The basic formula
For a plain rectangular room, square footage is simply length times width.
A room 12 ft long and 10 ft wide is 12 × 10 = 120 sq ft. That's the whole calculation for a rectangle — the challenge is almost never the math, it's measuring accurately and picking the right formula once the room isn't a plain box.
Formulas for every shape
Most real spaces are a rectangle, or close enough to treat as one. But circular patios, triangular gable ends, ring-shaped borders and walls with a window cut out all need their own formula.
| Shape | Formula |
|---|---|
| Rectangle / square | Length × Width |
| Rectangle border (frame) | Outer area − inner area |
| Circle | π × radius² (radius = diameter ÷ 2) |
| Circle border (ring) | Outer circle area − inner circle area |
| Triangle (base & height) | ½ × base × height |
| Triangle (three sides) | Heron's formula |
| Trapezoid | ½ × (base 1 + base 2) × height |
| Wall minus window | Wall area − (window area × count) |
You don't need to memorize Heron's formula by hand — enter the three side lengths into the square footage calculator and it solves it instantly, along with every shape above.
Splitting an irregular room into simple shapes
Most real rooms aren't perfect rectangles — L-shapes, bay windows, and alcoves are everywhere. The standard approach is to mentally split the space into rectangles, circles or triangles that don't overlap, work out each section's area, then add them together.
Worked example
A rectangular bedroom 14 ft long and 11 ft wide, plus a closet alcove 3 ft by 4 ft:
14 × 11 = 154 sq ft. 3 × 4 = 12 sq ft. Added together: 166 sq ft total.
That's exactly what the project total on the square footage calculator does automatically — measure each section, add it to the running total, and move to the next one.
Square feet, square yards and acres
Once you have a total, it's often easier to talk about it in a different unit — flooring is usually priced per square foot, but land is measured in acres. These conversions are exact, not estimates.
| Unit | Equals |
|---|---|
| 1 square yard | 9 sq ft |
| 1 acre | 43,560 sq ft |
| 1 hectare | 2.47 acres |
| 1 square metre | ≈10.76 sq ft |
The square footage calculator shows the acre equivalent automatically underneath any US-units result once the area is large enough to matter, so you don't need a separate converter.
How much extra to order for waste
Whatever you're covering — flooring, tile, sod, paint — the material you buy should exceed the raw square footage you calculated. Cuts, offcuts and mistakes all eat into coverage.
- 5–10% extra. A simple rectangular layout with square, straightforward cuts.
- 10–15% extra. Rooms with lots of corners, angles, or built-in obstacles like an island or a bay window.
- 15%+ extra. Diagonal, herringbone, or chevron patterns, which waste significantly more material at every cut.
The “+ Add waste calc” option in the square footage calculator's project total adds a percentage buffer directly on top of your measured area, so the number you take to the store already accounts for it.
Pricing your project
Once you know the square footage, multiplying by a price per unit area gives you a materials estimate before you buy anything. Enter a price per square foot (or per square metre) in the calculator and every area you add is priced automatically, with a running project cost at the bottom.
Pair the square footage total with the Flooring Calculator once you want an exact box count for a specific plank size, or the Tile Calculator for individual tile counts.
Common questions
How do I calculate square footage?
For a rectangular room, multiply the length by the width in feet. For an irregular space, split it into simple shapes — rectangles, circles, triangles — calculate each one and add them together. The square footage calculator does this automatically: add each shape to the project total and it sums the whole area for you.
How do I find the square footage of a circular area?
Measure the diameter, divide it by 2 to get the radius, then use Area = π × radius². A 10 ft diameter circle has a 5 ft radius, giving π × 5² ≈ 78.5 sq ft.
How do I calculate the area of an L-shaped or irregular room?
Break the room into rectangles (or other simple shapes) that don't overlap, calculate each section's area separately, then add them together. Add each section to the project total in the calculator above and the running total does the adding for you.
How many square feet are in an acre?
One acre is 43,560 square feet. The square footage calculator shows the acre equivalent automatically underneath any US-units result once the area is large enough to matter.
How much extra should I buy for material waste?
For straightforward rectangular layouts, 5–10% extra covers cuts and offcuts. For rooms with lots of angles, or diagonal/herringbone patterns, 10–15% is safer. Use the “+ Add waste calc” option in the calculator's project total to add a percentage buffer to your order.