Resolution
Least count, and why it decides your answer
The least count is the smallest difference the instrument can resolve. It is not a property of the part you are measuring — it is a property of the scale in your hand.
It comes from a deliberate mismatch. On a 50-division vernier, the fifty divisions are laid out to span forty-nine millimetres, so each one measures 0.98 mm — exactly 0.02 mm short of a main scale division. That shortfall accumulates along the scale, which is why only one vernier line can be level with a main scale line at a time. Which line it is tells you the fraction.
| Vernier type | Main division | Divisions | Least count |
|---|---|---|---|
| Standard metric | 1 mm | 50 | 1 ÷ 50 = 0.02 mm |
| Medium metric | 1 mm | 20 | 1 ÷ 20 = 0.05 mm |
| Coarse metric | 1 mm | 10 | 1 ÷ 10 = 0.1 mm |
| Imperial | 0.025 in | 25 | 0.025 ÷ 25 = 0.001 in |
Try it in the simulator
Load the ball bearing and switch the least count between settings. The ball is 15.88 mm. A 0.02 mm caliper reports 15.88; a 0.05 mm caliper can only land on 15.90. The part never changed — the instrument's resolution did. That gap between the true size and the reported size is the first thing to internalise about measurement.
Limits
When a caliper is the wrong instrument
A 0.02 mm vernier is a general-purpose workshop instrument, not a metrology lab tool. Four situations call for something else.
Tolerance under 0.02 mm
Bearing fits, ground shafts and reamed holes are specified far tighter than a caliper can resolve. Use a micrometer at 0.01 mm, or a digital micrometer below that.
Features narrower than the jaw
Caliper jaw tips are a few millimetres wide, so a narrow groove or a fine slot simply cannot be entered. Use feeler gauges or a depth micrometer.
Hot, soft or compliant parts
Steel grows roughly 0.012 mm per 100 mm per 10 °C. A part measured straight off the lathe reads large. Let it reach room temperature first.
Dimensions beyond 300 mm
Beam flex starts to dominate on long calipers. A height gauge or a beam caliper gives a more trustworthy result.
Try it
Everything on this page is easier to believe with the instrument in front of you.
The simulator has draggable jaws, three least counts, imperial mode, zero-error simulation and a practice mode that marks your readings.
Open the caliper simulatorKeep reading
How to read a vernier caliper
The five steps, the TR = MSR + (VSR × LC) formula worked through, and the six mistakes that spoil a reading.
Zero error
The fault that adds itself to every reading you take, why it is invisible in the results, and how to correct for it.
Parts of a vernier caliper
All twelve parts named and explained, plus the ISO, ASME, BS and DIN standards that govern them.
Frequently asked questions
Short answers to the questions that come up most often about reading and using a caliper.