Paint Calculator
Buying paint for a room? Enter your wall area, subtract the doors and windows, and set your coats and coverage to find out exactly how many gallons you need.
Paint You Need
How It Works
Paint is sold by the gallon, but walls are measured in square feet — so the job is converting one into the other. The calculator subtracts the area of doors and windows from your total wall area, multiplies what is left by the number of coats, and divides by the coverage printed on the paint can. The result is rounded up to whole gallons, because paint shops do not sell fractions of a can.
Coverage is the figure to get right. Most interior paints claim around 350 square feet per gallon per coat, measured on a smooth, primed surface. Rough plaster, bare drywall or a big colour change can cut real coverage noticeably, which is why the coverage field is yours to adjust.
Worked Example
Walls totalling 1,200 sq ft, minus 60 sq ft of doors and windows, leave 1,140 sq ft to paint. Two coats mean 1,140 × 2 = 2,280 sq ft of painting. At 350 sq ft per gallon that is 2,280 ÷ 350 = 6.51 gallons, so you buy 7 gallons — the extra half-gallon covers touch-ups later.
Assumptions
- Coverage is the same for every coat; in practice the first coat on bare surfaces uses more.
- Ceilings are not included — measure them separately if you plan to paint them.
- No allowance is added for spills or waste; rounding up to whole gallons provides the buffer.
Gallons needed at 350 sq ft per gallon, rounded up:
| Paintable wall area | 1 coat | 2 coats |
|---|---|---|
| 400 sq ft | 2 gallons | 3 gallons |
| 800 sq ft | 3 gallons | 5 gallons |
| 1,200 sq ft | 4 gallons | 7 gallons |
| 2,000 sq ft | 6 gallons | 12 gallons |
How to Use This Calculator
Measure each wall (length × height), add the areas, and enter the total. Subtract the doors and windows, set the coverage from your paint can, and choose the coats. The result updates live.
Tips and Practical Advice
Buy all your paint at once, from the same batch — colour can vary slightly between batches. Keep the leftover paint: a sealed can lasts years and is perfect for touch-ups. If you are painting over a strong colour, budget for a primer coat or a third coat. And measure twice: the most common painting mistake is forgetting a wall.
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