How to Estimate a Painting Job

Measure once, buy once. Which calculator to use for walls, ceilings, doors, cabinets and outside — and the three places people always come up short.

Nearly every painting job goes wrong in the same place: the second trip to the shop. Not because the maths is hard, but because people estimate the walls and forget that the ceiling, the doors, the primer and the second coat are all separate numbers.

This walks through them in the order you actually meet them.

Start with the surface, not the room

There is no single "how much paint for a room" answer, because a room is four or five different surfaces that cover at completely different rates. Work out each one on its own:

Walls → Paint Calculator
Ceiling → Ceiling Paint Calculator
Doors → Interior Door Paint Calculator
Cabinets → Kitchen Cabinet Paint Calculator
Outside → Exterior Paint Calculator
Wallpaper instead → Wallpaper Roll Calculator

Adding them up at the end gives you a shopping list. Estimating "the room" as one number gives you a second trip.

The three things people forget

The second coat. This is the big one, and it is not a rounding error — it is roughly double. The ceiling tool exists partly to make this obvious: a 12 × 14 ceiling is under a gallon for two coats, and the number that catches people is the second coat, not the first. Decide up front whether you are doing one or two, and be honest, because a dark colour over a light one is almost never one.

Primer is not paint. Primer covers 200–300 sq ft per gallon where paint covers about 350, so the same wall needs roughly 40% more primer than you would assume from your paint figure. That is what the Primer Calculator is for. Reusing your paint number for primer is the second most common way to come up short.

Openings cut both ways. Deducting doors and windows saves you paint on the walls — and then those doors need painting themselves, at a different coverage, on possibly two sides. The wall calculator takes the deduction; the door calculator gives you what the deduction created.

Cabinets are a different job

The Kitchen Cabinet Paint Calculator usually surprises people, and not in the direction they expect. A 20-door kitchen is about 156 sq ft — under a gallon for two coats.

The paint is never the expensive part of a cabinet job. The time is: doors off, hardware off, degreasing, sanding, priming, two thin coats a side, drying between each. If your budget for this job is mostly paint, the budget is wrong. Work out the paint in five minutes with the calculator, then spend your planning time on the sequence and the drying.

The drying is a number too

The Paint Drying Time Calculator puts figures on those waits — and on the fact that "dry" is three different claims.

At the conditions on the data sheet — 77°F and 50% humidity, the Sherwin-Williams baseline the tool is built on — dry to the touch is about 1 hour, ready for the next coat is 4 hours, and full cure is 14 days, counted from the last coat. Touchable and recoatable are hours apart; hard enough to scrub or rehang a door against is weeks away.

Cold and damp stretch all three. The same paint at 50°F is 1h 32m to the touch, over 6 hours to recoat, and 22 days to cure — which is why a job that was fine in July goes wrong in the same room in November.

Outside, measure the gables

The Exterior Paint Calculator asks for perimeter and wall height, and then it asks for the gable ends — the triangles above the eaves that people leave out because they are awkward to measure and easy to not see from the ground.

At the figures it opens with, a typical house comes to 9.49 gallons for two coats. Worth knowing: the substrate moves that number more than the brand does. Bare or porous surfaces drink the first coat, which is another reason the primer question is not optional outside.

If you are papering instead

Wallpaper does not work like paint at all, and the Wallpaper Roll Calculator is the one tool here where the arithmetic genuinely catches experienced people out.

You buy whole rolls, but you hang whole drops, and the pattern repeat decides how many drops you get out of one roll. A 21-inch repeat can cost you two extra rolls on the same wall — not because you use more paper, but because more of each roll becomes offcut. Always work this out before you buy, and buy the extra roll from the same batch while you are there.

The short version

Split the job by surface. Never estimate "the room".
Decide one coat or two before you calculate, not after.
Do primer separately — it covers less.
Deduct openings from walls, then calculate those openings as their own job.
Outside, do not skip the gables.
For paper, the repeat decides everything.
Dry to the touch is hours; cured is weeks — and cure counts from the last coat.

Each tool has a printable sheet with its own assumptions on it, so you can take the numbers to the shop and check them against what is actually on the tin.