Accurate measurements are the difference between a frame that fits and one that doesn't. This guide shows how to read a tape measure in millimetres and inches, how its sliding hook works, and the 10mm method for the most accurate measurements. Then try the interactive tape measure, and test yourself.
How to use a tape measure
Use a steel tape
Cloth and plastic tapes are cheap and flexible, but they aren't reliable for measuring a frame:
- they stretch and lose their shape over time;
- they sag over longer lengths;
- they change with heat and damp;
- their markings are often less precise.
A good steel tape measure stays straight and true, so your measurements are accurate.
Reading the markings
Millimetres
The top edge of most tapes is marked in millimetres, numbered every centimetre:
- every 1mm: the shortest marks;
- every 5mm: slightly longer marks;
- every 10mm (1cm): the longest marks, with a number. On many tapes every 10cm is in red.
To read it, take the last number before the reading and count on in millimetres. The 4 is 40mm; 7 small marks on from it is 47mm. We ask for sizes in millimetres, as they're the most exact.
Inches
The bottom edge is usually in inches, each divided down to 1/16 inch (or 1/8 on some tapes):
- 1/16 inch: the shortest marks;
- 1/8 inch: a little longer;
- 1/4 and 1/2 inch: longer again;
- whole inches: the longest marks, with a number.
One inch and three eighths is 1 3/8 inches. If you measure in inches, the interactive tape below will turn it into millimetres for you.
Interactive tape measure
Enter a measurement and choose Show on tape to see where it falls, in millimetres and in inches. Drag the tape (or click it and use the arrow keys) to move it along. Then choose Test yourself: the tape stops at a random reading, and you type in what it says.
In inches, type whole numbers and fractions, such as 12 or 4 3/8.
How the hook works
The metal hook at the end of a steel tape is loose on purpose: it slides by exactly its own thickness, so the tape reads correctly both ways.
- Outside measurements: hook it over the edge of the item and pull the tape tight. The hook slides out, and the 0 is right at the edge.
- Inside measurements: push the hook firmly against one side, such as the inside of a recess, and extend the tape to the other. The hook slides in, and the 0 is right at the surface.
A bent or worn hook throws every measurement out, so look after it, and keep it clean.
The 10mm method: the most accurate of all. Line up the 10mm mark with the edge instead of the hook, read the other edge, and take 10mm off. Here the tape reads 415mm, so the size is 405mm. The hook isn't used, so its movement can't affect the measurement.
Good habits
- Keep the tape straight and taut. A sagging or angled tape reads long.
- Measure twice, order once. Our frames are made to your measurements, so they can't be refunded or exchanged.
- Use the same tape for the whole job. Two tapes can differ by a millimetre or so.
- Keep your tape clean and dry. Dirt and rust hide the markings and stop the hook moving.
Now you're ready to measure: see our guides to measuring a stretched canvas and a canvas board, or all our measuring advice.