How the paint calculator works
This calculator estimates the amount of paint you need using a simple, standard formula:
- Find the wall area. Multiply the room perimeter (2 × length + 2 × width) by the wall height.
- Subtract openings. Deduct about 21 sq ft for each standard door and 15 sq ft for each window.
- Add the ceiling if you're painting it (length × width).
- Multiply by coats. Most rooms need at least two coats.
- Divide by coverage. A gallon of interior paint covers about 350 sq ft. We round up to whole gallons so you always have enough.
Paint coverage varies by brand, color, and surface. Dark colors and rough surfaces may need extra paint. When in doubt, buy 10–15% more than the estimate — unopened cans can always be returned at most home improvement stores.
Paint coverage reference
| Surface | Typical coverage (sq ft per gallon) |
|---|---|
| Smooth interior walls (latex) | 350–400 |
| Textured / rough interior walls | 250–300 |
| Primer | 300–350 |
| Exterior siding (smooth) | 300–400 |
| Exterior brick / stucco (porous) | 200–250 |
Pro tips
- One coat per color change is rarely enough. Painting over a darker color usually needs three coats.
- Buy extra in the same batch. Paint is tinted per batch — matching an exact color later can be difficult.
- A 5-gallon bucket is cheaper per gallon for large projects and reduces container waste.
- Use the share link to save your measurements or send them to a contractor or paint store.
Frequently asked questions
How many square feet does a gallon of paint cover?
A gallon of interior paint covers about 350 square feet with one coat. Rough or porous exterior surfaces cover less — around 250–300 square feet per gallon.
How do I calculate the paintable area of a room?
Measure the perimeter (2 × length + 2 × width) and multiply by wall height for the gross wall area. Subtract about 21 sq ft per door and 15 sq ft per window, then multiply by the number of coats.
Should I buy paint in 5-gallon buckets or 1-gallon cans?
For larger projects, 5-gallon buckets are usually cheaper per gallon and mean fewer trips to the store. The calculator recommends the combination of 5-gallon buckets and 1-gallon cans with the least waste.
How much paint do I need for two coats?
Roughly double the single-coat amount. If one coat needs 1.5 gallons, plan for 3 gallons. A second coat provides a more even, durable finish, especially over a dramatically different color.
Do I need primer plus paint?
Prime bare drywall, patched areas, stained walls, or big color changes. Primer covers about 300 sq ft per gallon and helps the paint adhere, which can actually reduce total paint use.
Does this paint calculator work in metric (metres and litres)?
Yes. Flip the Imperial / Metric toggle at the top of the calculator to enter your room in metres and see the result in litres and square metres. Coverage is shown in m² per litre, and the recommended purchase uses 5 L, 2.5 L and 1 L tins. Your choice is remembered on your device.