Updated July 18, 2026
The numbers this calculator uses
Three formulas cover almost every room this calculator will ever see, plus two fixed conversion factors on the output side.
| Shape | Formula this calculator runs |
|---|---|
| Rectangle or square | length × width |
| Triangle | 0.5 × base × height |
| Circle | π × radius² |
| Conversion | Factor used |
|---|---|
| Square feet to square meters | × 0.092903 |
| Square feet to square yards | ÷ 9 |
The area formulas and the two conversion factors are exact geometric and unit definitions, not figures pulled from a third party, so they carry no outside citation. The overage percentages further down the page do come from an outside source, cited where they appear.
Doing the math by hand
Rectangle = length × width. Triangle = ½ × base × height. Circle = π × radius². Results convert automatically to square meters (× 0.0929) and square yards (÷ 9).
The math, simplified
Square footage is just length times width for a rectangle, and almost every other shape reduces to that or to a couple of simple formulas. Get the area right and you can order flooring, paint, tile or sod without guessing. The trick is measuring carefully and adding the right overage for cuts.
Measuring a real room
Measure wall to wall at floor level, not at the baseboard, and take two readings on long walls because rooms are rarely truly square. For an L-shaped or notched room, draw it out and split it into rectangles, find each area, then add them. Subtract large fixed features only if they genuinely will not be covered, like a kitchen island footprint. Closets and doorway returns usually do get flooring, so leave them in.
How much extra to order
| Layout | Extra to add |
|---|---|
| Straight or grid flooring | 10% |
| Diagonal layout | 12 to 15% |
| Herringbone or chevron | 18 to 20% |
| Glazed ceramic or porcelain tile | 10% |
| Natural stone or patterned tile | 15% |
These overage figures track the NWFA guidance for wood and standard tile practice. Diagonal and angled patterns waste more because every cut at the wall leaves an offcut you cannot reuse.
Paint and coverage notes
For paint, a gallon covers about 350 to 400 square feet per coat on smooth interior wall, and most jobs need two coats. Primer covers less, around 200 to 300 square feet per gallon. So a 144 square foot room of wall, painted twice, is comfortably inside two gallons. Buy from one batch or lot number so the color matches across the room.
Sources: flooring overage guidance, Sherwin-Williams paint coverage.
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