How the paint calculator works
Paint covers a stated area per gallon for each coat. The calculator finds the paintable wall area, multiplies it by the number of coats, divides by your coverage rate, and adds the allowance you choose. The total rounds up to whole gallons (or whole litres in metric).
The breakdown shows what each coat uses, the allowance, and how much you would have left in the last can, so you can judge whether a quart or a five-gallon bucket makes more sense.
Formula and assumptions
- coverage
- square feet one gallon covers per coat (from the can)
- allowance
- extra percentage for touch-ups and uneven absorption
Assumptions and limits
- Coverage is your input. Use the can’s stated spread rate per coat.
- Rounds up to whole gallons; in metric, to whole litres (combine can sizes).
- Walls only: no ceiling, trim, or primer.
Worked example
A 12 × 10 ft room with 8 ft walls, 20 sq ft of openings, two coats, 350 sq ft per gallon, and a 10% allowance.
- Paintable area = 44 × 8 − 20 = 332 sq ft.
- One coat = 332 ÷ 350 = 0.95 gal; two coats = 1.90 gal.
- With 10% allowance = 2.09 gal.
- Round up: buy 3 gallons.
These are the calculator’s default values, so the live result above matches this example until you change an input.
How to measure correctly
- Measure wall height to the ceiling, not to the top of the trim.
- Measure large openings individually instead of guessing.
- Note texture: rough walls and new drywall soak up more paint.
- Paint the ceiling and trim as separate estimates.
Paint by wall area
| Net wall area | One coat | Two coats + 10% | Buy |
|---|---|---|---|
| 200 sq ft | 0.57 gal | 1.26 gal | 2 gal |
| 300 sq ft | 0.86 gal | 1.89 gal | 2 gal |
| 400 sq ft | 1.14 gal | 2.51 gal | 3 gal |
| 500 sq ft | 1.43 gal | 3.14 gal | 4 gal |
| 600 sq ft | 1.71 gal | 3.77 gal | 4 gal |
| 800 sq ft | 2.29 gal | 5.03 gal | 6 gal |
Paint Calculator FAQ
How many square feet does a gallon of paint cover?
Many interior wall paints list roughly 350–400 sq ft per gallon per coat on smooth, primed walls. Texture, porous new drywall, deep colors, and roller technique all reduce it, so enter the spread rate printed on your can.
Should I subtract doors and windows?
Subtract openings that will not get the same paint. For one or two openings the difference is small; for a room with a wall of windows, measure each opening and enter the total.
Do I need two coats of paint?
Two coats is the usual plan for an even finish and for color changes. A same-color refresh over sound paint may cover in one; the label and a test patch decide. Each extra coat adds the full wall area again.
Does the paint estimate include the ceiling or trim?
No. It covers the four walls. Ceilings and trim often use a different product or sheen, so estimate them separately with the ceiling paint calculator or as their own surface.
Does it include primer?
No. Primer is a separate product with its own coverage. Use the primer calculator when the surface is new drywall, patched, stained, or a big color change.
Why buy more paint than the calculated need?
Paint is sold in whole cans and the result rounds up so you are not short mid-wall. A little left over is useful for touch-ups, and buying it all at once keeps the tint batch consistent.