How this calculator works
Absolute uncertainty is non-negative and uses the measurement unit. Relative uncertainty divides it by the magnitude of the measured value, so a negative reading still has positive relative uncertainty. The interval shown is the reading plus or minus the supplied uncertainty. Its coverage depends on whether the input is a standard uncertainty, an expanded uncertainty or a stated bound.
Formula
u = |x| × ur; ur = u/|x|; |x| = u/ur. Interval: [x - u, x + u].
Worked example
A reading of 50 with 2% relative uncertainty has absolute uncertainty 1 and interval 49 to 51. A reading of -20 with absolute uncertainty 0.5 has relative uncertainty 2.5%.
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Frequently asked questions
Does plus or minus mean 95% confidence?
Only when the stated uncertainty and coverage factor justify that interpretation. A standard uncertainty alone is not a 95% interval.
What happens at zero?
Relative uncertainty is undefined for a zero reading. An absolute uncertainty can still be reported.
Can I recover a signed measurement?
The reverse calculation recovers its magnitude. Uncertainty alone does not identify its sign.