Calculating with measurements
Measure, compare, add, and subtract lengths (m/cm/mm), mass (kg/g), and volume/capacity (l/ml) using standard units
What a learner can do afterwards
- Measure a length in centimetres and millimetres
- Weigh an object using grams and kilograms
- Check attributes and units before combining measurements; add or subtract lengths, masses and liquid volumes after changing them to matching units
1 · Read
Line up one end with zero on the ruler and read the other end. Check cm or mm. If a label is missing, use the equal steps to find it. Each cm is 10 mm: 4 cm is 40 mm. For 4 cm 2 mm, change the centimetres first: 40 mm + 2 mm = 42 mm.
Match units and measurement kinds. Add hundreds, tens and ones: 900 g + 600 g = 1500 g (9 + 6 hundreds). Subtract removed amounts in parts: 900 ml - 200 ml = 700 ml. Find missing amounts by counting up to the target and adding jumps. Check a proposed amount: add it to the start and compare with the target. One litre is 1000 ml.
One kilogram (kg) is 1000 grams (g). Use 1000 g per kg; group each 1000 g into 1 kg: 1400 g is 1 kg 400 g. One litre is 1000 ml; change litres to ml before calculating. For 1000 ml - 625 ml, count up: 625 to 630 is 5, to 700 is 70, to 1000 is 300. Add the jumps: 375 ml more.
When units differ, check what they measure. You can change centimetres to millimetres because both measure length. For 5 cm + 20 mm, change 5 cm to 50 mm, then add: 50 mm + 20 mm = 70 mm. You cannot add grams to millilitres to find a mass or volume. They measure different things.
Read the number and unit. Add or subtract measurements of the same kind in matching units. Change units first when needed.
Readings, questions and flashcards written for this topic. Referenced videos are by Khan Academy.
2 · Watch
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Where it sits
Where this leads
17 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.