How to
How much paint do I need? A wall-area worksheet
Measure the walls, subtract the openings and keep each paint specification separate, with a worked example you can check before buying.
To estimate paint quantity, calculate the wall area you intend to cover, subtract the doors and windows you are excluding, then multiply by the required coats and divide by the selected paint's coverage per coat. Keep the measurements and product instructions beside the result. A room's floor area alone does not provide those inputs.
The worksheet below uses an imaginary rectangular room to show the arithmetic. It is a quantity-planning exercise, not a paint application guide or a tested coverage claim. Replace its dimensions, coverage and coat count with information for your own project before treating any figure as a purchasing estimate.
Make a wall list before opening the calculator
Give each surface a name you will recognise: window wall, door wall, opposite wall and bookcase wall, for example. Record width and height using the same unit. Multiplying metres by metres gives square metres; multiplying feet by feet gives square feet. Keep a note of the original dimensions so another person can check the calculation without measuring the room again.
For this hypothetical room, two walls are 4 metres wide and two are 3 metres wide. Every wall is 2.5 metres high. The first pair totals 20 m² and the second totals 15 m², making 35 m² of gross wall area. The ceiling is a separate surface and is not included. These dimensions illustrate the method; they are not standard room dimensions.
Sherwin-Williams' paint-quantity guidance describes adding wall width × height areas, subtracting large windows and doors, and checking the selected paint's label for coverage. Its example uses feet and gallons. Our worksheet uses metres and litres, without adopting a generic coverage figure or the manufacturer's rounding instruction.
Subtract each excluded opening once
List the openings separately from the walls. In our example, a window measuring 2 metres by 1.5 metres occupies 3 m². A door measuring 1 metre by 2 metres occupies another 2 m². Subtracting those 5 m² from the 35 m² of walls leaves 30 m² to estimate. The dimensions are deliberately simple so the subtraction is visible.
| Surface or opening | Illustrative calculation | Area |
|---|---|---|
| Two longer walls | 2 × 4 m × 2.5 m | 20 m² |
| Two shorter walls | 2 × 3 m × 2.5 m | 15 m² |
| Gross walls | 20 + 15 | 35 m² |
| Excluded window | 2 m × 1.5 m | 3 m² |
| Excluded door | 1 m × 2 m | 2 m² |
| Net walls | 35 − 3 − 2 | 30 m² |
Choose one way to enter that information. In the calculator, enter 35 as the gross area and 5 as openings. Alternatively, enter the already calculated net area of 30 and zero openings. Do not enter 30 and then subtract 5 again: that would count the exclusions twice and leave a different area.
A door you are painting with a different product needs its own estimate. Subtracting it from the wall calculation does not mean it needs no paint. Irregular surfaces also need an appropriate measured area rather than a guessed rectangle. Leave an uncertain dimension visibly unresolved instead of rounding it until the total looks convenient.
Attach coverage and coats to the exact product
Find the coverage for one coat and the required coat count in the instructions for the paint you plan to use. Record the product and finish with those figures. A coverage number copied from a different range cannot establish how much of your selected coating is needed. Confirm unclear instructions with the manufacturer or supplier before completing the estimate.
For arithmetic only, suppose the chosen inputs are 10 m² per litre and two coats. Our 30 m² of net walls then gives 30 × 2 ÷ 10 = 6 litres before an allowance. The 10 and the two are hypothetical inputs, not recommendations for an unselected product. The calculator supplies neither number automatically.
If another wall colour or a ceiling coating has a different coverage figure, keep it in a separate row and run a separate estimate. Do not combine surface areas and divide the total by whichever coverage number is easiest to find. A useful final sheet tells you which quantity belongs to which product and which surfaces.
Keep allowance distinct from container sizes
The paint calculator lets you add an optional percentage allowance. With the example above, zero allowance leaves 6 litres; a deliberately selected 10% scenario produces 6.6 litres. This article does not prescribe an allowance. The extra 0.6 litre follows from the entered percentage and is not a measurement of waste or absorption.
Container selection is another decision. The calculator reports volume rather than choosing local pack sizes or promising that a particular combination will finish the job. Check the available sizes and the complete project requirements with the supplier. Keep any primer or other coating separate instead of assuming it is included in the wall-paint figure.
If your measurements are in square feet, select that area unit. Select the coverage unit printed on the product separately: square metres per litre, square feet per US liquid gallon or square feet per Imperial gallon. The tool converts compatible units during the calculation. Its litre and gallon results describe the same quantity; they are not quantities to add together.
Finish the sheet before choosing what to buy
Write a final line with the surfaces, gross area, excluded openings, coverage unit, coat count, chosen allowance and resulting volume. Mark anything still awaiting confirmation. You now have a calculation someone else can inspect, including its assumptions, rather than an isolated number with no connection to the room or paint.
If colour is still undecided, use our paint-sampling guide before turning an illustrative quantity into an order. For the wider colour brief, the small-room colour planning guide separates the surfaces that stay from those you intend to change. Quantity planning starts when those decisions and the product instructions are clear.
