Paint Calculator

The arithmetic, and the part people skip

Wall area is perimeter times height: 2 × (length + width) × height. A 16 × 12 foot room with 9-foot ceilings has 504 square feet of wall before openings. A standard door is about 21 square feet and a window about 15, so a room with two doors and three windows loses 87 square feet — 17 per cent of the job, and enough to change how many tins you buy.

Coverage on the tin is a best case

Three hundred and fifty square feet per gallon assumes smooth, sealed, previously painted wall in a similar colour. Knockdown texture can drop that to 250. Bare drywall drinks the first coat regardless of what you use, which is what primer is for: it is cheaper than topcoat and it stops the expensive paint doing the sealing.

Two coats is the realistic default

One coat works only over a near-identical colour on a sealed surface. Any real colour change needs two, and a dramatic one — dark over light, or a saturated red over anything — may need three even with a tinted primer. It is worth buying the second gallon at the same time so the batch numbers match; colour can vary slightly between batches.

Coverage on the tin assumes a surface you probably do not have

A gallon nominally covers 350 to 400 square feet, and that figure assumes a smooth, sealed, previously painted wall in sound condition. New drywall, bare plaster, raw wood and masonry all absorb substantially more on the first coat, which is the entire argument for primer: it is cheaper per gallon than finish paint and it stops the expensive coat from disappearing into the substrate. Textured surfaces increase the actual painted area well beyond the measured area, with heavy texture or popcorn adding a large fraction.

Two coats is the realistic default rather than a premium option. One coat almost never gives uniform colour and sheen, and the failure is most visible on walls lit at a shallow angle by a window. Dramatic colour changes, particularly light over dark or any strong red, may need a tinted primer plus two finish coats to avoid a third. Buy the total quantity in one purchase so that every gallon comes from the same tint batch, and box multiple cans together before starting if the job spans more than one.

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