Which digit is uncertain on a digital display?

The last digit shown on a digital display is usually the uncertain one unless the device says otherwise.

Think of a digital display like a ruler with only numbers; every digit except the last is steady and reliable, and the final digit is the one that can wiggle a little because the instrument cannot resolve smaller parts. For example, if a balance shows three point two five grams, the three point two five is read and the five is the uncertain digit because the balance cannot tell you whether it is three point two five zero or three point two five nine; it only reports to that place. The manufacturer may give a stated uncertainty or a plus or minus value which tells you how much that final digit might vary, and for precise work you use that specification. When you record the measurement write all the displayed digits and, if you know the instrument uncertainty, include it or round appropriately.

Students often think every displayed digit is perfectly exact, which leads to reporting false precision; a simple fix is to always treat the last displayed digit as uncertain and avoid adding extra zeros. Another common mistake is ignoring the device manual where the manufacturer sometimes states a different uncertainty or resolution; check the specifications and use that stated uncertainty when available. Some learners try to guess more precision by averaging many readings without considering systematic errors; averaging helps random noise but does not fix a biased instrument, so also check calibration and instrument limits.

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Digital Display: Which Digit Is Uncertain?

1. Treat final digit as uncertain
2. Read and record all displayed digits
3. Consult manufacturer for stated uncertainty
4. Use specification for precise work
5. Include uncertainty or round appropriately

Common Measurement Mistakes with Digital Displays

1. Treating every digit as exact
2. Ignoring manufacturer specifications
3. Averaging without checking calibration

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