Square footage calculator

The arithmetic is trivial. Almost every wrong square footage comes from the measuring, and specifically from four habits.

Measure wall to wall, not skirting to skirting. Flooring runs under trim in most installations, and the difference around a room is easily a square foot or two — small, until it is the piece you are short.

Measure at the widest point. Walls bow, especially in older houses. Measure a long wall at both ends and in the middle; if they differ, use the largest and note it, because material has to reach the widest part.

Write each measurement against its shape. Not on a phone note in a column of numbers. A sketch with dimensions on it is what catches a transposed 12 for 21, and it costs nothing.

Measure twice, in the same units. Mixing a measurement in feet-and-inches with one in decimal feet is the classic way to produce a number that is 8 per cent wrong and looks completely plausible.

The single most expensive habit in measuring is averaging. Faced with a room that is 14 ft wide at one end and 10 ft at the other, the tempting move is to call it 12 and multiply. For a trapezoid that happens to be exactly right; for an L-shape, a bay or a room that steps, it is wrong — and wrong by more the bigger the room gets.

Divide instead. Every real floor plan is a handful of rectangles with the occasional triangle or trapezoid. Four shapes cover essentially everything a house contains.

A circle is the case where intuition fails hardest. A circular patio 20 ft across is 314 square feet; the 20 × 20 square it sits inside is 400. Treating one as the other buys you 21 per cent too much material, which on a paver patio is real money left on a pallet in the garage.

Almost no calculator in this market lets you subtract, which is why almost every result is a gross figure being used as if it were a net one. The rule is simple: subtract anything the material does not go under.

Subtract: a kitchen island or peninsula built on a plinth, a stairwell opening, a chimney breast, built-in cabinetry that sits on the subfloor, a fixed hearth, a column. On walls: window and door openings, when you are working out paint, drywall or siding.

Do not subtract: free-standing furniture, appliances that sit on the finished floor, rugs, or the swing of a door. Flooring runs underneath all of those, and leaving a gap under the fridge is how you discover that the fridge moves.

The same floor gets quoted to you in different units by different trades in the same week, and the conversions are where confidence outruns accuracy.

The trap is that area conversions are not the same as length conversions. A yard is three feet, so people reach for three — but a square yard is nine square feet, because area scales with the square of length. The same logic makes a cubic yard twenty-seven cubic feet.

All of these follow from one definition: the international agreement that fixes the inch at exactly 25.4 millimetres. Everything else — the foot, the yard, the acre, the square metre equivalence — is arithmetic from there, which is why these numbers do not drift and do not need updating.

Flooring, paint and tile are bought by area. Everything that goes in a space rather than on it is bought by volume: concrete, gravel, mulch, topsoil, sand. For those, the area is only the first half of the sum.

Put a depth in and the panel adds cubic feet, cubic yards and cubic metres. The cubic yard is the one that matters in practice — it is what a ready-mix plant and a landscape supply yard both quote in, and it is 27 cubic feet, not three.

Half of what a room costs is not sold by area at all. Baseboard, quarter round, transition strips, edging, gutters, fencing and countertop edge banding are all sold by the linear foot — a foot of length with no width involved.

So the panel reports the perimeter next to the area. For a single rectangular room that is all you need. For several areas, read the warning it prints: the perimeter is the sum of each shape’s own outline, and where two areas meet along a shared wall, that wall gets counted twice. Subtract it once before you buy trim.

Triangles and trapezoids do not get a perimeter here, and that is deliberate. Defined by base and height, their sloping sides are genuinely unknown — two triangles with identical base and height can have very different outlines. Returning a plausible number would be worse than returning none.

Every material calculator on this site adds a waste allowance, shows it, explains it and lets you change it. This one adds nothing, and the omission is the point.

Waste is not a property of the room. It is a property of the material and the way it is laid: tile on a diagonal wastes half as much again as tile laid straight; a hallway full of doorways wastes more flooring than an open room of the same area; a trench wastes concrete that a formed slab does not. A single “add 10 per cent” folded into the square footage would be wrong for all of them and invisible in every one.

So the number this page gives you is the measurement. What you buy is that number plus an allowance that belongs to the material, applied where you can see it — the methodology page sets out why that separation is the whole argument of this site.

Every conversion here follows from the international definition of the inch as exactly 25.4 millimetres, which fixes the foot, the yard and the square metre equivalence by arithmetic. The acre is defined as 43,560 square feet. None of these figures drift, which is why this is the one page on the site with no price to re-check.

Nothing you type is sent anywhere. The whole calculation runs in your browser.

Seven units for the same room, depending on who is quoting. These are the ones worth being able to tell apart.