Paint quantity estimate

The geometry is a strip unrolled. A rectangular room’s walls are its perimeter times its height, as if the four walls were peeled off and laid flat as one band; the ceiling, when included, adds the plan area on top. The engine reports these surfaces separately as well as combined, so a doubtful total can be traced back to the dimension that produced it.

Openings are the honest subtraction. Doors and windows occupy wall the roller never touches, and their combined area comes off before the litres are counted. The default is a modest allowance for an ordinary room; a glazed wall or a run of patio doors deserves its true measured figure, because glass is the one part of a wall paint cannot pretend to cover.

When the openings meet or exceed the surfaces, the engine clamps the paintable area at zero rather than letting it go negative — refusal-shaped honesty inside the arithmetic. A zero result is not a malfunction; it is the calculator reporting that the inputs describe more door and glass than room, which is almost always a wall measured in one unit meeting openings measured in another. Negative litres would be nonsense; zero litres is a message.

Coats multiply area, not paint. Each coat is the whole paintable surface passed over again, so the litre figure scales directly with the count — and the count is where judgement lives. Fresh plaster drinks its first coat; a deep colour over a pale one hides in fewer passes than the reverse; the coverage on the tin was measured on a sealed, smooth surface that most walls only resemble.

Coverage itself is the input to respect most. It is a property of the specific product at the manufacturer’s stated spreading rate, in area per litre, and it varies more between paints than rooms vary between houses. Entering the tin’s true figure — not a remembered one — is the single cheapest improvement to the estimate, because every litre in the answer is the area divided by exactly this number.

The geometry of a rectangular room is friendlier than it looks. Peel the four walls off and lay them flat, end to end, and they form one continuous band: the perimeter long, the wall height tall. That single mental move — the strip unrolled — is the whole wall calculation, and it explains the counter-intuitive fact that catches tall rooms out: every extra unit of height adds a full perimeter of wall, which is why lofty rooms surprise people at the paint counter.

The ceiling, when it joins the job, simply adds the plan area on top of the band. The engine reports the surfaces separately as well as combined, and that separation is deliberate: a doubtful total can be traced back to the dimension that produced it instead of being re-guessed whole. An L-shaped or broken plan is better run as two rectangles, each owning its own openings, than forced into one.

Doors and windows occupy wall the roller never touches, and their combined area comes off before any litre is counted. The default is a modest allowance for an ordinary room — one door, a window — and it exists so a quick estimate is not derailed by fiddly measuring. The moment glazing dominates a wall, the default stops being honest: a run of patio doors deserves its true measured figure, because pretending glass is wall buys paint for a surface that does not exist.

The subtraction has a floor, and the floor is the calculator’s quietest teaching. When the openings meet or exceed the surfaces, the engine clamps the paintable area at zero rather than letting it go negative. Zero litres is not a malfunction — it is the calculator reporting that the inputs describe more door and glass than room, which is almost always one measurement made in one unit meeting another made in a different one. Negative litres would be nonsense dressed as an answer; zero is a message worth reading.

A second coat is not a top-up; it is the entire paintable surface paid for again. The litre figure therefore scales directly with the coat count, and the count is where the estimate’s judgement lives. Fresh plaster and new board drink their first coat; a deep colour hides a pale one in fewer passes than the reverse; a patched wall telegraphs its repairs through thin coverage. None of this changes the room — it changes how many times the room must be crossed.

This is also why the coat count is the right place to be honest about surface condition, rather than quietly inflating the area or discounting the coverage. The area is a fact of geometry and the coverage is a fact of the product; the coats are the one input that is genuinely yours.

Coverage is the input to respect most, because every litre in the answer is the area divided by exactly this number. It is a property of the specific product at the manufacturer’s stated spreading rate, and it varies between products far more than most people expect — matt and silk, budget and premium, primer and topcoat all spread differently. Entering the datasheet figure of the paint actually being bought, rather than a remembered one, is the single cheapest improvement an estimate can receive.

The promise was also made on a sealed, smooth laboratory surface that most walls only resemble. The estimate takes the tin at its word and lets the coat count carry the honesty about texture and porosity — which is why comparing two products means running the same room at each tin’s own coverage, and why the result is often a better price comparison than the shelf labels themselves.

The engine below is the same certified engine the calculator page mounts, verified before it opens and pre-filled with the pack’s worked example: an ordinary room with its default door-and-window allowance, walls only. Flip the ceiling switch and watch exactly what the ceiling costs; push the openings past the walls and watch the paintable area clamp to zero — the pack declares that behaviour as a test vector of its own.

Read the surfaces before the litres: the wall band, the ceiling entry, and the paintable area after the openings come off. The litre figure is that area paid once per coat at the tin’s coverage — computed live by the verified engine, never stored on this page.