Metric Ruler
FIELD GUIDE / METRIC RULER
On this metric ruler, each small space is one millimetre. Ten small spaces make one centimetre.
Know the two units
A centimetre (cm) is one hundredth of a metre. A millimetre (mm) is one thousandth of a metre. This means 1 cm = 10 mm, and 1 mm = 0.1 cm. These are exact relationships, not approximations.
The ruler in the workshop labels the whole centimetres with numbers. Between one numbered mark and the next, there are ten equal millimetre spaces. The halfway line is 5 mm past the whole centimetre.
Read from the last whole centimetre
- Find the whole centimetres. A pointer between 2 and 3 starts with 2 cm.
- Count the extra millimetres. Four small spaces after 2 cm adds 4 mm.
- Use the requested unit. 2 cm and 4 mm is 24 mm, or 2.4 cm.
In millimetres, multiply the whole centimetres by 10 and add the remaining small spaces. In centimetres, the extra millimetres form the first decimal digit.
| What you see | In mm | In cm |
|---|---|---|
| 7 small spaces from zero | 7 mm | 0.7 cm |
| 1 cm plus 3 mm | 13 mm | 1.3 cm |
| Exactly the 3 cm mark | 30 mm | 3 cm |
| 5 cm plus 8 mm | 58 mm | 5.8 cm |
Avoid the ten-times mistake
The same position can have two different numbers, depending on the unit: 25 mm = 2.5 cm. Entering 25 when the tool asks for centimetres is ten times too large. Check the unit stamp beside the question and the label beside the answer box.
The diagram stays the same when you change the answer unit. Only the number you need to enter changes. This lets you practise connecting both ways of expressing the measurement.
Try a quick mental check
If the pointer sits between 4 cm and 5 cm, a millimetre answer must be between 40 mm and 50 mm. An answer of 4 mm cannot fit there. A simple range check catches many mistakes before you press Check reading.
On a real ruler
Look for the printed unit and inspect the interval between labelled marks. Some real rulers label millimetres directly or use other subdivision patterns. This workshop uses whole-centimetre labels and one-millimetre spaces; do not assume every real tool follows that exact layout.
Reference
The metric relationships used here follow the SI length prefixes. See NIST: SI Units — Length for metre-based length units and the exact relationship between inches and millimetres.