Calculateur de Superficie
Additionnez des pièces de toute forme et unité, et convertissez le total
Mix shapes and units, add up every room, convert to anything
Dernière mise à jour: septembre 7, 2026
Measuring a Real Room
Square footage is length times width, right up until the room stops being a rectangle — which is most rooms. Alcoves, bay windows, chimney breasts and L-shaped living rooms all break the simple formula, and the usual workaround of averaging the long and short walls systematically overestimates.
The reliable method is to split the space into shapes you can measure and add them:
- An L-shaped room is two rectangles. Draw the dividing line along a wall, not through the middle.
- A bay window is a trapezoid, or a rectangle plus two triangles if you want to be exact.
- A chimney breast is a rectangle you subtract — measure the room whole, then add the breast as a second area and mentally deduct it, or simply measure the two flanking alcoves separately.
- A path around a patio is a border: outer rectangle minus inner.
The calculator above takes as many areas as you need and sums them, each with its own shape and its own unit, so a room you paced in feet and a plot from a deed in metres can go in the same total.
The Unit Conversion Almost Everyone Gets Wrong
To convert square feet to square metres you multiply by 0.09290304, not by 0.3048.
0.3048 is the linear conversion — one foot in metres. Area conversions are the square of the linear one: 0.3048² = 0.09290304. Using the linear figure by mistake makes an answer 3.28 times too large, and it is an easy error to make because both numbers look plausible.
| From square feet | Multiply by | 1,000 sq ft becomes |
|---|---|---|
| Square inches | 144 | 144,000 |
| Square yards | ÷ 9 | 111.1 |
| Square metres | 0.09290304 | 92.90 |
| Acres | ÷ 43,560 | 0.02296 |
| Hectares | ÷ 107,639 | 0.00929 |
This calculator converts everything internally from a single square-foot figure, so the trap never arises no matter which unit you measured in.
How Much Waste to Add
Waste is not a fudge factor, it is the offcuts. Every plank or tile that meets a wall gets cut, and the remainder is usually too short to use elsewhere.
10% — the usual case
Straight-lay tile, laminate, vinyl plank or engineered wood in a simple rectangular room.
15% — the awkward cases
Diagonal or herringbone layouts, large-format tile, rooms with many angles, alcoves or curves. Every one of these multiplies offcuts.
Buy it all at once, too. Tile and laminate are made in dye lots, and a top-up bought weeks later can be visibly a different shade under daylight — a difference you will notice for as long as you own the floor.
What Counts as Square Footage Depends on Who Is Asking
The number you need changes with the job, and they are not interchangeable:
Flooring. Measure to the wall and ignore the skirting — the flooring runs under or behind it. Include closets if you are flooring them.
Paint. You want wall area, not floor: perimeter × ceiling height, minus doors and windows. That is a different calculation from this one — our paint coverage calculator handles it.
Real estate listings. "Gross living area" generally counts finished, heated space measured to the outside of exterior walls, and typically excludes garages, unfinished basements and anything with a ceiling under seven feet. It will not match your interior floor measurement, and it is not supposed to.
Land. Lot sizes are usually quoted in acres above about a fifth of an acre and in square feet below it. 43,560 square feet to the acre — an odd number because it descends from a chain by a furlong, the area a team of oxen could plough in a day.
Comment nous calculons cela
- Méthode
- Area by shape, summed across areas
- Formule
-
Rectangle = L x W. Circle = pi x (d/2)^2. Triangle = base x height / 2. Trapezoid = ((a + b) / 2) x height. Border = (L + 2w)(H + 2w) - LH. Each area converts its own linear inputs to feet first, so the area factor is the SQUARE of the linear one; totals then convert to m2 (x 0.09290304), sq yd (/ 9), acres (/ 43,560) and hectares. - Source
- Standard plane geometry. Unit factors are the international definitions: 1 ft = 0.3048 m exactly, 1 acre = 43,560 sq ft.
- Limites
- Areas are measured, not surveyed - this cannot know about bay windows, alcoves, sloped ceilings or wall thickness. For flooring, measure to the wall rather than the skirting. The waste allowance is a flat percentage and does not model cut optimisation, which for large-format or diagonal layouts can waste noticeably more than the percentage suggests.
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