# Area and volume

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.

- Category page: https://jianzhujisuan.com/en/c/area-volume
- Data checked: 2026-10-25

## Tools in this group

- [Room Area Calculator](https://jianzhujisuan.com/en/room-area): Floor, ceiling, walls, skirting and volume, all at once.
- [Wall Area and Paint Calculator](https://jianzhujisuan.com/en/wall-area): Net wall area with openings deducted, plus paint and filler.
- [Roof Area Calculator](https://jianzhujisuan.com/en/roof-area): True sloped roof area, rafter length and ridge rise from pitch.
- [Circle and Sector Calculator](https://jianzhujisuan.com/en/circle-calc): Circle area and circumference plus sector, arc, chord and segment.
- [Triangle Area Calculator](https://jianzhujisuan.com/en/triangle-area): Area from three sides, with angles, inradius and circumradius.
- [Battered Pit Volume Calculator](https://jianzhujisuan.com/en/pit-volume): Top dimensions and frustum volume from base size, depth and batter.
- [Cylinder Volume Calculator](https://jianzhujisuan.com/en/cylinder-volume): Volume, capacity, surface areas and weight for any cylinder.
- [Stair Calculator](https://jianzhujisuan.com/en/stair-calc): Riser count and height, run, slope length and slab area, with comfort check.

## Questions

### 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.