Year 4 maths · Key Stage 2 · Measurement
Year 4 area, perimeter, converting units and 24-hour time
Year 4 measurement pulls together four practical skills. Children convert between units such as kilometres and metres or hours and minutes, find the perimeter and area of shapes made of straight sides and right angles, calculate with money in pounds and pence, and read and write time on analogue and digital clocks, including the 24-hour clock.
Area, perimeter, units and time, explained
Converting units rests on place value. The metric system is built on tens, hundreds and thousands: there are 10 millimetres in a centimetre, 100 centimetres in a metre and 1,000 metres in a kilometre, 1,000 grams in a kilogram and 1,000 millilitres in a litre. Going from a larger unit to a smaller one means multiplying, because you need more of the smaller unit: 3 km is 3 × 1,000 = 3,000 m. Going the other way means dividing. Children also convert units of time, which don’t follow the pattern of tens: 60 minutes in an hour, 60 seconds in a minute, 12 months in a year and 7 days in a week.
Perimeter is the distance all the way round the outside of a shape. Children measure and calculate the perimeter of rectilinear shapes, which are shapes made only of straight sides and right angles, such as squares, rectangles and L-shapes, in centimetres and metres. For a rectangle you may see it written as 2(a + b), meaning two lengths and two widths, but with an L-shape it is safest to add every side and tick each one off as you go. Area is the amount of flat space inside a shape. At this stage children find it by counting squares on a grid, and they link rows of squares to multiplication: a rectangle 6 squares wide and 4 rows tall has 6 × 4 = 24 squares.
Money is measured with the same decimal ideas. Children estimate, compare and calculate amounts in pounds and pence, write £2.35 as 235p and back again, and solve problems involving change. Because there are 100 pence in a pound, they are really working with hundredths. The same habits apply to other measures: children estimate before they measure, for example judging whether a door is about 2 m tall or a bag of flour about 1 kg, then compare and calculate lengths, masses and capacities, such as the total of 1 kg 300 g and 850 g.
Time is read on both analogue and digital clocks, using 12-hour and 24-hour notation. On the 12-hour clock, morning times are written with am and afternoon and evening times with pm. On the 24-hour clock the hours keep counting after noon, so 3:25 pm is 15:25 and 11:50 pm is 23:50. Midnight is written 00:00 and noon is 12:00. Children practise translating between the two, and they solve problems such as how many minutes are in 3 hours, how many days are in 6 weeks, or how many months are in 4 years.
Words your child will hear
- Perimeter
- The total distance around the outside edge of a shape.
- Area
- The amount of flat space inside a shape, counted in squares.
- Rectilinear
- Made of straight sides that all meet at right angles, like an L-shape.
- Convert
- Change a measurement into a different unit without changing its size.
- Square centimetre
- A square with sides 1 cm long, used for measuring area.
- 24-hour clock
- A way of writing time with hours 00 to 23, so 6 pm is 18:00.
- Analogue and digital
- Analogue clocks have hands and a face; digital clocks show numbers.
What the National Curriculum says
In Year 4, pupils should be taught to:
- Convert between different units of measure [for example, kilometre to metre; hour to minute]
- Measure and calculate the perimeter of a rectilinear figure (including squares) in centimetres and metres
- Find the area of rectilinear shapes by counting squares
- Estimate, compare and calculate different measures, including money in pounds and pence
- Read, write and convert time between analogue and digital 12- and 24-hour clocks
- Solve problems involving converting from hours to minutes, minutes to seconds, years to months, weeks to days
Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.
A worked example
An L-shaped patio is made from two rectangles. Across the top it is 6 m long and 2 m deep. Below the top bar, on the left, a 3 m wide leg sticks down a further 3 m. Find its perimeter and its area.
- Sketch the shape and label every side. Each X below is one square metre.
XXXXXX XXXXXX XXX XXX XXX
- Perimeter: go round the outside, starting top left. Top 6 m, right side 2 m, then in 3 m, down 3 m, along the bottom 3 m and up the left side 5 m.
6 + 2 + 3 + 3 + 3 + 5 = 22
- Area: count the squares. The top bar has 2 rows of 6, which is 12 squares. The leg has 3 rows of 3, which is 9 squares.
12 + 9 = 21
- Check the area another way. Imagine the full 6 by 5 rectangle, 30 squares, then take away the empty 3 by 3 corner, 9 squares: 30 − 9 = 21. The perimeter of the full rectangle is 2 × (6 + 5) = 22 m, the same as the L-shape, which is a handy check.
The perimeter is 22 m and the area is 21 square metres.
Common mistakes and how to help
Mixing up area and perimeter
Both are found from the same sides, and the words sound similar. Children add when they should multiply or count, or the other way round.
What helps: Tie each word to an action. Perimeter is walking round the edge, like a fence. Area is covering the inside, like turf. Use a different colour for each when marking a drawing.
Missing a side when finding the perimeter of an L-shape
Only the sides with a number on them get added. Sides with no label are skipped, and the answer comes out too small.
What helps: Work out the unlabelled lengths first, then tick every side while adding. For an L-shape, the perimeter of the surrounding rectangle, as in the example, gives a quick check.
Multiplying when they should divide in unit conversions, or the reverse
Children remember that you ‘times by 1,000’ but not which way round it goes, so 4,000 m becomes 4,000,000 km.
What helps: Ask whether the number should be bigger or smaller. The smaller the unit, the more of them you need. A kilometre is a long way, so 4,000 m is only 4 km.
Getting the 24-hour clock wrong around noon and midnight
Adding 12 works for 1 pm to 11 pm, so children write 12:30 pm as 24:30, or 12:15 am as 12:15.
What helps: Keep a clock face with both sets of numbers nearby. 12:30 pm is 12:30, and just after midnight is 00:15. Talk through the day in order: 11:00, 12:00, 13:00.
How to help at home
- Measure the room. Pace out or measure the sides of a rug, table or garden bed and work out the perimeter. Squared paper helps if your child wants to draw it first and count the area.
- Draw L-shapes on squared paper. Take turns drawing a shape made of straight sides and right angles. The other person finds the perimeter and the area, and you compare answers.
- Spot the 24-hour clock. Many ovens, phones and timetables use the 24-hour clock. Ask your child to read out the train, bus or film time in both ways.
- Plan the day in minutes. How many minutes until bedtime? How many seconds in a three-minute egg timer? Everyday questions practise converting hours, minutes and seconds.
- Read labels. Look for litres and millilitres on bottles, or kilograms and grams on packets. Ask how many grams a 2 kg bag holds, or how many 250 ml glasses fill a litre jug.
Related topics
Builds on
- Year 3: Length, mass, money and time
Leads on to
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