How to estimate paint: walls, openings, coats & containers
A useful paint estimate starts with a list of surfaces, not the floor area of the room. This example measures four walls, subtracts two openings, and keeps coverage, coats, and container rounding separate.
Open the paint calculator1. Sketch the surfaces you will paint
Our example room is 4 m long, 3 m wide, and 2.5 m high. We will paint all four rectangular walls with the same product. The ceiling, trim, door, and window are excluded; primer is a separate estimate. Two walls measure 4 by 2.5 m and two measure 3 by 2.5 m.
For your room, measure each wall rather than assuming opposite walls match. Break irregular surfaces into non-overlapping rectangles where practical. Measure openings that you will leave unpainted, and avoid subtracting one twice. This follows the wall-area approach in Sherwin-Williams’ paint quantity guidance, which also points you to the chosen paint’s coverage label.
| Surface | Calculation | Area |
|---|---|---|
| Two long walls | 2 times 4 times 2.5 | 20 m² |
| Two short walls | 2 times 3 times 2.5 | 15 m² |
| Door opening | 0.9 times 2 | 1.8 m² |
| Window opening | 1.2 times 1.2 | 1.44 m² |
Total wall area is 35 m². Openings total 3.24 m², leaving 31.76 m² to paint. Enter four walls and two openings in the paint quantity calculator; its initial two-wall example is smaller than this worked room.
2. Apply coats and the label’s coverage
For this illustration, assume 2 coats and coverage of 10 m² per litre per coat. These are example inputs, not a promise for a particular paint or surface.
Read whether a quoted coverage figure is per coat. Entering a two-coat coverage figure and multiplying by two again would count coats twice. Porous or textured surfaces and a color change may alter how much product the job needs. Follow the product’s preparation, primer, and application instructions.
3. Add an allowance, then round containers
We choose a 10% extra allowance for this example. It is an explicit planning choice, not a universal recommendation. The estimate becomes 6.352 times 1.10 = 6.9872 L.
If the chosen product comes in 2.5 L containers, 6.9872 / 2.5 = 2.79488 containers. Round up to 3 containers, or 7.5 L total. The planner only rounds one container size; it does not find the cheapest mix of sizes.
Leaving openings out of the calculation is another way to leave spare paint. If you do that and also add an allowance, recognize that both increase the estimate. Unused paint may still be useful for later touch-ups, but estimate deliberately.
4. Keep metric and US units together
In metric mode, use metres, square metres per litre, and litres. In US mode, use feet, square feet per US gallon, and US gallons. For feet and inches, first convert the inches to a fraction of a foot: 8 ft 6 in is 8.5 ft, not 8.6 ft.
The same example is approximately 341.862 square feet net area, with equivalent coverage of 407.458 square feet per US gallon. Its 6.9872 L estimate is about 1.84582 US gallons. These are conversions of the example, not a second product recommendation.
Switching units in the calculator converts all existing measurements, coverage, and container volume. The 2.5 L container becomes about 0.66043 US gallons. Change that field to the actual retail size afterward: with 1-US-gallon cans and otherwise equivalent inputs, this example rounds up to 2 cans.
The tool uses 1 ft = 0.3048 m and 1 US gallon = 3.785411784 L. UK imperial gallons are different and are not supported. NIST’s unit conversion tables distinguish the units. Check your measurements and chosen product before buying.