Why use this calculator
Paint is sold in fixed tin sizes and tinted to order, so getting the quantity wrong costs either way. Buy too little and the second batch may not match the first, leaving a visible line halfway along a wall. Buy too much and a tinted tin cannot be returned.
Most rule-of-thumb figures also ignore the things that actually change the answer: the spread rate differs by product and brand, textured and rendered walls drink a quarter more paint, new plaster needs a sealer coat before any colour goes on, and a strong colour change needs a third coat.
This calculator uses your tin's own spread rate, allows for the surface, adds primer where needed and turns litres or gallons into the tins you will carry out of the shop.
How to use it
- Choose metric or imperial to match your tape measure and the tins in your local store.
- Enter the room length and width measured corner to corner along the skirting, and the wall height from floor to ceiling.
- Count the doors and windows so their area is taken off the walls.
- Pick the wall surface: previously painted, new bare plaster, or textured, rendered or brick. Set the number of topcoats, normally two.
- Copy the spread rate from the tin or the product data sheet.
- Choose whether to include the ceiling, and enter the price per litre or gallon to see a cost.
What each result means
- Wall paint to buy
- The volume of topcoat to buy for the walls, rounded up to whole tins so you are never short on the last wall.
- Wall paint left over
- The difference between the whole tins you buy and the exact amount the walls need, which is the paint you will have spare for touch-ups once the job is done.
- Suggested tins
- The combination of standard tin sizes that covers the need with the least leftover, then the fewest tins to carry.
- Exact wall paint needed
- The unrounded volume for your walls and coats. The gap between this and the tins bought is your touch-up supply.
- Primer or sealer
- One coat of sealer or primer for new bare plaster or plasterboard, walls and any included ceiling, rounded up to tins.
- Ceiling paint
- Flat ceiling paint for the room's ceiling area and the same number of coats, bought separately because ceiling paint is a different product.
- Paintable wall area
- The wall surface left after the doors and windows are taken off. One coat covers this area once.
- Doors and windows deducted
- The area removed for openings using typical sizes. Enter fewer if your doors or windows are small or you will paint the doors too.
- Coverage rate used
- The spread rate the sums used after any textured surface allowance. Compare it with the tin if the answer looks high.
- Estimated paint cost
- Everything bought, topcoat, sealer and ceiling paint, multiplied by your price per litre or gallon, before brushes, rollers and tape.
The formula
Gross wall area = 2 x (length + width) x height, in square metres or square feet
Paintable area = gross wall area minus doors x 1.9 m² (20 sq ft) minus windows x 1.5 m² (15 sq ft)
Effective spread rate = the tin's spread rate x the surface factor, which is 1.0 for smooth painted or plaster walls and 0.75 for textured surfaces
Wall paint = paintable area x coats / effective spread rate, in litres or gallons
Sealer for new plaster = (paintable area + ceiling area) / (spread rate x 0.8), one coat
Ceiling paint = length x width x coats / effective spread rate
Each quantity is then rounded up to the smallest combination of 10, 4 and 1 litre tins, or 5 gallon, 1 gallon and quart cans.
How this number is used in practice
Spread rates on tins are measured on smooth, sealed, non-porous test panels, and real walls rarely match them. Australian and UK interior acrylics typically quote 12 to 16 square metres per litre; US latex paints usually quote 350 to 400 square feet per gallon, which is about 8.6 to 9.8 square metres per litre.
Rough render, bagged brick and textured coatings can use a quarter to a third more, rollers with a long nap load more paint than short ones, and a spray gun loses a share to overspray. Painters treat the tin figure as a best case and round up.
New plaster and plasterboard are porous and patchy in suction. Painting colour straight on gives a blotchy, flashing finish and wastes topcoat, so manufacturers call for a sealer undercoat or, in the UK, a mist coat of diluted emulsion before the finish coats. Bare timber, stain-blocking over water marks, and going from a dark colour to a light one also call for a primer. When the colour change is large, a tinted primer close to the new colour often saves a third topcoat.
Buy all the paint for one room at once and ask for it from the same batch, or box the tins by pouring them into one bucket before you start, so the colour is consistent from wall to wall. Keep the leftover tin labelled with the colour name, formula and room for touch-ups.
In homes built before about 1970 in Australia or 1978 in the United States, test old paint for lead before sanding it.
