Area and volume calculators

8 area and volume calculators: room area, wall area and paint, roof area, circle and sector, triangle, pit volume, cylinder and stairs.

8 tools, most used first

◱Room Area CalculatorFloor, ceiling, walls, skirting and volume, all at once.▯Wall Area and Paint CalculatorNet wall area with openings deducted, plus paint and filler.⌂Roof Area CalculatorTrue sloped roof area, rafter length and ridge rise from pitch.◯Circle and Sector CalculatorCircle area and circumference plus sector, arc, chord and segment.△Triangle Area CalculatorArea from three sides, with angles, inradius and circumradius.◣Battered Pit Volume CalculatorTop dimensions and frustum volume from base size, depth and batter.⬭Cylinder Volume CalculatorVolume, capacity, surface areas and weight for any cylinder.⌐Stair CalculatorRiser count and height, run, slope length and slab area, with comfort check.

What this group solves

Area and volume sit under every take-off: floor area drives flooring, wall area drives paint and filler, roof area drives waterproofing and tiles, and circular areas drive pipe insulation and concrete. These tools are deliberately thin — pure geometry, no materials or workmanship — so they can sit at the front of any calculation chain.

Three things get missed repeatedly. Wall area must deduct openings but add the reveal: roughly 0.3-0.5 m² per door and 0.2-0.4 m² per window. Roof area is the plan area divided by cos(pitch), not a tape measure along the slope. And stairs need rise, going and number of risers checked together — residential 260-300 mm going and 150-175 mm rise is what the code expects.

This is the most-used group on the site and the one most exposed to convention: does wall area deduct openings, does room area deduct wall thickness, is roof area measured on plan or on the slope? Every answer depends on what the area is for — pricing, material take-off or load calculation all use different conventions, and the same element can differ by over 12% between them. Ask the purpose first, then pick the tool.

Which one for your case

Room net area, wall area and ceiling area	Room Area Calculator
Wall area with door and window openings deducted	Wall Area and Paint Calculator
Roof area converted from plan to slope	Roof Area Calculator
Circle, sector, segment and annulus areas	Circle and Sector Calculator
Triangle area from three sides	Triangle Area Calculator
Pit or tank volume with side slopes	Battered Pit Volume Calculator
Cylinder, pipe and tank capacity	Cylinder Volume Calculator
Stair riser, going and flight length	Stair Calculator

Where this group goes wrong

Measuring rooms on centreline dimensions with no wall deduction. Deducting 240 mm each way turns a 3.6×4.2 room into 3.36×3.96 = 13.3 m², not 15.1 m² — 12% out, and flooring and paint are both over-ordered.
Deducting all openings or none. The convention deducts individual openings over 0.3 m² and does not add reveal area; tenders often deduct everything and price reveals separately. The two approaches differ by about 5%.
Reporting roof area on plan. At 30° the slope area is 1.155 times the plan, at 45° it is 1.414; flat roofs still carry a 2% structural fall, and membrane laps add another 10%.
Mixing the three stair conventions. Concrete is sloped length × slab thickness, finishes are developed area, formwork is plan area — on one staircase those differ by more than 20%, so mixing them guarantees at least one wrong figure.

FAQ

How do I measure wall area properly?

Floor area × 2.5 to 3.0 is only a rule of thumb. Measure each wall: length × height, less openings, plus the reveals. Reveals are the opening perimeter times half the wall thickness — about 0.3-0.5 m² per door and 0.2-0.4 m² per window. Paint is then converted at 0.25-0.35 kg/m² over two coats.

How much bigger is the roof than the plan area?

By a factor of 1/cos(pitch): 1.035 at 15°, 1.103 at 25°, 1.155 at 30°, 1.221 at 35° and 1.414 at 45°. So a 100 m² plan at 30° is 115.5 m² of roof, and that is the number to order membrane against — 15% is real money.

How do I size stair treads and risers?

The rule of thumb is 2h + b = 600-630 mm, with h the rise and b the going. Residential uses 150-175 mm rise and 260-300 mm going, public buildings 140-160 and 280-340. Storey height divided by the number of risers gives the rise, and every riser must be identical or the last step becomes a trip hazard.

How far apart are net room area and gross floor area?

Gross floor area includes walls and shared circulation; net area is the usable internal space. High-rise housing achieves 72-80%, low-rise 80-88%, and retail over 85%. Price on gross area, order flooring and paint on net — never mix the two.

How is sloped roof area converted?

Plan area ÷ cos(pitch). At 30° divide by 0.866 for 1.155 times, at 45° divide by 0.707 for 1.414. Where the fall is given as a percentage i, the factor is √(1+i²): 2% gives 1.0002 and 5% gives 1.0012, so a flat roof's fall barely changes the area — only pitched roofs are affected significantly.

What riser and going are required for stairs?

Dwellings: riser 150-175 mm, going 260-300 mm. Public buildings: 140-160 mm and 280-340 mm. Kindergartens and primary schools: 120-150 mm and 260-300 mm. All must satisfy 2h + b ≈ 600-620 mm, and risers within one flight must not vary by more than 10 mm.

How is the area of an irregular shape calculated?

Break it into rectangles, triangles and sectors and add them up. Truly irregular outlines yield to the coordinate (shoelace) method: list the boundary points in order and the area falls out, far more accurately than counting squares. Without coordinates, overlay a grid and count whole plus half squares — accurate to within about 5%.

Related data tables

Area and volume formulasThe bottom layer of every take-off: plane areas and solid volumes with the symbols spelled out.

Other groups

Concrete Rebar Masonry Finishes Earthwork Temporary Works Construction Costing M&E Material weights Unit conversion Code Lookup

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.