A gallon of good interior paint covers about 350–400 ft² per coat on smooth, primed walls; textured or porous surfaces take more. Two coats give even colour, and a primer coat is worth it over new drywall, patches or strong colour changes. Buy a little extra for touch-ups.
How to use the Paint Calculator
- Room length
- Room width
- Wall height
- Doors
- Windows
- Paint the ceiling too
- Coats
- Coverage
- Prime first (one coat)
- Can size
- Price per can
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the room size, the number of doors and windows, whether to paint the ceiling, the number of coats, the coverage, whether to prime, the can size and price.
Tips for an accurate estimate
- Take measurements at floor level and at the ceiling – walls and rooms are often slightly out of square.
- Use the manufacturer’s coverage, spacing and installation data when it differs from the typical values here.
- Keep a little extra of every finish material for touch-ups and repairs later.
- Check local building codes and permits before structural, electrical, plumbing or gas work.
Common mistakes to avoid
- Sizing heating or cooling equipment by area alone when insulation, windows and climate make a big difference.
- Buying too little paint, stain or flooring and ending up with mismatched batches.
- Overlooking the extra material for steps, railings, gables and trim.
Inputs explained
| Field | What to enter |
|---|---|
| Room length | Units: ft, m, in, yd, cm |
| Room width | Units: ft, m, in, yd, cm |
| Wall height | Units: ft, m, in, yd, cm |
| Doors | A number |
| Windows | A number |
| Paint the ceiling too | Choose one: No, Yes |
| Coats | A number |
| Coverage | Choose one: 350 ft²/gal (typical), 400 ft²/gal (smooth, premium), 250 ft²/gal (rough / textured), 10 m²/L … |
| Prime first (one coat) | Choose one: No, Yes |
| Can size | Choose one: 1 US gallon, 5 US gallons, 1 US quart, 1 L … |
| Price per can | Units: /piece |
What each result means
- Area to paint = max(2×([Room length] + [Room width])×[Wall height] – [Doors]×1.9509638400000002 – [Windows]×1.3935456000000002, 0) + [Paint the ceiling too]×[Room length]×[Room width]
- Paint needed = [Area to paint]×[Coats]÷[Coverage]
- Primer needed = [Prime first (one coat)]×[Area to paint]÷[Coverage]
- Cans of paint = ceil([Paint needed]÷[Can size] – 0.000001)
- Cans of primer = ceil([Primer needed]÷[Can size] – 0.000001)
- Cost = ([Cans of paint] + [Cans of primer])×[Price per can]
Example values
How the main result changes when Room length changes and everything else stays at the example values above:
| Room length (ft) | Paint needed (US gal) |
|---|---|
| 7 | 1.45 |
| 10.5 | 1.77 |
| 14 | 2.09 |
| 17.5 | 2.41 |
| 21 | 2.73 |
| 28 | 3.37 |
Formula
Area = 2 (L + W) × H − doors × 21 ft² − windows × 15 ft² (+ ceiling)Paint = area × coats ÷ coverage; primer = area ÷ coverageCans = ceil(volume ÷ can size)Worked example
Inputs — Room length: 14 ft; Room width: 12 ft; Wall height: 8 ft; Doors: 1; Windows: 2; Paint the ceiling too: No; Coats: 2; Coverage: 350 ft²/gal (typical); Prime first (one coat): No; Can size: 1 US gallon; Price per can: $40 /piece.
Results — Area to paint = 365 ft²; Paint needed = 2.09 US gal; Primer needed = 0 US gal; Cans of paint = 3; Cans of primer = 0; Cost = $120.00.
Frequently asked questions
How much paint for a 12 × 14 room?
About 380 ft² of wall after a door and two windows – 2.2 gallons for two coats, so buy 2–3 gallons.
How many square feet does a gallon of paint cover?
350–400 ft² per coat on smooth walls; 250–300 ft² on rough surfaces.
Do I need primer?
On new drywall, bare wood, patches, stains or big colour changes, yes. Paint-and-primer products can skip it on previously painted walls.
What formula does the Paint Calculator use?
Area = 2 (L + W) × H − doors × 21 ft² − windows × 15 ft² (+ ceiling).