Paint estimating is area × coats ÷ coverage. The skill is in choosing a realistic coverage figure for the surface in front of you, and in remembering the parts of a room that aren't flat walls: ceilings, woodwork, doors and the primer under it all.
Measuring a room
Add the lengths of all the walls to get the perimeter, then multiply by the wall height. Take off doors and large windows. Ceilings are length × width. For stairwells and sloped ceilings, split the surface into rectangles and triangles. Measure the woodwork separately: baseboards and trim in running feet, and doors by count.
Coverage rates in the real world
| Surface | Typical coverage |
|---|---|
| Smooth, previously painted wall | 350–400 sq ft per 3.78 L |
| New drywall (after primer or mist coat) | 300–350 sq ft per 3.78 L |
| Textured ceiling, rough stucco | 200–300 sq ft per 3.78 L |
| Brick, block, textured stucco | 150–250 sq ft per 3.78 L |
| Bare or weathered wood | 250–300 sq ft per 3.78 L |
The can's figure is a best case, measured on a smooth, sealed surface with a fresh roller. Assume a bit less, especially on your first job with a new product.
How many coats
- Same colour refresh: often one coat, but two gives a more even finish.
- Colour change: two coats, and three for strong colours or dark to light. A tinted primer can save a coat.
- New plaster or drywall: a coat of primer, then two finish coats.
- Patched walls: spot-prime the repairs first, or they'll show through as dull patches ("flashing").
Deductions
Deduct doors and large windows. Don't bother with small openings: cutting in around them uses as much paint as the area you'd save.
Woodwork and trim
Doors, frames and baseboards take a different product (trim enamel or semi-gloss) and are often missed. As a rough guide, one 946 mL can covers the trim of a typical bedroom with two coats. Allow about half a 946 mL can per door, both sides, for two coats.
Spraying vs rolling
Airless spraying uses 20–30% more paint than rolling because of overspray, but it's much faster on large, empty areas such as new builds and ceilings. Masking takes longer, so spraying pays off most on big, open jobs.
Exterior work
Outside, the surface makes a huge difference. Rough stucco, split-face block and brick can use twice the paint of smooth siding or boards. Gable ends are triangles: ½ × width × height. Soffits and fascias are extra, and so are the access costs: ladders, towers or scaffold. Weather windows matter too. Most paints need dry conditions and a minimum temperature to cure properly.
Buying the right sizes
Paint is sold in 946 mL cans, 3.78 L cans (the old gallon) and 18.9 L pails. A pail is usually cheaper per litre and lets you box (mix) several cans for consistent colour. Our calculator picks a sensible combination so you don't overbuy.
A worked example
A 14 × 12 ft bedroom with 8 ft walls, one door and two windows: walls 366 sq ft, ceiling 168 sq ft. Two coats at 350 sq ft per 3.78 L can: about 2.1 cans (8 L) for the walls and 1 can for the ceiling. Buy three 3.78 L cans of wall paint and one of ceiling white, plus a 946 mL can of trim paint.
Prep is the real cost
On most repaint jobs, preparation (filling, sanding, caulking, masking, protecting floors and furniture) takes longer than painting. Price it as its own labour line, and the client will understand why two painters quote very different prices.
Use the paint calculator for rooms and the exterior paint calculator for outside walls with gables and textured surfaces.
Common questions
How much paint for one wall?
Multiply the wall's width by its height, double it for two coats, and divide by the coverage. A 12 × 8 ft wall with two coats at 350 sq ft per 3.78 L can needs about half a can (2 L), so buy one 3.78 L can.
Do I need primer?
On new plaster or board, bare wood, stained areas and big colour changes, yes. On a sound, previously painted wall in a similar colour, usually not.
Checklist before you send the price
- Walls, ceilings and woodwork measured separately
- Coverage set for the real surface, not the can's best case
- Primer or mist coat where needed
- Number of coats agreed, especially for colour changes
- Preparation, protection and clean-up priced as labour
- Access equipment for high walls and stairwells
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.