Measurement Uncertainty Calculator
This measurement uncertainty calculator converts an absolute uncertainty into a relative, percentage uncertainty and works out the full range a measurement could realistically fall within, so you can see how precise a reading actually is once its margin of error is taken into account. You enter the measured value and its absolute uncertainty, the plus or minus figure attached to the reading, and the calculator instantly returns three results: the value shown with its uncertainty, the relative uncertainty as a percentage, and the value range, the lowest to the highest figure the true measurement could plausibly sit within. Relative uncertainty matters because a small absolute error can be significant on a tiny measurement and trivial on a large one, so expressing it as a percentage of the value is the fair way to compare the precision of different instruments, readings or experiments. This is standard practice in physics and chemistry lab work, engineering tolerances and any setting where you need to report how much confidence to place in a number. Simply type in your value and its uncertainty and the results update as you go, with a worked example, the formula and the assumptions used explained further down the page. Use it to check lab results, prepare a report, or understand how tight or loose a measurement really is before drawing conclusions from it.
The formula
Relative uncertainty is the absolute uncertainty divided by the value, as a percentage: (uncertainty / value) times 100. The range is the value minus the uncertainty up to the value plus the uncertainty.
Worked example
A length of 10 with an uncertainty of 0.2 has a relative uncertainty of 0.2 / 10 = 2 percent, and a range of 9.8 to 10.2. Enter 10 and 0.2 to confirm.
Frequently asked questions
What is relative uncertainty?
The absolute uncertainty divided by the measured value, usually as a percentage. It shows precision relative to size.
What is the difference from absolute uncertainty?
Absolute uncertainty is the plus or minus in the same units. Relative uncertainty expresses that as a fraction of the value.
How do I report a measurement?
As value plus or minus uncertainty, for example 10.0 plus or minus 0.2, ideally with the relative uncertainty too.
Who this calculator is for
This calculator is for science students and anyone reporting the precision of a measurement.
What this calculator assumes
- You enter a value and its absolute uncertainty.
- Relative uncertainty is uncertainty over value.
- Results are rounded for display.
Formula and sources
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