rulerWhat do you have available?
- Phone + card or coinCalibrate for small objects →
- Known-size object or paperCompare physical edges directly →
- No reliable referenceUnderstand what you can estimate →
- Object too long for the screenChoose a tool that fits the length →
| Method | Useful for | Main limit |
|---|---|---|
| Calibrated screen | Small, flat objects | Needs a physical scale check; object must fit |
| Known-size card or paper | Comparing edges without a screen | Judged fractions are approximate |
| Finger or hand | A rough comparison | No universal length |
| Ruler or tape measure | Longer lengths or a closer fit | Requires a physical tool |
Use your phone as a ruler
Open the online ruler and calibrate against a card or supported coin. Those instructions cover choosing the reference, matching its edges and saving. You do not need a camera or a photo of your card.
After checking the scale, hold the object beside the visible ruler with its edges parallel to the ticks. Look straight at each edge and move A and B to those positions. Turn off Snap to ticks if an edge falls between marks. Freeze holds the selected span for reading or copying.
A worked example: a small paper strip
Suppose a strip starts at 10 mm and ends at 53 mm on your calibrated screen. Its length is 53 − 10 = 43 mm, or 4.3 cm. The right endpoint alone is not the length.
The tool’s large readout subtracts A from B for you. See how endpoint measurements work or the one-inch walkthrough. Calibration improves the screen scale, but reference quality and alignment still limit the result; decimal digits do not guarantee physical accuracy.
Compare with a known-size object
A standard ID-1 bank card has a nominal long edge of 85.60 mm, as documented in ICAO Doc 9303, Part 5, section 2.1. Use an undamaged standard card without a sleeve; do not assume every membership card or ID has this size.
A worked example: about one and a quarter cards
Lay the card beside a paper piece and align their starting edges. Suppose the paper extends one full card length plus what looks like a quarter of that length. Calculate 85.60 × 1.25 ≈ 107 mm, or about 10.7 cm. The quarter is judged by eye, so this is an estimate, even though the calculation produces a tidy number.
Known-size paper offers another reference: confirm its dimensions on the pack and use an untrimmed sheet. Folding an edge in half gives a shorter reference, although imperfect folds add error. Do not guess whether an unidentified sheet is A4 or US Letter, and do not treat a printed web image as an actual-size ruler.
What if you have no reference?
A finger width or hand span can describe “roughly this big,” but people’s sizes vary. An iPhone model estimate can improve the screen’s starting scale; it still needs a physical comparison. Without one, treat its readings as unverified.
A US quarter is not exactly an inch: its diameter is 24.26 mm according to the United States Mint, while an inch is 25.4 mm. Select the matching coin calibration instead of substituting an approximate inch.
When the object is too long
Full screen or a larger display may provide enough room; recheck the scale after changing screens or zoom. If the object still cannot fit beside the visible ruler, use a physical ruler or tape measure. Repeatedly moving a card can accumulate gaps and overlaps.
For a precise fit, use suitable physical measuring equipment. This browser ruler measures visible spans on a calibrated screen; it does not measure distance from a camera, photograph or device movement.