How to use the floor area ratio calculator
- Choose square feet or square meters and enter the lot area.
- List the floor area of each level or space, such as the first floor, second floor and garage. Use the areas your own rules count.
- To compare with a limit, enter a ratio limit, such as 0.5. No limit is built in.
- Read the floor area ratio to three decimals, and with a limit the floor area still available or the amount over, shown in parentheses.
Formula
floor area ratio = total floor area / lot area
floor area allowed = lot area x ratio limit
floor area available = allowed - total floor area
A ratio of 0.5 means the total floor area is half the size of the lot. The ratio has no unit because both areas use the same unit.
A two story house with a garage on a 6,000 square foot lot
A first floor of 1,500 square feet, a second floor of 1,300 and a garage of 420 give 1,500 + 1,300 + 420 = 3,220 square feet of floor. On a lot of 6,000 square feet the floor area ratio is 3,220 / 6,000 = 0.537.
With a limit of 0.5 the lot allows 6,000 x 0.5 = 3,000 square feet, so the plan is over by (220 square feet). With a limit of 0.6 the allowance is 3,600 square feet, which leaves 380 square feet available. If the rules exclude the garage, enter only the two floors and the ratio becomes 2,800 / 6,000 = 0.467.
What the ratio tells you
Floor area ratio, often shortened to FAR, compares the size of a building with the size of its lot. A ratio of 1.0 means the building has as much floor area as the lot has ground, whether that is one story covering the whole lot or four stories covering a quarter of it. It describes bulk rather than shape, which is why it appears alongside rules about height and coverage.
To see how much of the ground the building covers, use the Lot Coverage Calculator. The two measures answer different questions, and a plan can meet one while missing the other.
What counts as floor area
Definitions vary. Some rules count every enclosed level, some leave out basements, garages, attics or stair openings, and some measure to the outside of the walls while others measure the inside. This tool adds exactly the areas you list, so decide first what your rules count, then enter only those spaces. List excluded spaces separately if you want to see both totals.
The Square Footage Calculator and the Area Calculator help measure each level. The DIY Materials List Builder turns the finished area into a shopping list.
Assumptions and limits
- The lot and every floor use the same unit, chosen at the top.
- The total is the sum of the areas you enter. Nothing is excluded automatically.
- A ratio limit, if you enter one, comes from your own rules. The tool contains no legal limits and gives no advice about permits.
- Nothing is stored or uploaded.
Frequently asked questions
How do you calculate floor area ratio?
Add the floor area of every level the rules count and divide by the lot area. A building with 3,220 square feet of floor on a 6,000 square foot lot has a ratio of 0.537.
What is a good floor area ratio?
There is no single good value, because it depends on the place and the rules that apply. The calculator gives the number and, if you enter your own limit, shows how far you are from it.
Do garages and basements count?
Some rules count them and some do not. Check the definition that applies to you, and list only the spaces it counts. You can list the others in a separate run to see both ratios.
Can the ratio be more than 1?
Yes. A multi story building can have more floor area than its lot has ground, so a ratio of 1.5 or 2 is possible. It simply means the floor area is one and a half or two times the lot area.