Paint Calculator
Work out how much paint a room or wall needs, for any number of coats, plus the cost.
How to use this calculator
- 1Measure the area. For an irregular space, split it into rectangles, work each out, and add them.
- 2Set the unit you measured in — the calculator converts everything for you.
- 3Check the coverage figure against the bag or tin you are actually buying; it varies between brands.
- 4Leave the waste allowance on. Running short mid-job costs far more than one spare unit.
How the calculation works
Units = ⌈ (Area × (1 + waste)) / Coverage per unit ⌉- Area
- Surface area of the job
- waste
- Allowance for cuts, breakage and spillage
- ⌈ ⌉
- Round up — you cannot buy a fraction of a unit
Every dimension is converted to metres internally, so feet, yards and metres can be mixed with inches or centimetres without error.
For multiple coats, multiply the area by the number of coats before dividing by coverage — or simply divide the coverage figure by the number of coats.
Manufacturer coverage figures assume a smooth, sealed, previously painted surface. Bare plaster, masonry and textured wallpaper can halve them.
Worked example
A 20 × 12 ft area
- 1.20 ft × 12 ft = 240 sq ft, which is 22.30 m².
- 2.Divided by the coverage per litre, that gives the raw requirement.
- 3.A 0% waste allowance is added, then the figure is rounded up to whole litres.
Result: See the result panel — it updates live as you change the numbers.
What determines how much paint a job needs
Paint coverage comes down to area, the number of coats, and how absorbent the surface is. The figure printed on a tin — coverage per litre or gallon — is measured under close-to-ideal conditions: a smooth, sealed, already-painted surface. Real walls are rarely that cooperative, so the printed figure is closer to a best case than an average.
Texture and porosity matter more than most people expect. Bare plaster, new drywall, masonry, and heavily textured surfaces can soak up nearly twice as much paint on the first coat as a smooth, previously painted wall — which is exactly the gap that a primer or mist coat is designed to close before the finish coats go on.
Paint types and sheen levels
- Matte / flat — hides surface imperfections well and gives the least reflective finish, but is generally the least washable, which makes it a common choice for ceilings and low-traffic walls.
- Eggshell / satin — a soft, low-sheen finish that’s more washable than flat paint, often used as a middle-ground choice for living areas and bedrooms.
- Semi-gloss — noticeably reflective and more resistant to moisture and scuffing, a common choice for trim, doors, kitchens, and bathrooms.
- Gloss — the most reflective and most durable finish, but also the most revealing of any surface imperfection underneath it.
- Primer — not a finish coat at all — a preparatory layer that seals the surface, improves adhesion, and evens out absorbency so the finish coats go on consistently.
Estimating a paint job
A methodical approach avoids both the under-order that means a mid-job trip to the store and the over-order that leaves half-used tins behind.
- 1Measure the perimeter and height — multiply the wall perimeter by the ceiling height to get the gross wall area.
- 2Decide whether to subtract openings — subtracting windows and doors is more accurate, but leaving them in provides a built-in safety margin on a typical room — worth doing the subtraction on a wall that’s mostly glazing, though.
- 3Divide by the coverage rate, per coat — using the figure on the specific tin being bought, not a generic estimate, since coverage varies noticeably between products and finishes.
- 4Multiply by the number of coats — two coats is standard for a colour change on a previously painted wall; budget for three when covering a dark colour with a lighter one, or vice versa.
Where estimates commonly go wrong
- Trusting the tin’s coverage figure on an unsealed surface — bare plaster, new drywall and masonry can use noticeably more paint on the first coat than the printed figure assumes.
- Forgetting primer is a separate product — with its own coverage rate, not simply "paint used more thinly."
- Underestimating a strong colour change — going from a dark wall to a light one, or the reverse, often needs a third coat that a same-tone repaint wouldn’t.
What this assumes, and where it stops
Assumptions
- The area is flat and regular as described. Slopes, steps and obstructions change the quantity.
- Coverage figures are the manufacturer’s stated values, which assume good conditions and an even surface.
- One coat unless you adjust the coverage figure.
- The surface is prepared and sealed.
Limitations
- This estimates material only — not labour, tools, delivery, hire or disposal.
- Manufacturer coverage varies between brands and batches. Always check the packaging.
- Does not subtract windows and doors. On a typical room that is a useful safety margin; on a wall that is mostly glazing, measure the painted area directly.
- Primer and undercoat are separate products with their own coverage rates.
Common questions
How much paint do I need for a room?
Measure the perimeter of the room and multiply by the wall height for the wall area, then divide by the coverage rate on the tin. A typical 4 × 4 m room with 2.4 m ceilings has about 38 m² of wall — roughly 3.5 litres for one coat, or 7 litres for two.
Why did I run out of paint?
Almost always surface absorbency. Coverage figures on the tin assume a smooth, sealed, previously painted wall. Bare plaster, new render and textured surfaces can use nearly twice as much on the first coat — which is exactly what a mist coat or primer is for.
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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