Buying paint is one of those jobs where getting the quantity wrong costs you either money or a second trip. Too little and you’re back at the shop mid job, hoping the batch matches. Too much and you’ve got half a tin of a colour you’ll never use again sitting in the shed.
Here’s how to work it out properly before you buy.
The Basic Formula
Everything starts with wall area.
Measure the length of each wall and multiply by the ceiling height. Add those together for the total wall area. Then subtract the doors and windows.
A standard internal door takes up around 1.6 square metres. An average window is around 1.5 to 2 square metres, though large sliding doors and picture windows will be considerably more.
Once you’ve got your net wall area, divide by the coverage rate on the tin.
What Coverage Rate Really Means
Most interior paints quote coverage somewhere between 12 and 16 square metres per litre.
That figure assumes a smooth, sealed, previously painted surface applied at the recommended thickness. Real world conditions rarely match the tin.
Things That Reduce Coverage
Bare plasterboard soaks up paint. Expect your first coat to cover 30 to 50 percent less than the stated rate.
Textured or rough render increases surface area significantly. A heavily textured wall can eat 40 percent more paint than a flat one.
Dark to light colour changes need more coats. Going from navy to white might take three coats even with a good primer.
Rollers with a longer nap hold and deposit more paint, which improves coverage on rough surfaces but uses more product.
Spraying wastes more than rolling due to overspray, typically 20 to 30 percent more.
Room by Room Estimates
These assume standard 2.4 metre ceilings, two coats, and reasonably smooth walls in good condition.
Small Bedroom (3m x 3m)
Wall area is around 28.8 square metres before deductions. Take out a door and a window and you’re at roughly 25 square metres.
Two coats needs about 50 square metres of coverage, so around 4 litres. A 4 litre tin will do it with a little left for touch ups.
Standard Bedroom (3.5m x 4m)
Wall area comes in around 36 square metres, dropping to about 32 after a door and window.
Two coats is 64 square metres, so 5 litres is comfortable. A 4 litre tin will be tight.
Living Room (5m x 6m)
Around 52 square metres of wall, roughly 46 after openings.
Two coats needs 92 square metres of coverage. Budget 8 litres, which usually means a 10 litre tin.
Kitchen
Kitchens are deceptive. Cabinetry, splashbacks and appliances remove a lot of paintable wall.
A typical kitchen of 3.5m x 4m might only have 20 to 24 square metres of actual paintable wall after the cabinets are accounted for. Around 4 litres for two coats.
Use a washable finish here. Low sheen or satin handles steam and splatter far better than flat.
Bathroom
Small area, but tiles reduce it further. Most bathrooms need 2 to 4 litres.
Use a bathroom specific paint with mould resistance. It costs more per litre but the alternative is repainting in two years.
Hallway
Hallways are easy to underestimate because they’re long. Measure both walls plus any end walls.
A 6 metre hallway with two walls at 2.4 metres high is nearly 29 square metres before the doors come out. Doors help a lot here, since a hallway might have four or five.
Budget 4 to 5 litres for two coats.
Ceilings
Ceiling area is simply length times width. A 4m x 5m room has a 20 square metre ceiling.
Ceiling paint typically covers a bit less than wall paint because it’s applied more thickly to avoid drips. Two coats on a 20 square metre ceiling needs around 3 to 4 litres.
Whole House
For a rough starting point on a three bedroom home with standard ceilings, most people need somewhere between 30 and 45 litres for walls and ceilings in two coats, plus separate product for trim and doors.
Don’t Forget Trim, Doors and Skirting
Trim is calculated by length rather than area, and enamel or trim paint covers differently.
As a rough guide, 1 litre of trim paint covers around 25 to 30 metres of standard skirting board in two coats. A single door, both sides plus edges, uses roughly 250 to 350 millilitres for two coats.
Add up your total skirting, architrave and door count before buying.
Primer and Undercoat
Skipping primer is the most common false economy in painting.
You need primer when painting bare plasterboard, bare timber, previously unpainted render, patched areas, glossy surfaces, or when covering a strong colour or stain.
Primer coverage is usually similar to topcoat, around 10 to 14 square metres per litre. Budget one coat over the same area.
The upside is that primed surfaces make your topcoat go much further, so the primer often pays for itself in reduced topcoat usage.
Practical Buying Tips
Buy slightly more than you calculate. Ten percent extra covers the awkward bits, the second coat that needed a touch, and the inevitable patch up in eighteen months.
Buy in the largest tin size that suits your quantity. A 10 litre tin is almost always cheaper per litre than three 4 litre tins, and you get consistent colour across the whole job.
Check batch numbers if you’re buying multiple tins of a tinted colour. Slight variations between batches show up on large walls. If you have to use two batches, box them together in a clean bucket and stir before starting.
Keep leftovers sealed properly with the lid hammered down, stored somewhere that doesn’t freeze or bake. Label the tin with the room and date.
Get the Numbers Checked
Bring your measurements into Discount Paints and we’ll work out exactly what you need, including primer and trim. Getting it right before you start saves a return trip and usually saves money too.