Room square footage
Square footage is the area of a surface measured in square feet. For a rectangular room it is the length times the width, with both measured in feet. A room that is 12 feet by 12 feet has 144 square feet, and one that is 12 feet 6 inches by 10 feet has 125. The same number answers many everyday questions: how much flooring to buy, how much paint a ceiling needs, how big a patio or lawn is, or whether a piece of furniture will leave room to walk around it. This calculator takes the arithmetic one step further by letting you combine many sections, mix units, subtract cut-outs and add an ordering allowance, while showing every line so you can check it. Write each room's length and width on a quick sketch as you measure, so you can recheck a number later without walking the room again.
Feet and inches versus decimal feet
Tape measures read in feet and inches, but the formula needs one number. There are two ways to enter a length. Choose the feet-and-inches option and type whole feet and inches separately, or type a decimal number of feet. Twelve inches make a foot, so 6 inches is 0.5 feet, and 12 feet 6 inches is the same as 12.5 feet. The mistake to avoid is typing 12.6 for 12 feet 6 inches, which means 12 feet and about 7 inches. Inches must be at least 0 and below 12, and the calculator rejects 12 or more rather than carrying it over silently.
Each dimension has its own unit, so one side can be in feet and another in inches or metres. The options are feet and inches together, feet, inches, yards, metres, centimetres and millimetres. A blank field is never treated as zero: the calculator asks for a value instead, because an empty length would otherwise shrink the area to nothing without telling you.
The nine shape formulas
Rectangles and parallelograms
A rectangle is length times width. A rectangle border is the outer rectangle minus the inner one, where the inner sides are each shorter by twice the border width. For a 10 by 8 foot frame with a 1 foot border that is 80 − 48 = 32 square feet. A parallelogram is base times the perpendicular height, not the slanted side: a base of 10 feet and a height of 6 feet gives 60 square feet.
Circles, rings and sectors
A circle's area is π times the diameter squared, divided by 4, so a circle 10 feet across covers 78.54 square feet. A ring is the outer circle minus the inner one, which uses the border width; with a 10 foot outer diameter and a 1 foot border it is 28.27 square feet. A sector is a slice of a circle: π times the radius squared times the angle in degrees, divided by 360. A 90 degree sector of radius 6 feet is a quarter circle, 28.27 square feet.
Triangles and trapezoids
With a base and a height, a triangle is half of base times height: 30 square feet for 10 and 6. With three side lengths, the calculator uses Heron's formula, which finds the area from the sides alone; the classic 3, 4, 5 triangle gives 6. The three sides must be able to meet, meaning each is shorter than the other two added together. A trapezoid is the average of its two parallel bases times the height. Bases of 10 and 6 feet with a height of 4 feet make 32 square feet.
| Shape | Formula | Example (ft²) |
|---|---|---|
| Rectangle | length × width | 12.5 × 10 = 125 |
| Rectangle border | outer − inner rectangle | 10 × 8, 1 ft = 32 |
| Circle | π × D² ÷ 4 | D 10 = 78.54 |
| Ring | π × (D² − (D − 2b)²) ÷ 4 | D 10, b 1 = 28.27 |
| Triangle, three sides | Heron's formula | 3, 4, 5 = 6 |
| Triangle, base and height | base × height ÷ 2 | 10, 6 = 30 |
| Trapezoid | (base 1 + base 2) × h ÷ 2 | 10, 6, h 4 = 32 |
| Sector | π × r² × angle ÷ 360 | r 6, 90° = 28.27 |
| Parallelogram | base × perpendicular height | 10, 6 = 60 |
Multiple rooms and cut-outs
Most real spaces are not one shape. Add a section for each part and the calculator sums them. An L-shaped room made from a 14 by 10 foot rectangle and a 6 by 4 foot alcove is 140 + 24 = 164 square feet. To remove something, add a section and mark it Subtract. A 20 by 15 foot lawn (300 square feet) with a round pond 6 feet across leaves 271.73 square feet to seed. Each section can repeat with a quantity, such as four identical flower beds, and the quantity is multiplied once. Cut-outs are plain subtraction. The calculator does not check that a cut-out sits inside an added area, and if the cut-outs exceed the added areas it shows an error instead of reversing the sign.
Unit conversion
The net area is shown in square feet, square metres, square yards and acres. One square yard is 9 square feet, one acre is 43,560 square feet, and one square foot is exactly 0.09290304 square metres. A 30 by 20 metre plot is 600 square metres, which is 6,458.3 square feet and 717.6 square yards. Because each length is converted to metres before multiplying, a mixed entry such as feet by inches gives the same answer as converting everything first. Choose the area unit that matches how the material is sold: flooring and tile in square feet or square metres, turf and carpet often in square yards, and land in acres.
Allowance and cost
Measured area and the area you order are different numbers. Flooring and tile are cut around corners and patterns, so people often add a percentage for waste. Enter your own figure; the room above at a 10% allowance becomes 137.5 square feet. If you also enter a unit price, you choose whether it applies to the net area or the order area. At 3 per square foot, the order area costs 412.50 and the net area 375.00. The price is used exactly as typed, so include any tax in it yourself if you want it counted. The calculator does not know which materials come in boxes or rolls. For tile and paint quantities, the tile calculator and the paint calculator take it from here, and for loose materials the gravel calculator turns an area into a volume.
Worked examples and questions
Measure each part twice, sketch the shape and label every side before you enter anything. For a patio that includes a semicircle, treat it as a rectangle plus half a circle: enter a circle and note that it counts in full, so use a sector of 180 degrees for the half. Concrete slabs and footings are better estimated with the concrete calculator, which works from volume.
How do I calculate the square footage of a 12 by 12 room?
Multiply 12 by 12 to get 144 square feet.
How do I enter 12 feet 6 inches?
Use the feet-and-inches unit and type 12 and 6 in the two boxes, or type 12.5 as decimal feet. Do not type 12.6.
How do I measure an L-shaped room?
Split it into two rectangles that do not overlap, add a section for each and let the calculator add them. The example above gives 164 square feet.
How do I subtract an area such as a cut-out?
Add a section for the cut-out, enter its dimensions and mark it Subtract. The net area cannot be negative.
Is the area the same as the amount of flooring to buy?
Not always. Waste, cuts and pattern matching call for an allowance, which you can enter separately so the net and order areas stay visible.
Can I use this for a property listing?
The result is a measuring aid. Listings and valuations follow their own measurement rules, so apply the relevant standard separately.
Measurement limits
The tool does not read plans, maps or roof pitch, and it does not verify that sections lie inside one another or do not overlap. Walls have thickness, and whether an area is measured to the inside or outside of the wall changes the answer. Use the same convention for every section, round up when buying and keep the sketch for later checks. Measure twice, and choose a larger allowance when the pieces have to line up with a pattern.