Guide
Objects of Known Size to Use for Scale in Photos
Updated · RefScale
Quick answer
The best everyday objects for scale have a standardized size, a flat shape and sharp edges. A bank card (85.60 × 53.98 mm) is ideal for small items; a sheet of A4 (210 × 297 mm) or US Letter (8.5 × 11 in) paper, or an iPhone without its case, suits larger ones.
Standard sizes of everyday objects
| Object | Millimeters | Inches | Note |
|---|---|---|---|
| Bank card, driver's license, ID card (ID-1) | 85.6 × 53.98 | 3.37 × 2.13 | ISO/IEC 7810 ID-1 |
| Passport booklet (ID-3) | 125 × 88 | 4.92 × 3.46 | ISO/IEC 7810 ID-3 |
| A4 paper | 210 × 297 | 8.27 × 11.69 | ISO 216 |
| A5 paper | 148 × 210 | 5.83 × 8.27 | ISO 216 |
| US Letter paper | 215.9 × 279.4 | 8.50 × 11.00 | 8.5 × 11 in |
| US dollar bill | 156 × 66.3 | 6.14 × 2.61 | All denominations |
| €20 banknote | 133 × 72 | 5.24 × 2.83 | Euro banknotes differ by value |
| €50 banknote | 140 × 77 | 5.51 × 3.03 | Euro banknotes differ by value |
| US business card | 88.9 × 50.8 | 3.50 × 2.00 | 3.5 × 2 in; common, not a standard |
| US quarter | ⌀ 24.26 | ⌀ 0.955 | Diameter |
| €1 coin | ⌀ 23.25 | ⌀ 0.915 | Diameter |
| €2 coin | ⌀ 25.75 | ⌀ 1.014 | Diameter |
For phones, see the full iPhone dimensions table; for cards, the credit card size guide.
How to choose a good reference
- Known exactly. Standardized objects (cards, paper sizes, banknotes) beat "about this big" objects like pens or fruit.
- Flat and rigid. A reference that bends, curls or has thick rounded edges is hard to mark precisely. Paper should lie completely flat.
- Sharp, visible edges. Put a light reference on a dark surface or the other way round, so its corners stand out.
- Large enough. The longer the reference is in pixels, the smaller the error from placing its edges a pixel or two off. Match the reference to the object: a card for a phone case, a sheet of paper for a box, a door for a wall.
- In the same plane. The reference must be the same distance from the camera as what you measure — lay it right beside the object, on the same surface.
Using your own reference
Anything you can measure once with a ruler can become a reference: a floor tile, a notebook, a sheet of plywood. Measure it carefully, then use those numbers for every later photo. In RefScale you can enter a custom reference size and save it for reuse, alongside the built-in bank card and iPhone presets.
Next: how to measure an object from a photo.
Frequently asked questions
What is the best object to use for scale in a photo?
A bank card is the best everyday choice for small objects: it is standardized to 85.60 × 53.98 mm, flat, rigid and has sharp corners. For larger objects, use something longer, such as a sheet of A4 or US Letter paper or an iPhone without its case.
Can I use a coin for scale?
Yes, but a coin is small and round, so it is harder to mark precisely than a rectangle. Measure across its full diameter, and prefer a card or paper if you have one.
Can I use a ruler or tape measure as a reference?
Yes. A ruler lying next to the object works well: use a long, clearly marked span, such as 0 to 20 cm, as the known length.
Is a banana a good scale reference?
It is a popular joke on the internet, but bananas vary too much in size to give real measurements. Use an object whose dimensions are standardized.