Percentage error
Not an uncertainty budget.
Method
Worked case: 98 against 100 is 2%.
How to read a percentage error
This page answers a comparison question: how far is a measured or modelled value from a reference value, as a percentage of that reference. It is a relative difference. It is not a tolerance in a specification and not a pass or fail.
Error is (value − reference) / reference. 98 against 100 is 2% low. The sign matters. A page that reports only the absolute value hides whether the model is over or under. This page keeps the comparison against the reference you typed, so the reference must be the number you intend to judge against.
A small reference makes a large percentage. 2 mm on a 10 mm gauge is 20%. 2 mm on a 10 m span is 0.02%. Quoting the percentage without the reference is how a coordination error gets misread. Units must match before the division. Millimetres against metres is not a percentage error. It is a unit mistake.
Use it when a survey, a schedule and a model disagree and you want the relative gap on one line. Do not use it to accept a structural utilisation or a setting-out tolerance. Those limits live in the project specification, not in this arithmetic.
- Related: unit converter
- common calculations
What to write beside the result
A review can only check this figure if the inputs sit next to it. Write the value, the unit and the assumption the form used. A diameter in millimetres and a diameter in metres are not the same reading. A force in kilonewtons divided as if it were newtons is a thousand-fold error.
The result is the arithmetic on this page. It is not a utilisation, not a selected product, and not a clause from a standard. If the number is going into a shared container, name the page, the date and the inputs. The file-name checker does not read the calculation.
Last checked 3 October 2026. See the common-calculations note and the disclaimer.