How do I calculate the wall area of a room?

Add the length and the width, double it to get the perimeter, and multiply by the ceiling height. A five by four metre room is a perimeter of 18 metres, which at 2.4 metres high gives 43.2 square metres. Then subtract the doors and windows at their true sizes. Doing it this way rather than measuring four walls separately gives the same answer with a quarter of the opportunity for a mistake.

How do I calculate the area of a gable end?

Half the base times the rise. A gable is a triangle, and a triangle is always exactly half the rectangle that would contain it, regardless of the pitch. An eight metre wide gable rising two and a half metres is ½ × 8 × 2.5 = 10 square metres. The most common mistake in a domestic take-off is measuring the gable as if it were a rectangle, which doubles that part of the order — on the example above, about 590 bricks too many.

How do I find the rise of a gable from the roof pitch?

Multiply half the base by the tangent of the pitch angle. A 32 degree pitch on an eight metre span gives 4 × tan(32°) = 2.5 metres of rise. The calculator works the other way too — enter a base and a rise and it tells you the equivalent pitch, which is the quickest way to check that a measurement taken from the ground is plausible. If the pitch comes out at 60 degrees on a house, something has been measured wrong.

Should I deduct doors and windows?

Yes for anything of meaningful size. There is a trade convention of ignoring openings under about half a square metre, on the basis that the extra material roughly covers the reveals around the opening, which do need finishing. For a large window that convention breaks down badly — a 1.2 by 1.2 metre opening is 1.44 square metres of material you would otherwise buy for nothing. Deduct the big ones and let the small ones cover the reveals.

How many bricks per square metre of wall?

About 59.3 for a half-brick wall in standard 215 by 65 mm bricks with 10 mm joints, which most people round to 60. The figure comes from the module rather than the brick: each brick occupies 225 by 75 mm of wall once its joint is counted. For a one-brick wall, double it. Blocks are far fewer at about 9.9 per square metre for a standard 440 by 215, which is the main reason blockwork goes up so much faster.

How much paint do I need for a wall?

Divide the net area by the coverage on the tin and multiply by the number of coats. Coverage varies more than any other figure in a take-off — around 10 square metres per litre on bare plaster that is drinking it in, up to 14 on a previously painted and sealed surface. Two coats is the normal assumption, and bare plaster usually wants a mist coat first, which is effectively a third pass at thinned paint.

Do I measure walls before or after plastering?

For the plaster itself, measure the bare wall. For anything applied over the plaster, such as paint or tiles, the difference is negligible — a 12 mm render coat changes a room perimeter by under a hundred millimetres in total, which on a wall of any size is a fraction of a percent and well inside the wastage allowance. The one case where it matters is a small enclosure such as a shower recess, where the finished internal dimensions can be noticeably smaller than the structural ones.

What about walls that are not rectangular?

Break them into shapes you can measure. A wall with a sloping top is a rectangle plus a triangle. A wall under a staircase is a rectangle minus a triangle. A curved wall can be measured along its centre line and treated as a rectangle of that length, which is accurate enough for ordering. The general rule for take-off is that any shape can be decomposed into rectangles and triangles, and both are trivial once separated.

Why do different calculators give slightly different brick counts?

Almost always because of how the joint is treated. Working from the bare brick face rather than the module gives 10.57 blocks per square metre instead of 9.88, or 66 bricks instead of 59 — around seven percent too many. Rounding also differs: some quote 60 bricks per square metre as a round figure where the exact value is 59.26. The figures here use the module throughout and match the block and mortar calculators on this site exactly.

Wall Area Calculator

Wall area is the measurement everything else on a job depends on, which is why getting it slightly wrong is expensive in a way that compounds. The same figure decides how many bricks to order, how much mortar to mix, how much render to buy and how much paint to open. An error of five percent in the ...

THE WALLS

ROOM LENGTH (m)

ROOM WIDTH (m)

HEIGHT (m)

Perimeter 18.00 m × 2.4 m high — all four walls in one measurement.

GABLE ENDS — THE TRIANGLE ABOVE THE EAVES

HOW MANY

BASE (m)

RISE (m)

OPENINGS TO DEDUCT

DOORS

WIDTH (m)

HEIGHT (m)

WINDOWS

WIDTH (m)

HEIGHT (m)

NET WALL AREA

38.43 m²

43.20 gross less 4.77 of openings · 413.7 ft²

WALLS

43.2 m²

GABLES

—

PERIMETER

18.0 m

Gross area

43.20 m²

Openings deducted

4.77 m²

Net area

38.43 m²

Net as a share

89.0%

Wall area

43.20 m²

Gable area

None

Bricks

2,278

Blocks

380

Render at 12 mm

0.461 m³

Paint, 2 coats

7 L

These material figures are estimates from the area alone. They use the correct modular coverage, but they carry no waste allowance and they assume a single skin. For an order, take the net area to the block, mortar or render calculator, which handle joints, waste and mix properly.

THE ELEVATION

A room is one measurement, not four. The perimeter of 18.00 m times the 2.4 m height gives 43.20 m² directly. Measuring each wall separately gives the same answer with four times as many chances to mistype something.

Live calculation · updates as you type

Created with❤️byeaglecalculator.com

HOW TO USE

1
Choose whether you are measuring a whole room or separate walls. For a room, enter its length, width and height and the calculator works from the perimeter — one measurement instead of four.
2
For separate walls, fill in as many of the four rows as you need and leave the rest at zero length. The walls do not need to be in the same plane; their areas simply add up.
3
Add gable ends as a count, a base width and a rise. The calculator shows the equivalent roof pitch beside it, which is worth checking against the roof you actually have.
4
Enter doors and windows to deduct. The result gives gross, deducted and net separately so you can check each against a drawing, plus material estimates from the net figure.

THE FORMULAS

One walllength × height

A whole room2 × (length + width) × height

Gable end½ × base × rise

Gable pitchatan(rise / (base ÷ 2))

Net areagross − openings

Bricksnet × 59.26

Blocksnet × 9.88

Paintnet × coats / coverage

WHAT A SQUARE METRE OF WALL TAKES

Material	Per m²	Per 100 ft²	Note
Bricks, half-brick wall	59.26	550.5	Standard 215 × 65 with 10 mm joints
Blocks, 100 mm	9.88	91.8	Standard 440 × 215 with 10 mm joints
Render at 12 mm	12.00 L	111.5 L	Wet volume before waste
Plaster at 2 mm skim	2.00 L	18.6 L	Finish coat over board
Paint, one coat	0.09 L	0.8 L	At 11 m² per litre

All figures are before wastage. Brick and block coverage comes from the module — the unit plus one joint — which is the same basis the block and mortar calculators use, so the three pages give consistent answers rather than three slightly different ones.

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Last updated: July 29, 2026 · Formula verified · Eagle-eyed accuracy for every calculation.