Math // Measurement

Metric & Imperial Conversion

Understand and use approximate equivalences between metric units and common imperial units (inches, pounds, pints)

Ages
9 to 10
Type
Concept
Builds on
1 topic
Leads to
1 topic
Standards
1 code

Quick check

If the learner hears that a baby weighs 7 pounds, can they give a rough idea of that in kilograms — using the approximate conversion they know?

Ask this to see whether the topic is secure.

Topic // Mastery

What mastery looks like

What a learner can do once this topic is secure.
  • State that 1 inch ≈ 2.5 cm and use this to estimate length in inches
  • Know that 1 kg ≈ 2.2 pounds and estimate a person's weight in pounds
  • Convert approximately between litres and pints (1 litre ≈ 1.75 pints)

Topic // Map

Where this fits

What to learn first and what this topic opens up, with the reason for each link. Required links come first.
  • RequiredMust come first
  • HelpfulHelps, but not essential

Before this

1 topic

You are here

Metric & Imperial Conversion

Ages 9 to 10 // Concept

Leads to

1 topic
  • Miles & KilometresRequired

    Measurement // Ages 10 to 11

    Miles↔km conversion extends Y5 imperial/metric approximation

Topic // Research-informed

Teaching notes

From the Marble Skill Taxonomy. These notes have not been validated in classrooms, so treat them as a starting point for your own judgement.

The approach

Builds a working, approximate sense of common imperial units through key facts (1 inch ≈ 2.5 cm, 1 kg ≈ 2.2 lb, 1 litre ≈ 1.75 pints) used for estimation and sense-checking. Learners meet the ≈ symbol and the idea that these cross-system facts are used only roughly: the conversion factors are fixed but rounded for everyday use (e.g. 2.54→2.5, 2.20462→2.2, 1.75975→1.75), so answers are phrased as 'about …'. Relationships purely within imperial — pound↔ounce, stone↔pound, gallon↔pint, foot↔inch — are exact. Teaching anchors each fact to a familiar referent object and a dual scale, emphasises which unit is larger (and hence whether the count goes up or down), and treats conversion as scaling a constant multiplicative ratio rather than isolated one-off sums. Includes a hands-on/tool-based estimation activity, a static dual-scale fact activity, a check-and-practice activity, and vocabulary support; each with a distinct purpose and limits.

Common misconceptions

  • Treats an approximate equivalence as exact (writes 1 inch = 2.5 cm and cannot say the answer is only roughly right).
  • Confuses which unit is larger, so converts in the wrong direction (multiplies when they should divide, or vice versa) because the larger number of cm suggests a cm is the bigger unit.
  • Struggles with unfamiliar imperial vocabulary and relationships (e.g. knows feet and inches measure length but not that 1 foot = 12 inches).

Useful representations

  • Dual-scale ruler (inches and cm side by side)
  • Kitchen/bathroom scales showing kg and lb together
  • Double number line
  • Ratio / conversion table
  • Familiar referent objects (sugar bag, ruler, bottle)

Teaching practices

  • Introduce ≈ explicitly and contrast 'approximately equal' with 'equal'; expect answers phrased as 'about …'.
  • Anchor each fact to a familiar referent (a 30 cm ruler ≈ 12 inches; a bag of sugar ≈ 2.2 lb; a large bottle ≈ 1.75 pints) and to a dual-scale tool (ruler with inches and cm; kitchen scales showing kg and lb).
  • Use a double number line or ratio table so the constant multiplicative relationship is visible and children scale up (2 inches ≈ 5 cm) rather than treat each conversion in isolation.
  • Emphasise which unit is the larger, and therefore whether the count of units should be more or fewer after converting; use estimation to sense-check direction.
  • Pre-teach imperial vocabulary (inch, foot, pound, ounce, stone, pint, gallon), since many children have little prior exposure to these units.

Activities to try

  1. 1

    Dual-scale estimation playground

    Why it works: Converting between systems means scaling by a fixed multiplier (pounds ≈ 2.2 × kg; inches ≈ cm ÷ 2.5), and because the taught factor is rounded the cross-system answer is used approximately, so it is phrased as 'about …'.

    1. Place a bag of sugar (about 1 kg) on a kitchen scale reading kilograms, then estimate its weight in pounds before checking.
    2. Drag a pointer on a blank pounds dial to your estimate, then reveal the true value and compare it against a double number line showing 1 kg ≈ 2.2 lb.
    3. Load 3 kg and watch both scales climb together; describe in words how the pounds number relates to the kg number (about 2.2 times), with a ratio/conversion table recording each kg–lb pair.
    4. Use a ruler tool that overlays inches on centimetres: line up 1 inch and read off that it spans about 2.5 cm, then scale up (2 inches ≈ 5 cm).
    5. Toggle 'is it exact?' to reveal the fuller value (e.g. 1 kg ≈ 2.20462 lb) and explain why the everyday answer is only 'about'.
  2. 2

    Dual-scale fact set with referent objects

    Why it works: The key approximate equivalences become memorable when tied to objects you can picture and to a scale where both unit systems sit side by side.

    1. Look at a dual-scale ruler (inches and cm side by side) and read off that a 30 cm ruler is about 12 inches.
    2. Match familiar referent objects to facts: a 1 kg bag of sugar ≈ 2.2 lb; a 1-litre bottle ≈ 1.75 pints; an adult's thumb-width ≈ 1 inch (a child's is usually a bit narrower).
    3. Tap each ≈ fact to reveal and read it aloud, saying 'about' each time.
    4. After reading a fact, say aloud how you would scale it up (e.g. two bags of sugar ≈ 4.4 lb) even though the display does not recompute.
  3. 3

    Estimate and sense-check practice

    Why it works: Estimating and checking whether an answer is sensible relies on knowing which unit is larger and therefore which direction the count moves.

    1. Estimate on a slider: 'A 10 kg dog is about ___ lb' (expect about 22 lb).
    2. Answer 'About how many inches is 15 cm?' and justify the direction of conversion.
    3. Choose which unit is larger — 1 inch or 1 cm — and say whether the count of inches should be more or fewer than the count of cm.
    4. Decide whether 2 kg of flour is more or fewer pounds than 2, and give a rough figure (about 4.4 lb).

Topic // Standards

Curriculum standards

Codes for the curriculum statements this topic maps to. Each publisher owns its standard text.
The national curriculum in England: Key stages 1 and 2 framework document
Ma5/3.1b

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