Area & volume helper

Most rooms aren't a clean rectangle. There's a chimney breast eating into one wall, a bay window pushing out, or the whole room is L-shaped because of how the building was extended over the years. Working out the floor area or the volume of air in a room like that means splitting it into simpler shapes first — usually two rectangles — working out each one separately, and adding them together. This tool does that split for you: enter the length and width of the main rectangle, the length and width of the smaller section, and a shared ceiling height, and get the total floor area and total volume back in one go.

Why split into two rectangles

Area and volume formulas only work cleanly on rectangles — length times width for area, and that area times height for volume. An L-shaped or irregular room doesn't fit either formula directly, so the standard trick is to draw a single straight line across the awkward bit and treat what's left as two ordinary rectangles sitting next to each other. Measure each one on its own, add the two areas together for the total floor area, then multiply that total by the ceiling height for the total volume — assuming the ceiling is flat and the same height right across both sections, which is true for most standard rooms.

This is useful for costing flooring, screed or plaster by area, or for sizing a heater, extractor fan or dehumidifier by the volume of air it has to work with. Both jobs need the real total, not just the figure for the main rectangle, and it's easy to forget the smaller section entirely when you're working it out on paper.

A worked example

Take an L-shaped bedroom: a main rectangle 4.0m long by 3.0m wide, with a smaller return along one wall measuring 2.0m by 1.5m, and a ceiling height of 2.4m throughout. Rectangle A comes to 4.0 × 3.0 = 12.0m², and rectangle B comes to 2.0 × 1.5 = 3.0m², giving a total floor area of 15.0m². Multiply that by the 2.4m ceiling height and the total volume of the room is 36.0m³. Set rectangle B's length or width to 0 and the tool collapses back down to a single rectangle — handy if you start out with a simple room and only need to add the awkward section later.

What this isn't

This only handles rooms that split cleanly into two rectangles — a genuinely irregular shape needing three or more sections, or one with an angled wall, will need working out by hand or split into more pieces than this tool covers. It doesn't subtract anything out either: a chimney breast, built-in cupboard or stairwell eating into the floor area will still be counted as part of whichever rectangle it sits inside, so knock that off separately if it matters for the job. And it assumes a flat ceiling at one constant height — a sloped ceiling under a roofline changes the volume sum rectangle by rectangle, so treat the volume figure here as a starting point rather than the final number on anything with restricted headroom.