Can a Stopwatch Really Measure Hundredths of a Second?
The digits on a stopwatch describe its display resolution, not automatically its accuracy. Here is how to separate elapsed time, resolution, and measurement uncertainty.
A stopwatch may display hundredths of a second, but those digits do not by themselves prove hundredths-of-a-second accuracy. NIST defines a time interval as the elapsed time between two events. A stopwatch therefore reports an interval created by the chosen start and stop events; the readout is only one part of the measurement.
Resolution is not accuracy
NIST’s stopwatch calibration guide distinguishes what an instrument can display from how close its result is to a reference. A digital display with two decimal places has 0.01-second resolution. That describes the displayed increment, not an accuracy guarantee of 0.01 seconds.
NIST gives a useful example in its calibration guide: a device specified at ±5 seconds per day would imply about 1.7 milliseconds over a 30-second measurement if the daily rate were scaled down. Yet its display resolution would still be 0.01 seconds. The example is not a specification for every stopwatch. It shows why a rate specification, a display, and a short interval cannot be treated as interchangeable.
A more detailed display can make a result easier to read without establishing equivalent accuracy. For a short event, the start and stop action, the display resolution, and the device’s time base all belong to the measurement process. That is why the number of visible digits is a poor substitute for knowing the measurement uncertainty.
What should be checked before trusting a reading?
Three questions make a stopwatch result easier to interpret:
- Which two events define the interval? The start and stop boundaries determine what was actually measured.
- What does the device specification cover? Display resolution tells you what can be shown. An accuracy specification describes a different property and must be read with its stated interval and conditions.
- Is the result informal or calibrated? For a formal check, NIST’s guide discusses comparison with a traceable reference and accounting for measurement uncertainty. Direct comparison can include effects from operator reaction and display resolution, so the comparison method matters as much as the displayed number.
This changes the practical question from “How many digits does the stopwatch show?” to “Which part of this measurement limits the conclusion?”
A stopwatch and a clock answer different questions
A stopwatch measures elapsed time between events. A clock answers what time it is. NIST’s What Time is It? FAQ describes time.gov as a time-of-day service and says it should not be used to measure time intervals or to establish formal traceability to NIST.
That distinction prevents a common category error: a timestamp can tell you when something happened, while a stopwatch is intended to describe the duration between two events. They are related measurements, but they are not interchangeable tools.
What a lap button changes
The interface can also change what gets recorded. For example, Apple’s U.S. iPhone guide instructs users to open Clock, choose Stopwatch, tap Start, tap Lap to record a lap or split, tap Stop, and tap Reset to clear the stopwatch. Apple also says the stopwatch continues running when another app is opened or the iPhone goes to sleep.
A lap is therefore a checkpoint during a running measurement, not proof that the underlying timing is more accurate. It adds structure to the interval being recorded; it does not change the difference between display resolution and accuracy.
The useful way to read a stopwatch
Treat the displayed digits as a description of the stopwatch’s resolution, not as an automatic uncertainty statement. Keep the start and stop events clear, read any accuracy specification with its stated interval, and use a calibration method when the result must be compared with a reference.
The central lesson is simple: a stopwatch can tell you an elapsed interval, but the confidence you can place in that interval depends on more than the last digit on the screen.