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.
Worked example: a 12 × 10 ft bedroom
Here is the whole calculation for a typical bedroom, worked through line by line so you can copy the method for any room. The room is 12 ft long, 10 ft wide, with an 8 ft ceiling, one door, two windows, and it needs two coats over existing off-white paint.
- Find the perimeter. 2 × (12 + 10) = 44 ft.
- Gross wall area. 44 ft × 8 ft = 352 sq ft.
- Subtract the openings. One door at 21 sq ft plus two windows at 15 sq ft each is 51 sq ft. 352 − 51 = 301 sq ft of surface to paint.
- Multiply by coats. 301 × 2 = 602 sq ft to cover in total.
- Divide by coverage. 602 ÷ 350 = 1.72 gallons, which rounds up to 2 gallons.
Two gallons leaves a comfortable margin for cutting in around the trim and for what a roller absorbs. That is the figure the calculator returns, and it is what you take to the store.
The same room with the ceiling painted
Painting the ceiling the same colour is a common request and it changes the answer. The ceiling adds 12 × 10 = 120 sq ft, and that surface also needs two coats, so a further 240 sq ft. The total becomes 842 sq ft, and 842 ÷ 350 = 2.4 — you would buy 3 gallons instead of 2.
If the ceiling is a different colour, which is the usual choice, work it out separately: 120 sq ft × 2 coats ÷ 350 = 0.69 gallons. That means one extra gallon for the ceiling on top of the two for the walls.
Where the answer moves
- Dark to light usually needs a third coat. Three coats on these walls is 903 sq ft, which is 3 gallons.
- Bare drywall or fresh patches drink far more paint. Prime those areas first — about a gallon for this room — and the topcoats then stay close to the calculated figure.
- Rough texture or porous brick drops coverage from 350 to around 250 sq ft per gallon. The same room would need 3 gallons of exterior paint rather than 2.
Type any of these variations into the calculator and the result updates as you go. The worked example is here to show where the numbers come from, not to replace your own measurements.
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 |
What paint costs in 2026
Buying by the gallon is almost always cheaper than by the quart: a quart typically costs 40–60% as much as a gallon while holding only a quarter of the paint. These are the ranges most US homeowners are seeing in 2026.
| Product | Typical price per gallon | Worth knowing |
|---|---|---|
| Basic interior latex (store brand) | $20 – $35 | Fine for low-traffic rooms; often needs three coats |
| Mid-range interior latex | $35 – $55 | The usual choice; covers properly in two coats |
| Premium / paint-and-primer-in-one | $55 – $90 | Can remove the separate priming step on moderate colour changes |
| Primer | $20 – $40 | Needed on bare drywall, stains, and big colour changes |
| Exterior paint | $40 – $80 | More resin and mildewcide for weather exposure |
For the 12 × 10 bedroom in the example above, materials come to roughly $70–$110: two gallons of mid-range paint plus rollers, brushes, tape and a tray liner. Choosing a paint-and-primer product at the upper end can remove the separate primer purchase for a straightforward colour change.
Hiring the work out is a different calculation. Professional painters in the US typically charge $1.50 to $3.50 per square foot of wall area including materials, or roughly $2 to $6 per square foot of floor area, which is how many quotes are written. That puts the same bedroom somewhere between $450 and $1,000 depending on where you live, how much preparation the walls need, and whether trim and ceiling are included.
Four things move the total more than the price of the paint itself: your region, since labour rates vary by more than double between markets; the size of the colour change; how much patching and sanding the walls need; and ceiling height, because anything much above 9 ft usually carries a surcharge for scaffolding or extension poles.
These are planning figures, not quotes. Prices shift with brand, region and season, and holiday-weekend sales are common. Use the calculator to get the quantity right, then price the job locally.
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.