Paint Calculator for a Room or House
Litres of emulsion, primer and wall putty for a room or any wall area, with the tins to buy and the material cost.
How the paint calculation is done
Paint is sold by the litre, and every tin says how much area one litre covers. The calculator turns your room into a paintable area and then into litres.
- Wall area. 2 x (length + width) x height. This is the area of the four walls of the room.
- Ceiling. If you paint it, length x width is added.
- Openings. Each door is taken as 7 x 3 ft (21 sq ft) and each window as 4 x 4 ft (16 sq ft), and these are subtracted.
- Rooms. The area is multiplied by the number of rooms of the same size. For other shapes, or for the outside of a building, choose “Total area I already know” and type the area.
- Paint. Litres = area x number of coats / coverage per litre per coat. The tin or the product data sheet gives the coverage.
- Primer and putty. Primer is one coat at 100 sq ft per litre. Wall putty is two coats at 12 sq ft per kg. Both are common label values; change them in the script if your product says otherwise.
Paint litres are also shown as tins of 20, 10, 4 and 1 litre, the usual pack sizes, so you know what to buy. The cost is the litres and kg bought (rounded up) x the prices you enter.
Worked example (default inputs)
One room 12 ft x 10 ft, 10 ft high, ceiling painted, one door and one window, two coats of interior emulsion at 120 sq ft per litre per coat, with putty. Paint at Rs 300 a litre, primer Rs 200 a litre, putty Rs 25 a kg.
- Walls: 2 x (12 + 10) x 10 = 440 sq ft.
- Ceiling: 12 x 10 = 120 sq ft.
- Less one door (21) and one window (16): 523 sq ft to paint.
- Paint: 523 x 2 / 120 = 8.7 litres. Buy 9 litres: two 4 litre tins and one 1 litre tin.
- Primer: 523 / 100 = 5.2 litres, so 6 litres.
- Putty: 523 / 12 = 43.6 kg, so 44 kg.
- Material cost: 9 x 300 + 6 x 200 + 44 x 25 = Rs 5,000.
For the outside of a house with 2,500 sq ft of wall, two coats of exterior emulsion at 70 sq ft per litre per coat need about 71.4 litres. That is three 20 litre buckets, one 10 litre bucket and two 1 litre tins, plus 25 litres of primer.
Coverage: what the numbers mean
Coverage (also called spreading rate) is the area one litre covers in one coat on a smooth, primed surface. It is lower on rough plaster, on new walls that soak up paint, and for textured finishes. Labels often give a range. Use the lower end of the range for new plaster and the outside of a building.
| Item | Default used here | What changes it |
|---|---|---|
| Interior emulsion | 120 sq ft per litre per coat | Brand, dilution, smooth or rough wall |
| Exterior emulsion | 70 sq ft per litre per coat | Rough plaster and texture lower it a lot |
| Primer | 100 sq ft per litre, one coat | New plaster soaks up more |
| Wall putty | 12 sq ft per kg for two coats | How uneven the wall is |
These are starting values, not the figure for your product. The tin or the company’s data sheet is the source to use.
Common mistakes when estimating paint
- Using the floor area. A 12 x 10 ft room has 120 sq ft of floor but about 440 sq ft of wall. Paint is for walls and ceiling.
- Forgetting the second coat. Almost every emulsion needs two coats for an even colour. A dark colour over a light one can need three.
- Skipping primer on new plaster. Without primer, the wall soaks up the first coat of paint, and you buy more paint than primer would have cost.
- Taking the best coverage on the label. The top of the range is for ideal surfaces. Most sites get less.
- Not curing and drying the plaster. Paint on damp plaster peels and shows patches. Let new plaster dry out fully before painting.
- Different batches of a tinted colour. Buy all the paint for one room together, or ask for the same tint code on every tin.
Paint in the finishing budget
Paint is the last coat on the plaster, so estimate the plaster first with the plaster calculator, the tiled areas with the tile calculator, and the rest here. For a whole-house figure use the house construction cost calculator. Painting is often given to a contractor at a rate per sq ft; record the measured area before you pay, using a measurement book format.
In the Trackovo app, the paint, primer and putty are requested from the site, approved and ordered by the office, and received with a photo of the material and the bill, so each site’s stock is clear. The painting contractor’s measured work and payments can be kept with the site: see material management and labour management. The free Excel templates page has a stock register if you are starting on paper.
Common questions
How much paint is needed for a 10 x 12 room?
A 12 x 10 ft room with 10 ft walls has 440 sq ft of wall and 120 sq ft of ceiling. After one door and one window, about 523 sq ft. Two coats at 120 sq ft per litre per coat need about 8.7 litres, so 9 litres of emulsion, plus about 6 litres of primer.
How much area does 1 litre of paint cover?
It depends on the product and the wall. Interior emulsions often cover around 100 to 140 sq ft per litre per coat on a smooth primed wall, and exterior emulsions less on rough plaster. The tin gives the figure for that product.
How many coats of paint are needed?
For emulsion: one coat of primer and two coats of paint on new plaster, with putty before the primer if you want a smooth finish. For repainting in the same colour, two coats are usually enough without fresh putty.
How much wall putty is needed per sq ft?
A common label value is about 10 to 12 sq ft per kg for two coats, so 100 sq ft needs about 8 to 10 kg. Very uneven walls take more.
Is the paint calculator for interior or exterior walls?
Both. Choose exterior emulsion to use a lower coverage, and use “Total area I already know” to type the outside wall area. The method is the same: area x coats / coverage.
Request the paint, primer and putty from these numbers and check what was used against the area painted: track material and labour in Trackovo. Trackovo is a construction site management app with material, labour attendance, DPR and payments, with a 14-day free trial.
