Why do two tape measures give different readings?

It is unsettling the first time you notice it: two tapes side by side, both new, reading a millimetre or two apart at three metres. Neither is faulty. Tapes are made to an accuracy class, that class permits a tolerance, and the tolerance is larger than most people assume.

Step by step

1. Find the class marking on the blade.Near the hook or the case there is usually a Roman numeral in a box — I, II or III — along with a CE or UKCA mark and a temperature, normally 20 degrees. That numeral is the accuracy class the tape is certified to.
2. Understand what the class permits.Class I is the tightest, Class II is the common consumer grade and Class III the loosest. The permitted error is not a fixed number: it is a small base amount plus a further allowance for every metre, so the tolerance widens as the tape extends. Two metres apart in class is easily a few millimetres at five metres.
3. Check the two tapes against each other before blaming either.Extend both, lay them alongside each other on a bench with the hooks over the same edge, and read across at one metre, three metres and five. You will see where and how they diverge, and whether it is a hook problem or a blade problem.
4. Check the hook separately.Compare an internal measurement and an external measurement of the same thing. The hook is supposed to slide by its own thickness so both read true. A hook that has been bent, or riveted tight, throws every reading by about a millimetre.
5. Look for blade wear at the beginning.The first 100mm of a well-used tape takes all the abuse — the printing wears off, the blade kinks, and the hook rivets slacken. That is exactly the region used for small measurements.
6. Accept the temperature effect and then ignore it.Steel expands with heat. The class is certified at 20 degrees, and a tape in the sun on a hot roof is genuinely slightly longer than the same tape in a cold garage. Over domestic distances this is smaller than the class tolerance and not worth managing.
7. Use one tape for a whole job.This is the practical answer to all of the above. If everything on a job is measured with the same tape, any error the tape has is common to every measurement and cancels out where it matters — in the relationship between parts.
8. Mark your tape so you can tell it apart.Tape on the case, a paint mark, initials. On a job with more than one person, tapes get swapped without anyone noticing, which reintroduces the problem you just solved.

Tips

• A tape that has been run over, dropped repeatedly or had its blade kinked should be replaced rather than trusted. The hook rivets are the usual casualty and the damage is invisible.
• If you own several tapes, nominate one as the reference, keep it in good condition, and check the working tapes against it occasionally.
• Standing the blade out unsupported over three or four metres is a genuinely useful specification, because a sagging blade reads long and a stiff blade lets you measure alone.
• Never use two different tapes for the two halves of the same measurement. That is where the class difference turns into a visible gap.

Common mistakes

• Bending the sliding hook flat because it wobbles — The movement is deliberate and equals the hook's thickness. Removing it makes external measurements short and internal ones long, permanently.
• Comparing two tapes hook to hook rather than over an edge — Hooking one tape onto another loads both hooks in a direction they were not designed for, and the comparison measures the hooks rather than the blades.
• Assuming an expensive tape is a more accurate tape — Price mostly buys blade stand-out, coating durability and case robustness. Check the class marking, which is the only statement about accuracy on the tool.

Questions people ask