You are about to order flooring, paint, mulch or concrete, and the number you get wrong is the number you pay for twice. Here is the arithmetic first, then the six searches that usually follow it.
Rectangles are easy. Everything else is rectangles you have not split up yet.
Length in feet times width in feet — a room 12 ft by 10 ft is 120 sq ft. If your measurements are in inches, divide each one by 12 first rather than dividing the final answer.
Convert each side to feet before multiplying, or multiply in inches and divide the result by 144. Dividing the inch answer by 12 is the classic error — it leaves you twelve times too big.
Where the numbers stop being intuitive.
43,560 — an oddly specific figure inherited from a medieval furlong-by-chain strip of ploughed land. A square acre is about 209 ft on each side.
Multiply by 10.764, so 50 m² is about 538 sq ft. Note it is not the 3.28 you would use for length — area conversions square the factor.
Multiply the area by the depth in feet, then divide by 27 — a 200 sq ft bed at 3 in deep is 200 × 0.25 ÷ 27, about 1.85 cubic yards. This is the calculation for mulch, gravel and concrete.
The area is not the order quantity, and the difference costs money.
Add 10% for straight layouts and 15% for diagonal or herringbone, to cover cuts and the odd damaged plank. Ordering exactly your square footage guarantees a second order at a different dye lot.
A gallon covers roughly 350–400 sq ft in one coat, and you should plan on two coats. Paint is quoted per wall area, not floor area — measure the walls, then subtract the doors and windows.
Material coverage figures below are typical manufacturer numbers and vary by product and surface. Read the bag or the box before ordering — this page tells you how much area you have, not how much the specific product covers.
You just walked 7 searches for “square feet calculator”. The extension maps the trail right on your results page — for anything you search, not just the journeys we’ve written.