Paint Calculator
Estimate how much paint a job needs in metric (m², litres) or imperial (sq ft, US gallons) units. Enter a wall area directly or build it from room dimensions, subtract doors and windows, add an optional ceiling and a primer coat, and get the volume, the number of cans to buy and an optional cost.
Formula
Worked example
Metric: a 4 × 3 m room, 2.4 m walls is 2 × (4 + 3) × 2.4 = 33.6 m². Subtract one door (1.8 m²) and one window (1.4 m²) for 30.4 m². Two coats is 60.8 m²; at 11 m²/L that is 5.5 L, about 6.1 L with 10% waste, so three 2.5 L tins.
How the Calculator Works
The calculation divides total wall area by the coverage rate of your chosen paint, then multiplies by the number of coats you plan to apply. Coverage rate is the key variable: most standard emulsions cover 10-12 m² per litre per coat under normal conditions, while specialist paints like primers or textured finishes can fall as low as 5-8 m². The result is the minimum volume needed, and most painters add a 10% buffer for touch-ups and unavoidable waste.
How to Use It
Measure the height and width of each wall surface you intend to paint, multiply them together to get the area of each wall, and add all wall areas to produce the total. Subtract the area of doors and windows if you want a tighter estimate, a standard interior door is roughly 1.8 m² and a typical window around 1.4 m². Enter your total area, select the number of coats (two coats is standard for a colour change), and enter the coverage rate from your paint tin's label.
What Affects the Result
Surface texture is the biggest variable: bare plaster, bare masonry, and previously unpainted wood are porous and absorb significantly more paint than a sealed, previously painted surface. Colour changes from dark to light frequently require three coats rather than two to achieve full opacity. Application method also matters, roller application is generally more efficient than brush application, and spraying can increase consumption by 20-30% due to overspray. Always check the specific coverage figure on your paint tin rather than relying solely on the typical range.
Limitations to Be Aware Of
This calculator assumes uniform, flat surfaces and does not account for architectural features such as cornices, window reveals, or skirting boards, which add surface area and complexity. It cannot predict how a specific paint product will perform on your particular substrate, manufacturer coverage claims are measured under controlled laboratory conditions. If you are painting over a significantly different colour or a highly absorbent surface for the first time, treat the output as a floor estimate and purchase an additional 15-20% to be safe.
Cans, cost and the ceiling
Beyond the raw litres or gallons, the calculator rounds up to whole cans using the tin size you buy, since paint is sold in fixed sizes such as 1 L, 2.5 L and 5 L, or 1 quart and 1 gallon cans. Turn on the cost estimate and enter a price per can to see a rough material total; this is a planning figure, as prices and finishes vary. If you are painting the ceiling as well, switch to room dimensions and toggle the ceiling on, which adds the floor footprint (length times width) to the area. Primer is handled as a single extra coat over the bare area, on top of your chosen number of colour coats.
Typical coverage and opening sizes
| Item | Metric | Imperial |
|---|---|---|
| Emulsion coverage | 10-12 m² per litre | ~350 sq ft per US gallon |
| Primer / bare plaster | 6-8 m² per litre | ~250 sq ft per US gallon |
| Standard door | 1.8 m² | 19 sq ft |
| Standard window | 1.4 m² | 15 sq ft |
| Waste allowance | 10% typical | 10% typical |
Defaults used here; always check the coverage figure printed on your paint tin.
Frequently asked questions
How many coats of paint does a typical interior wall need?
Most interior repaints in a similar colour require two coats for even, durable coverage. Significant colour changes, particularly going from a dark shade to a lighter one, often need three coats. Bare or newly plastered walls should receive a diluted mist coat first, which counts as an additional layer before your finish coats begin.
What does coverage rate mean on a paint tin?
Coverage rate, sometimes called spreading rate, states how many square metres one litre of that paint will cover in a single coat under standard conditions. It is measured per coat, not per finished job, so a two-coat application halves the effective coverage per litre. Rough or porous surfaces will reduce the rate printed on the tin; smooth, sealed surfaces may reach or slightly exceed it.
Should I add extra paint to the calculator's estimate?
Yes, a margin of 10-15% above the calculated figure is standard practice for interior work. This accounts for roller and brush waste, the edges and corners that require more careful application, and the ability to do minor touch-ups months later from the same batch. Paint colour can vary slightly between production batches, so having leftover paint from the original purchase is worth more than it might seem.
How many cans of paint should I buy?
Take the volume the calculator gives after the waste allowance and divide it by the size of can you buy, then round up to the next whole can. This calculator does that for you and shows the number of cans directly. Buying whole cans from one batch also gives you matching paint for later touch-ups.
Does this calculator include the ceiling?
Only if you ask it to. Switch to room dimensions and turn on the ceiling toggle, which adds the floor footprint (length times width) to the wall area. In wall-area mode it assumes you have already measured exactly the surfaces you intend to paint.