---
title: "Using Simple Formulae"
description: "Use simple formulae expressed in words or symbols to calculate values (e.g. perimeter = 2 × (length + width))"
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retrieved: 2026-10-19
---

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# Using Simple Formulae

Use simple formulae expressed in words or symbols to calculate values (e.g. perimeter = 2 × (length + width))

Subject: Mathematics · Area: Algebra · Ages 10 to 11
Page: https://lightmysky.com/learn/mathematics/using-simple-formulae-mt_Jvg8X0N5u0

## Ready when they can

- Substitute values into P = 2(l + w) to find the perimeter of a rectangle
- Use the formula for area of a triangle (A = 1/2 × b × h) given base and height values
- Interpret a formula expressed in words and use it to compute an output

## Lesson: Using Simple Formulae

A formula is a short rule that tells you how to work out a value. It uses letters to stand for numbers you don't know yet. Perimeter = 2 x (length + width) is a formula. To use it, you swap each letter for a real number, then work out the answer.

**Example.** A rectangle has a length of 7 cm and a width of 3 cm. Using P = 2 x (l + w), put the numbers in: P = 2 x (7 + 3). Add first: 7 + 3 = 10. Then multiply: 2 x 10 = 20. The perimeter is 20 cm.

*(drawing: This is the rectangle from the example. Its perimeter works out to 20 cm.)*

**Example.** A triangle has a base of 8 cm and a height of 5 cm. The formula is A = 1/2 x b x h. Put the numbers in: A = 1/2 x 8 x 5. Multiply 8 x 5 = 40 first, then take half: 40 divided by 2 = 20. The area is 20 square cm.

**Tip.** Some formulae are written in words, like 'cost = number of tickets x price per ticket.' Read the words carefully, turn them into a calculation, then work it out in the right order: brackets first, then multiplication.

**Recap.** A formula is a rule with letters for numbers. Substitute the numbers in, then calculate step by step in the right order.

## Practice

17 questions on this page, each with its working shown.

## Needs first

- [Writing Number Sentences](https://lightmysky.com/learn/mathematics/writing-number-sentences-mt_4WaKWECpcv)
- [Order of operations](https://lightmysky.com/learn/mathematics/order-of-operations-mt_jHgRQ4hR0g)

## Opens up

- [Algebraic Notation](https://lightmysky.com/learn/mathematics/algebraic-notation-mt_KwdjWEmMNo)
- [Area of Triangles & Parallelograms](https://lightmysky.com/learn/mathematics/area-of-triangles-and-parallelograms-mt_ML5t7n2-U8)