeters on my phone?▼
Yes. ANRuler is responsive and touch-enabled for iPhone and Android smartphones. You can switch to CM or MM mode, align an object (such as a key, screw, coin, or piece of jewelry) along your phone screen edge, and use the draggable calipers to read the measurement.
Can I measure inches with fractional divisions (1/8″, 1/16″)?▼
Yes. In Inch mode, ANRuler provides true drafting subdivisions down to 1/16th of an inch, as well as decimal inch readouts (e.g. 3.375″ = 3 3/8″), making it easy to measure both imperial hardware and drafting plans.
Is my calibration saved when I return to the site?▼
Yes. ANRuler stores your calibrated pixel density locally in your browser's localStorage. You will not need to recalibrate on future visits unless you change monitors, switch browser zoom, or clear browser site data.