Paint Calculator
Enter your room dimensions to get gallons of paint to buy — doors, windows and coats are all accounted for.
Enter your measurements to see what to buy.
How to calculate it
Enter your room dimensions to get gallons of paint to buy — doors, windows and coats are all accounted for.
Source: Paintable area × coats ÷ coverage. Standard coverage ~350–400 ft² per gallon (check the can); openings ~21 ft² per door, 15 ft² per window.
A 12×12 ft room with 8 ft walls, 1 door, 2 windows, 2 coats: walls = 384 ft² − 51 ft² = 333 ft² × 2 = 666 ft² ÷ 350 = 1.9 → 2 gallons.
Want the full step-by-step walkthrough? Read our How to Estimate Paint for a Room.
Paint coverage per gallon
How much area a gallon covers depends on the surface and how many coats you apply. Use this table to estimate gallons for common wall conditions.
| Surface type | Coverage per gallon (1 coat) | Gallons for 400 ft² (2 coats) |
|---|---|---|
| Smooth, primed wall | 350–400 ft² | ~2.3 gal |
| Textured wall | 250–300 ft² | ~3.2 gal |
| Bare drywall (unprimed) | 200–250 ft² | ~4.0 gal |
| Primer coat | 350–400 ft² | ~2.3 gal |
Coverage varies by brand, finish and roller nap — always check your paint can. Buy all gallons at once and mix them together for consistent color.
How to use this calculator
- Enter the room length, width and wall height in feet and inches.
- Add the number of doors and windows — the calculator deducts standard openings automatically.
- Set the number of coats (two is standard) and your paint’s coverage per gallon.
- Tick “Also paint the ceiling” if you’re painting it, and enter a price per gallon for a cost estimate.
Purchasing & waste guidance
- One gallon covers about 350–400 ft² of smooth, primed wall — check your can.
- Textured walls, bare drywall and drastic color changes can cut coverage by a third.
- Two coats is standard; plan on a primer coat for new drywall or dark-to-light changes.
- Buy all your paint at once and mix gallons together for perfect color consistency.
Waste factor guidance
- 10% covers roller waste and touch-ups for a standard two-coat room.
- Use 15% for textured walls or drastic color changes that need extra coats.
- Skip extra waste for a single-coat refresh in the same color.
Common mistakes to avoid
- Forgetting to deduct doors and windows, which overstates the paintable area.
- Buying paint one gallon at a time — mix all gallons together for consistent color.
- Skipping primer on new drywall or dark-to-light changes, which cuts coverage.
Assumptions & what’s included
- Room is rectangular; wall height is uniform.
- Openings use standard sizes: 21 ft² per door, 15 ft² per window.
- Coverage defaults to 350 ft²/gal — adjust to match your paint can.
- Gallons are rounded up so you have leftover for touch-ups. Price is per gallon and optional.
Sources & references
- Coverage of 350–400 ft² per gallon follows typical manufacturer product data (e.g., Sherwin-Williams, Behr) for smooth, primed walls — always check your paint can.
- Standard opening sizes (21 ft² per door, 15 ft² per window) follow common residential opening dimensions.
- Coverage drops on textured, bare, or dark-over-light surfaces; primer is recommended per manufacturer guidance for new drywall and dramatic color changes.
Learn more about how every calculator works on our methodology page.
Frequently asked questions
How much paint do I need for a 12×12 room?
A 12×12 ft room with 8 ft walls, one door and two windows needs about 2 gallons for two coats at 350 ft²/gal coverage. The calculator adjusts for your exact coverage and openings.
How many square feet does a gallon of paint cover?
One gallon typically covers 350–400 ft² of smooth, primed wall. Textured or bare drywall and dramatic color changes can reduce coverage to around 250 ft² per gallon.
Do I need to count the ceiling?
Only if you’re painting it. Tick “Also paint the ceiling” and the calculator adds the floor area (length × width) to your paintable surface.
Do I need primer before painting?
Plan on one primer coat over new drywall, repaired areas, or any dark-to-light color change. Primer seals the surface so your topcoat covers in fewer coats and less paint.
How much paint do I need for 500 square feet?
At 350 ft² per gallon and two coats, 500 ft² needs about 2.9 gallons — round up to 3 gallons so you have leftover for touch-ups. Enter your paint’s exact coverage in the calculator for a precise number.
How much paint do I need for a 10×10 room?
A 10×10 ft room with 8 ft walls, one door and one window has about 237 ft² of paintable wall. Two coats at 350 ft²/gal needs roughly 1.4 gallons — buy 2 gallons to cover touch-ups.
Related guides
Step-by-step project guides that use this calculator.