MeasureSparkScreen & ruler
Actual Size Online Ruler
Place an object at zero, then tap or drag the marker to read its length. Calibrate with a bank card or A4 short edge when you need better physical accuracy.0.0 mm · 0.00 in
Calibrate rulerImprove physical accuracy
Match a standard bank card
Place a physical card over this outline. Adjust until both long edges line up. Changes save automatically.
How this estimate works
A bank card is 85.60 mm wide. The matched pixel width determines your screen scale.
Measure twice and use the same unit throughout. Product coverage, material density, installation patterns, and local requirements can change the quantity you need.
Quick answer
An on-screen ruler is only physically accurate after calibration. Keep browser zoom at 100%, match the card guide to a real bank card, then place the object at zero and move the marker to its far edge.
Worked example
If the 85.60 mm card guide matches a standard payment card and the marker reaches 42.8 mm, the object is approximately half a card-width long.
Assumptions
- The reference card is the standard 85.60 mm wide ID-1 format.
- Browser zoom remains at 100% after calibration.
- The browser window stays on the calibrated display.
Why is calibration necessary?
CSS pixels are a display convention, not a guaranteed physical size. Screen pixel density, operating-system scaling, browser zoom, and external monitors can all change the apparent ruler.
How does card calibration work?
The tool stores a scale factor after you resize the guide to the width of a standard ID-1 card. That factor converts screen pixels to inches or millimeters for this browser and display setup.
When should a physical ruler be used?
Use a traceable physical measuring tool for fabrication tolerances, medicine, safety checks, legal dimensions, or any decision where a screen mismatch could cause harm or cost.
Can I calibrate with A4 paper instead of a card?
Yes. An A4 short edge is 210 mm. Align that edge to the on-screen guide if you do not have an ID-1 card. Recalibrate after changing zoom, display, or browser window.
Last reviewed September 22, 2026. Source: ISO/IEC 7810 identification card dimensions. Product instructions take priority over a general estimate.
Common questions
Short answers for the measurement details people most often need before using this calculator.
How do I calibrate an online ruler?
Keep browser zoom at 100%, resize the on-screen guide to a real ID-1 bank card (85.60 mm) or the 210 mm short edge of A4 paper, then measure on the same display.
Is an online ruler accurate without calibration?
No. CSS pixels are not a guaranteed physical size. Uncalibrated readings can be off after OS scaling, zoom, or moving the window to another monitor.