Calculator · Paint quantity
How much paint do I need?
Area, minus the openings, divided by the spreading rate, times the coats, plus an allowance for how thirsty the surface is. That is the whole calculation, and it is shown line by line below so you can see which assumption is doing the work. The spreading rate is almost always the one.
The calculator
Enter what you can measure
Total wall length means the run of wall you are painting — for a whole house, the perimeter. If you do not know it, estimate the area first and come back with a square-metre figure.
Worked by hand
The same calculation, on paper
So the calculator is checkable even with scripting turned off. A single-storey weatherboard house, 40 m of wall at 2.4 m, eight openings averaging 1.8 m², previously painted and sound, two coats at 12 m²/L.
- Gross wall area: 40 × 2.4 = 96.00 m²
- Openings: 8 × 1.8 = 14.40 m²
- Net area: 96.00 − 14.40 = 81.60 m²
- Profile uplift for bevel-back board, 10%: 81.60 × 1.10 = 89.76 m²
- One coat: 89.76 ÷ 12 = 7.48 L
- No porosity allowance on a sound painted surface, so both coats are the same: 7.48 × 2 = 14.96 L
- Tins: 1 × 15 L = 15 L, with 0.04 L to spare
Change one thing — bare timber instead of sound paint — and the first coat carries the 40% allowance: 10.47 + 7.48 = 17.95 L, which becomes 1 × 10 L + 2 × 4 L. One line of the calculation, three extra litres.
Where this calculation goes wrong in real life
- The spreading rate was optimistic. The most common cause by a wide margin. The datasheet number assumes a smooth sealed surface and a controlled film thickness.
- The wall length was measured on the plan, not the building. Bays, returns, recesses and a garage wall you forgot all add up.
- Fascia, soffit, joinery, downpipes and the front door were not counted. They are usually a different product in a different colour, and they are easy to leave out of the sums and then buy in a panic.
- The colour change was underestimated. Going from dark to light, or into a strong or bright colour, can need a third coat regardless of arithmetic.
- It was sprayed. Add 15% to 30%, or lower the spreading rate before you calculate.
And the honest caveat: getting the litres right tells you nothing about what the job costs. Product is a minority of an exterior repaint; the labour of preparation is the majority. Here is why this site will not give you a rate per square metre.
Questions people actually ask
What spreading rate should I use?
Whatever the tin says, and then be a little pessimistic. Most waterborne architectural topcoats state somewhere between 10 and 14 m² per litre per coat on a smooth, sealed, non-porous surface. That number is a laboratory figure achieved on a test panel, and it is the best case. Real walls are not test panels: profile, texture, absorption, application method and the weather all cost you litres. This calculator defaults to 12 m²/L because it sits mid-range, and it lets you change it because your product is the authority, not us.
Does spraying use more paint than brushing and rolling?
Generally yes, and sometimes a lot more. Airless spraying loses material to overspray and to the hose and pump at the start and end of each session, so 15% to 30% more product than a rolled equivalent is normal depending on the setup and the amount of masking. Spraying buys speed and a better finish on profiled or textured surfaces, not economy. If you are pricing a sprayed job, nudge the spreading rate down rather than adding a fudge factor at the end.
Why does the first coat need extra paint on a bare surface?
Because a porous surface absorbs paint rather than holding it on the surface. Bare timber, new plaster and especially bare block drink the first coat — the stated spreading rate assumes a sealed substrate, and on open block the first coat can deliver less than half the nominal coverage. That is what the porosity allowance in this calculator is for, and it is applied to the first coat only, because the second coat is going onto a surface the first one sealed. On very porous surfaces the right answer is usually a purpose-made sealer or block filler rather than extra topcoat.
Should I deduct windows and doors from the area?
Deduct them, but do not be greedy about it. Cutting in around an opening uses more paint per square metre than the open wall it replaces, and the reveals, sills and jambs around the opening are also being painted, often in a different product. The convention that works is to deduct the glazed area and the door leaf and ignore the frame. If the reveals are being painted in the wall colour, deduct nothing at all for them.
How many coats do I actually need?
Two topcoats over an appropriate primer or sealer is the standard specification, and it is what almost every manufacturer’s warranty assumes. One topcoat over an existing sound coating of the same colour is sometimes defensible as maintenance and often looks fine on the day and thin within two years. Three is genuinely needed for a strong colour change, for deep or bright colours whose pigments have poor hiding power, and where the substrate has a lot of repair patching to bury.
Why does the result round up to odd tin combinations?
Because you cannot buy 18.3 litres. This calculator assumes the standard New Zealand architectural range of 1, 4, 10 and 15 litre tins, and picks the combination that reaches your figure with the least surplus, then the fewest tins. Some product lines also come in 500 ml and 2 L, and some deep bases only come in certain sizes, so treat the tin list as a shopping guide rather than an instruction. One more thing: buy your topcoat for a whole elevation in one batch, or ask for the tins to be boxed together, because batch variation in a tinted colour is visible on a large flat wall.
Next
Estimate the exterior area if you do not have it yet, check what your substrate actually needs before you choose a product, and see how many days this month you can apply it.