Using known area, perimeter, or volume to work backwards and find a missing length, width, or height.
Where your child meets this in real life: Determining the width of a room given the floor area and length, or height of a box given volume
SEAGReady breaks find missing dimensions into 3 steps, taught in order so each skill builds on the last.
Find a missing side of a rectangle when given the area and one side length
Find a missing side of a rectangle when given the perimeter and one side length
Find a missing dimension of a cuboid when given the volume and two other dimensions
Three free sample questions from our find missing dimensions course. Every question comes with a full explanation, and hints that guide without giving the answer away.
A rectangular rug has an area of 20 m² and a length of 5 m. What is the width of the rug?
Answer: A. 4 m
Area = length x width 20 = 5 x width To find width, divide area by length: Width = 20 ÷ 5 = 4 m
Stuck? Start here: Remember the area formula: Area = length x width
A rectangular picture frame has a perimeter of 24 cm and a length of 8 cm. What is the width of the frame?
Answer: A. 4 cm
Step 1: Halve the perimeter 24 ÷ 2 = 12 cm This is the sum of one length and one width. Step 2: Subtract the length 12 - 8 = 4 cm The width is 4 cm.
Stuck? Start here: Perimeter counts both lengths and both widths. First, halve the perimeter.
A fish tank has a volume of 60 cm³. The length is 5 cm and the width is 4 cm. What is the height of the fish tank?
Answer: A. 3 cm
Step 1: Find the base area Base area = length x width Base area = 5 x 4 = 20 cm² Step 2: Find the height Volume = base area x height 60 = 20 x height Height = 60 ÷ 20 = 3 cm
Stuck? Start here: Volume = length x width x height. First find the base area.
This is the exact interactive worked example your child sees in SEAGReady. Step through it and watch the method build up.
Aoife is buying a rug for her bedroom. The rug has an area of 24 m² and is 6 m long.
What is the width of the rug?
24 ÷ 6 = ?
Step 1 of 4
Aoife is buying a rug for her bedroom. The rug has an area of 24 m² and is 6 m long.
What is the width of the rug?
The rug is 4 m wide.
The key insight: Division is the opposite of multiplication - use it to work backwards!
Watch out: 24 x 6 = 144 m. Multiplying makes the number bigger, but we need to find a smaller piece of the area.
These are the misconceptions we see most often in find missing dimensions, including the ones our practice questions are specifically designed to catch.
Struggling with find missing dimensions? The real gap is often in one of these earlier topics.
SEAGReady finds the exact step where your child gets stuck, teaches it with worked examples like the one above, and brings it back for review so it sticks.