Floor area calculator: in short
Add up the area of every room on a floor, measured inside the walls, then add the floors. Keep a garage, a shed, a balcony and loft space you cannot stand in apart. Measured inside the outer walls instead, the walls between rooms count too, so that number is a little larger.
Floor by floor: an example
The two totals differ because the second one includes the walls between the rooms and the stairs on both floors. Neither is wrong; say which one you give.
| Part of the home | Rooms inside the walls | Floor inside the outer walls |
|---|---|---|
| Ground floor | 40.3 m² | 42.6 m² |
| First floor | 35 m² | 42.6 m² |
| Home | 75.3 m² | 85.1 m² |
| Garden shed (apart) | 4.8 m² |
Inside the rooms, or inside the outer walls
Adding up the rooms gives the area you can use: what flooring, furniture and most room lists go by.
Measuring each floor inside its outer walls takes in the internal walls, the stairs and the hall. Many estate agents and valuers work this way. Measuring outside the outer walls gives a larger number again and is used for building costs, not for a listing.
What usually stays apart
- A garage, a shed or an outbuilding: its own line.
- Balconies, terraces and roof terraces: outdoor space, not floor area.
- Loft space or parts under a sloping roof that are too low to stand in: many measuring codes leave out parts lower than about 1.5 m. Check the rule that applies to you.
- The space taken by the stairs: counted once on the floor where they start, in most methods.
Measuring codes differ per country
A measuring code decides exactly where to hold the tape and what counts. The UK, Ireland, Australia, New Zealand and Canada each have their own habits, and an agent may follow a professional code. When your number has to match a listing, a lease or a valuation, ask which code they use.
Squaremetry draws every room and floor to scale and keeps the parts apart, so you can give the total either way.
Figure 1. One floor of 7.5 × 5 m. The rooms add up to 37.5 m².
About this page · Written by the Squaremetry team. Last checked on October 5, 2026. Found a mistake? Send us a message.