Unit conversions — for parents
By the end of Year 4 your child has met four families of units: length, mass, volume and time. Conversions look small at first ("1 m = 100 cm — what is hard about that?") but they trip children up because they pull together three skills at once: knowing the equality, choosing direction (× or ÷), and handling round numbers like 1 000.
What your child should master
- The four core ladders:
- Length: mm — cm — m — km
- Mass: g — dag — kg — t
- Volume: ml — cl — dl — l
- Time: s — min — h — day (with 60 and 24, not 10)
- Convert with whole numbers in both directions: 3 m → 300 cm; 5 000 g → 5 kg.
- Compare two measurements in different units (e.g. 1.2 km vs 950 m) by converting both to the same unit first.
- Read the 24-hour clock and work out elapsed time across the hour boundary.
Common mistakes
Multiplying when they should divide
A child sees "300 cm = ? m" and writes 30 000 instead of 3. The pattern they have learned ("just add zeros") was for the other direction.
Help: ask "which one is the bigger unit — cm or m?" Then "if I switch to the bigger unit, do I get more or fewer of them?" The answer is "fewer", so the number must shrink — divide.Using ten everywhere
Time does not scale in tens. Children sometimes write 1 h = 100 min by analogy with length and mass.
Help: post a small reminder on the kitchen wall: "Time uses 60 and 24, not 10."Skipping a step on the ladder
g → kg is ÷ 1 000, not ÷ 100, because there is one extra rung (dag) hiding between them. Same for ml → l (cl and dl in between).
Help: draw the full ladder once, then have your child point to each rung when converting.Mixed units within one problem
"1 m 25 cm − 40 cm = ?" — children often subtract 25 − 40 and get stuck.
Help: convert everything to a single unit first, then compute.Activities at home
Kitchen-scale game
Pull random items from the cupboard. Have your child estimate the mass in grams, then weigh them on the kitchen scale. Anything over 1 000 g — ask them to read it as both 1 250 g and 1.25 kg.
Tape-measure scavenger hunt
Give your child a tape measure and a list: "find something about 30 cm, something 1 m long, something less than 5 mm." After each find, ask "and how many mm is that?"
Cooking together
Recipes are full of conversions: "375 ml of milk" → measure with a jug marked in ml. "200 g of flour" → weigh on the scale. Talk about why the recipe uses ml here and g there.
TV-guide stopwatch
Pick a programme in the TV guide that starts at, say, 18:45 and ends at 20:10. Ask: "how long is it?" If the child finds it hard, count up step by step: 18:45 → 19:00, 19:00 → 20:00, 20:00 → 20:10.
Race the clock
Set a kitchen timer for 90 seconds. Ask: "how many minutes and seconds is that?" Then 150 s, then 600 s.
Why this matters
Unit conversions appear in every later year of maths — fractions, decimals, ratio, speed and area all use them. They are also the most directly useful piece of school maths in adult life: cooking, DIY, sports timing, travel distances. Time invested in fluency at Year 4 pays off.