A measurement has two parts: a number read from the scale and a unit that says what the number means. Both must be right for the mark.
This lesson is part of laboratory safety and investigation. It supports identifying variables and fair comparisons, because the responding variable is always a measurement.
How do I read a scale correctly?
Follow the same routine every time. It takes ten seconds and prevents careless slips.
- Find what one small division is worth. Count the gaps between two labelled numbers.
- Put your eye level with the mark you are reading.
- Read to the nearest small division, or half of one if the question allows it.
- Write the number with the unit, such as 23.5 cm or 14 cm³.
Step one matters most. A thermometer labelled every 10 °C with 10 gaps between labels has divisions of 1 °C, not 10.
Worked example: three instruments
A student records these readings from a practical.
| Instrument | Scale detail | Reading seen |
|---|---|---|
| Ruler | Marked every 1 mm | Pencil ends 6.3 cm from zero |
| Measuring cylinder | Marked every 2 cm³ | Bottom of the meniscus at 38 cm³ |
| Thermometer | Marked every 1 °C | Liquid level at 27 °C |
Ruler. The smallest division is 1 mm, which is 0.1 cm. So the reading is written to one decimal place: 6.3 cm.
Cylinder. The divisions are 2 cm³ apart, so readings are even numbers, or odd numbers if you read half a division. 38 cm³ fits the scale.
Thermometer. With 1 °C divisions, 27 °C is correct. Writing 27.0 °C suggests a precision the instrument does not have.
The mistake that costs marks
The slip is recording a value that the instrument cannot give, or leaving off the unit. Both lose marks even when the reading is nearly correct.
| Step | Wrong | Right |
|---|---|---|
| Ruler reading | 6.34 | 6.3 cm |
| Volume | 38 | 38 cm³ |
| Temperature | 27.00 °C | 27 °C |
| Time from a stopwatch in seconds | 12.5 | 12.5 s |
If the question gives a table, put the unit in the column heading, for example “Volume (cm³)”. Then you need not repeat it on every row.
Converting units before reporting
Some answers need a different unit from the reading. Change the unit first, then report.
A rod measures 6.3 cm. To convert to millimetres, multiply by 10: 6.3 × 10 = 63 mm. To convert 2 500 g to kilograms, divide by 1 000: 2 500 ÷ 1 000 = 2.5 kg.
Use the units and significant-figure checker to test your own conversions.
Check yourself
A measuring cylinder has a scale marked every 5 cm³. The bottom of the water curve sits exactly halfway between the 40 and 45 marks. Write the volume with the correct unit, and say why writing 42 cm³ would be wrong.
Answer
The halfway value between 40 and 45 is 42.5, so the reading is 42.5 cm³.
Writing 42 cm³ ignores the half division you can see. The recorded value should match what the scale lets you read, so 42.5 cm³ is the better report.
What to study next
Readings become evidence when you compare them. Continue with evaluating a science claim from a supplied dataset, or test the whole cluster in the practice set.
To go through your own recording mistakes with a teacher, see online one-to-one Science tuition.