Worked examples
A 4 m by 3.5 m bedroom, 2.4 m walls, one door, one window, previously painted, two coats at 12 m² per litre
32.6 m² of paintable wall needs 5.43 litres, so buy 6 litres as one 4 L and two 1 L tins
The same room freshly plastered, with the ceiling included
Walls still need 6 litres, a sealer coat adds 5 litres and the 14 m² ceiling needs 3 litres, 14 litres at a cost of $420.00
A 12 by 10 ft US room with 8 ft walls, one door, two windows, textured walls, two coats at 350 sq ft per gallon
302 sq ft of wall needs 2.30 gallons at the reduced textured spread rate of 262.5 sq ft per gallon, so buy 2.50 gallons: two gallon cans and two quarts
When people use it
- Buying the right number of tins for a bedroom or living room repaint
- Pricing the sealer and topcoats for a new plasterboard extension
- Comparing the cost of two coats against a primer and two coats
- Checking a painter's quote for the quantity of paint allowed
- Planning a whole-house repaint room by room
Tips
- Use the spread rate on the actual tin you plan to buy. Premium and budget paints can differ by 30 per cent or more.
- Measure at skirting height. Walls are often slightly out of square, and the skirting line gives the true run.
- Seal new plaster before any colour goes on, otherwise the topcoat soaks in unevenly and you end up applying an extra coat.
- For a big change from dark to light, use a tinted primer instead of a third topcoat. It is usually cheaper and hides better.
- Keep at least half a litre or a quart of leftover paint for touch-ups, labelled with the colour code and the room.
- Buy one batch for the whole job, or mix all the tins together in a bucket before starting, so every wall matches.
What it assumes, and where it stops being accurate
- Rooms are treated as rectangles with walls of equal height. Sloped ceilings, bulkheads and stairwells need measuring separately.
- Doors are 1.9 m² (20 sq ft) and windows 1.5 m² (15 sq ft). Painting the doors themselves, frames and skirting is not included.
- Textured surfaces use 75 per cent of the tin's spread rate. Very rough brick or render can need even more paint than this allows.
- Sealer on new plaster is one coat spreading 80 per cent as far as the topcoat. Follow the sealer's own data sheet where it differs.
- Tin sizes are 10, 4 and 1 litres or 5 gallons, 1 gallon and 1 quart. Local ranges vary, especially in the UK.
- Cost uses one price for every product. Sealer and ceiling paint are often cheaper per litre than premium wall paint.
Terms used on this page
- Spread rate
- The area one litre or gallon of paint covers in a single coat, printed on the tin and data sheet.
- Sealer undercoat
- A first coat that evens out the suction of new plaster or plasterboard so topcoats dry uniformly.
- Mist coat
- UK practice of thinning emulsion with water for the first coat on new plaster, used in place of a sealer.
- Topcoat
- The finish coats of coloured paint applied over a sound or sealed surface.
- Boxing
- Mixing several tins of the same colour together before painting so every wall matches exactly.
Questions people ask
- How much wall does a litre of paint cover?
- Most interior acrylic wall paints in Australia and the UK cover 12 to 16 square metres per litre per coat on smooth, sealed walls. That is the best case: textured, porous or dark surfaces cover less. Check the spread rate on the tin or data sheet and enter it here rather than relying on a generic figure.
- How many coats of paint do I need?
- Two topcoats is the standard for repainting a sound wall, even in the same colour, because one coat rarely hides evenly. Use three for strong reds, yellows and deep colours, or for a large change from dark to light, unless you use a tinted primer first. New plaster needs a sealer before the two topcoats.
- Do I need primer on new plaster?
- Yes. New plaster and plasterboard are porous and absorb paint unevenly, which leaves a patchy finish and wastes topcoat. Use a sealer undercoat made for plasterboard, or in the UK a mist coat of diluted emulsion, and let fresh plaster dry fully first. This calculator adds one sealer coat when you choose new plaster.
- How much paint do I need for a ceiling?
- Multiply the room length by the width for the ceiling area, then divide by the spread rate for each coat. Choose yes to include the ceiling and the calculator shows it as a separate purchase, because ceiling paint is a flat, low-spatter product sold separately from wall paint.
- Why does my textured wall need more paint?
- A textured or rendered surface has much more real surface area than its flat measurements suggest, and the peaks and hollows hold extra paint. Manufacturers typically say to expect a quarter to a third less coverage on rough surfaces, so this calculator reduces the spread rate by 25 per cent when you choose textured.
- Why are US figures quoted per gallon?
- Paint in the United States is sold in US gallons of 3.785 litres, quarts and 5 gallon pails, and coverage is quoted as square feet per gallon, typically 350 to 400. Switch to imperial to work in those units. The UK sells paint in litres despite often measuring rooms in feet.
Use this with
Paint Calculator answers one part of the question. These pick up where it stops: Flooring Calculator, Tile Calculator, Area Calculator and Renovation ROI Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- Dulux Australia. Product spread rates and surface preparation for new plaster
- Dulux UK. UK tin sizes, mist coats and coverage guidance
- US Environmental Protection Agency: Lead. Testing and safe work practices for older painted surfaces
- Standards Australia. AS/NZS 2311, the guide to painting of buildings
Paint Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.