The Room Size Calculator works backwards and forwards between a room’s area and its dimensions. Find the missing side when you know the area and one side, find length and width from an area and a length-to-width ratio, or get area, perimeter, diagonal and proportion from both sides.
It is useful when a listing gives only square footage, when planning a room of a target size, or when checking whether a room’s shape is square or long and narrow.
Room Size Calculator
Work out room dimensions: the missing side from area, size from area and a length-to-width ratio, or area, perimeter and diagonal from the sides.
Formula
- Missing side: W = A ÷ L
- From area and ratio k = L ÷ W: L = √(A × k), W = √(A ÷ k)
- From sides: A = L × W, P = 2(L + W), d = √(L² + W²)
How the Calculation Works
Step 1: Choose what you know: area and one side, area and a proportion, or both sides.
Step 2: The calculator solves for the missing dimensions.
Step 3: Review perimeter and diagonal for trim or furniture planning.
Example Calculation
Missing side
Given: 180 ft² room, one side 15 ft
Other side = 180 ÷ 15 = 12 ft; perimeter = 54 ft
From area and ratio
Given: 180 ft², length 1.25 × width
L = √(180 × 1.25) = √225 = 15 ft; W = √(180 ÷ 1.25) = √144 = 12 ft
From dimensions
Given: 15 × 12 ft
Area 180 ft², perimeter 54 ft, diagonal ≈ 19.21 ft, ratio 1.25
Shape Properties
- A ratio of 1 is a square room; the larger the ratio, the longer and narrower the room.
- For a fixed area, a square has the smallest perimeter — less baseboard and wall area.
Frequently Asked Questions
What size is a 200 sq ft room?
Many shapes are possible — for example 10 × 20, 12.5 × 16 or about 14.1 × 14.1 ft. Enter a ratio to get one.
How do I find the length of a room from square footage?
Divide the area by the known width.
What are the dimensions of a square room of 144 ft²?
√144 = 12 ft on each side.
What is a room’s aspect ratio?
Length divided by width.
Does this work in meters?
Yes — enter the area in m² and sides in m.