How to Read a Vernier Caliper (Without Getting It Wrong)

A Vernier caliper reading has two parts. You read the main scale just before the zero of the sliding scale, then you find the one line on the sliding (Vernier) scale that lines up exactly with a line on the main scale. Main scale plus that line number times the least count is your measurement. That's how to read a Vernier caliper. The rest of this article is about doing it without the slips that catch nearly everyone the first time.

What the two scales are actually doing

The main scale is just a ruler in millimetres. On its own it can tell you that a coin is "somewhere between 24 and 25 mm". The Vernier scale is the clever bit: it squeezes 10 divisions into 9 mm, so each of its divisions is 0.9 mm long, a tenth of a millimetre shorter than a main-scale division.

Because of that tiny difference, only one Vernier line can sit exactly on top of a main-scale line at any moment. If it's the 3rd line, the jaws have opened 0.3 mm past the last whole millimetre. The caliper does the dividing for you.

That difference between one main-scale division and one Vernier division is the least count. Here is the Vernier caliper least count for the three common scales:

Vernier scale	Least count
10 divisions over 9 mm	1 − 0.9 = 0.1 mm
20 divisions over 19 mm	0.05 mm
50 divisions over 49 mm	1 − 0.98 = 0.02 mm

If your exam paper doesn't tell you the least count, work it out like this before you read anything. A 0.02 mm caliper read as if it were 0.1 mm is wrong by a factor of five.

Reading it, step by step

Say you're measuring the diameter of a coin.

1. Close the jaws gently on the coin. Don't squeeze. If you tighten the lock screw while pushing hard, you're measuring a squashed coin and a bent jaw.
2. Find the Vernier zero. Look along the main scale to where the 0 of the sliding scale sits. Suppose it's just past 24 mm. Your main-scale reading is 24 mm, the last mark before the zero, even if the zero is very close to 25.
3. Find the line that matches. Run your eye along the Vernier scale and look for the one line that sits perfectly in line with a main-scale line (any main-scale line; it doesn't have to be 24 or 25). Say it's line 3.
4. Add. 24 + (3 × 0.1) = 24.3 mm.

With a 0.02 mm caliper the Vernier numbers usually run 0 to 10 with four small lines between each. Line "3" on that scale is really the 15th division, so it's worth 15 × 0.02 = 0.30 mm. Count divisions, not printed numbers, until you're used to it.

Zero error: the bit everyone forgets

Checking for Vernier caliper zero error takes five seconds. Close the jaws completely with nothing between them. The Vernier zero should line up exactly with the main-scale zero. On a lot of school calipers it doesn't, because the jaws are worn or a bit of grit is stuck in them.

• If the Vernier zero sits slightly to the right of the main zero, every reading is too big. That's a positive zero error, say +0.2 mm.
• If it sits to the left, every reading is too small: a negative zero error.

The fix is always the same: true value = reading − zero error. With a +0.2 mm error, a reading of 24.3 mm becomes 24.1 mm. With a −0.2 mm error, it becomes 24.5 mm, because subtracting a negative adds.

To read a negative zero error, find the matching line and count it from the 10 end, not the 0 end. If line 8 matches on a 0.1 mm scale, the error is −(10 − 8) × 0.1 = −0.2 mm.

The mistakes that cost marks

Reading the main scale at the wrong mark. Students often take the nearest main-scale mark, which is sometimes the one after the Vernier zero. It's always the one before.

Adding the Vernier line as millimetres. "24 + 3 = 27 mm" happens more often than you'd think. The Vernier line is worth least count × line number.

Picking a line that's "nearly" lined up. The lines either side of the right one look close. Look for the one where the two lines become a single straight line, and check the neighbours look equally off in opposite directions.

Adding the zero error instead of subtracting it. Write the formula down every time: reading − zero error.

Using the wrong jaws. The big lower jaws measure outside sizes, such as a coin or a ball. The small upper jaws measure inside sizes, such as the hole in a washer. The thin rod that slides out of the end measures depths.

Dropping the units or the precision. A 0.1 mm caliper gives answers like 24.3 mm, never 24.30 mm and never 24 mm. Write the value to the precision of the instrument.

Frequently asked questions

What is the least count of a Vernier caliper?

The least count is the smallest length a Vernier caliper can measure. It equals one main-scale division minus one Vernier division. Most school calipers have a least count of 0.1 mm (10 Vernier divisions over 9 mm); more precise ones read to 0.05 mm or 0.02 mm. Always check which one you have before taking a reading.

How do you correct a zero error?

Subtract it from every reading: true value = reading − zero error. A positive zero error (the Vernier zero sits right of the main zero) makes readings too big, so subtracting it brings them down. A negative zero error makes readings too small, and subtracting a negative number adds it back on.

Is a Vernier caliper more precise than a micrometer?

No. A typical Vernier caliper reads to 0.1 mm or 0.02 mm, while a micrometer screw gauge reads to 0.01 mm. Use a caliper for larger objects, inside diameters and depths, and a micrometer for small, thin things like wire or paper thickness. Precise lengths matter in other practicals too, such as when you find g with a simple pendulum.

Practise until it's automatic

Reading a Vernier is a skill, like reading an analogue clock. The first few take a minute each; after twenty you'll do it in seconds. Open the Vernier caliper simulator, close the jaws on a coin or a mystery block, and type your reading. If you're wrong, the checker tells you how you went wrong: wrong main-scale mark, zero error added instead of subtracted, and so on. Once you're confident, turn the helper lines off and switch on zero error. That's exam conditions.

The same page has a micrometer screw gauge, which reads ten times finer. Its idea is similar, with a sleeve scale and a rotating thimble instead of a sliding scale.