Every serve of a recipe needs the same amount, so amounts scale by multiplying, never by adding. If the new number of serves is a multiple of the old one, multiply by that many. Otherwise use the unitary method: find the amount for one serve, then multiply by the serves you need. You can also find how many times bigger the new recipe is, such as 6 ÷ 4 = 1.5 times.
Explore this idea with maths tools
Maths you can move
Build 8 + 5
Tap two gold counters to fill the ten-frame.
8 in the frame + 5 waiting. The total stays the same.
Words to know
- proportion
- amounts that grow or shrink by the same number of times
- unitary method
- find the amount for one, then for as many as you need
Worked examples
A pancake recipe for 4 serves uses 300 g of flour. How much flour is needed for 8 serves?
- 1.8 serves is 2 times as many as 4 serves.
- 2.300 × 2 = 600.
= 600
A pancake recipe for 4 serves uses 300 g of flour. How much flour is needed for 6 serves?
- 1.One serve: 300 ÷ 4 = 75 g.
- 2.Six serves: 75 × 6 = 450.
= 450
A recipe that makes 4 serves uses 1.5 kg of potatoes. How much potatoes is needed for 6 serves, in kilograms?
- 1.The new recipe is 6 ÷ 4 = 1.5 times as big.
- 2.1.5 × 1.5 = 2.25.
= 2.25
For parents
Method: Unitary method
What to say: Do we need more or less? How much is one serve?
Common mistakes:
- Adds the change in serves to the amount instead of scaling it (Hart's onion soup error).
- Multiplies when the recipe should shrink, or divides when it should grow.
- Finds the amount for one serve and stops there.
- Works out how many times more ingredient there is and gives that as the number of serves.
- Multiplies the amount by the new number of serves, forgetting the recipe already makes several serves.