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How many significant figures should one give in stating a experimental uncertainty? For this purpose I found three rules:
Experimental uncertainties should be always stated to 1 significant figure.
For example: $3.45 \pm 0.015$ should be $3.45 \pm 0.02$ [doc1].
The number of significant figures in the experimental uncertainty is limited to one or (if the uncertainty starts with a one, e.g., ± 0.15) to two significant figures.
For example: $3.45 \pm 0.015$ should be $3.45 \pm 0.015$ [doc2].
One significant figure should be used to report the uncertainty or occasionally two, especially if the second figure is a five.
For example: $3.45 \pm 0.035$ should be $3.45 \pm 0.035$ [doc3].