Converter (cm / mm / inch)
Type in any box and the other two update instantly. 1 in = 2.54 cm and 1 cm = 10 mm.
Open the full converter →Actual Size Online Ruler
Calibrate with a real bank card, battery or A4 paper, then measure mm, cm and inches on any screen.
Common settings
Unit
Orientation
Pick a reference object
Not sure what to use? See everyday objects you can measure with →
Add a custom object: enter its name and real length. After adding, it appears above for you to select and calibrate with.
How to use
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1
Pick an object
Choose an item of known size, like a bank card, AA battery, or A4 paper.
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2
Align to 0
Line up the object's left edge with the ruler's 0 mark.
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3
Drag the handle
Drag the green handle so it matches the object's right edge.
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4
Done
Click "Calibrate". Your on-screen ruler now shows at true size.
Tip: Keep your browser zoom at 100%.
Guides & tools
Why this ruler asks you to calibrate
A normal on-screen ruler assumes a default pixel size, which is rarely your real one — so its “1 cm” is often not a true centimeter. This ruler uses a real object (a bank card is 85.6 mm wide) as a reference, works out your screen’s actual pixel size, and then draws the ruler at true scale.
How calibration works →How to measure accurately
On-screen rulers depend on your display's pixel density and your operating-system zoom. After calibrating with a real object, the ruler shows true size — then use Mark and Measure to capture lengths, and Lock to keep the scale while you scroll.
Read the full guide →Mark & measure
After calibrating, use Marker to drop points on the ruler to remember positions (e.g. an object's start and end); use Measure to click two points and draw a line that shows the true length. Marker and Measure are mutually exclusive—only one is active at a time.
How to use →Frequently asked questions
Is the online ruler accurate?
After calibrating with a real object, it shows the true size for the current screen and zoom level. Before calibrating it is only approximate, because different screens have different pixel densities.
Why do different screens give different results?
Each display renders pixels at a different physical size, and OS scaling (for example 125% or 150% on Windows) changes the effective size — so calibration is specific to a device and zoom level.
Do I need to know my screen's PPI or DPI?
No. The ruler calibrates by lining up a known object, which avoids the common mistake of entering the wrong DPI value.
Reference objects and their exact sizes
Calibration works with any object whose size is fixed by a standard. These are the most reliable ones, and the values are exact:
| Object | Exact size | In inches |
|---|---|---|
| Bank / credit card | 85.60 × 53.98 mm | 3.37 × 2.13 in |
| Passport (ID-3) | 125 × 88 mm | 4.92 × 3.46 in |
| AA battery | 50.5 mm long, 14.5 mm across | 1.99 in |
| AAA battery | 44.5 mm long | 1.75 in |
| A4 paper | 210 × 297 mm | 8.27 × 11.69 in |
| US Letter paper | 215.9 × 279.4 mm | 8.5 × 11 in |
| CD / DVD | 120 mm diameter | 4.72 in |
| US dollar bill | 155.96 × 66.29 mm | 6.14 × 2.61 in |
Use the long edge where possible — the longer the span, the easier it is to spot a scale error. Want to measure with these instead of a ruler? See how to measure without a ruler.
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