Interior Paint Calculator
Enter a room size or wall area to get the exact gallons of interior paint and primer to buy — coats, coverage and door/window openings all handled, with every formula shown.
Your paint job
Paint to buy
2 gal
- Paintable area
- 334 ft²
- Suggested container mix
- 2 × 1 gal can
- Always buy a little extra — running short forces a second store trip and risks a batch color mismatch. We round up, never down.
How this is calculated
Wall area
perimeter × height − openings
= 2 × (12 + 12) × 8 − 50
334 ft²
Paint per coat
paintable area ÷ coverage rate
= 334 ÷ 350
0.95 gal
Total paint
paint per coat × coats
= 0.95 × 2
1.91 gal
Round up to containers
smallest whole-container combination ≥ need
= 2 × 1 gal can
2 gal
How much paint do I need for a 12×12 room?
A 12 ft × 12 ft room with 8 ft walls has a perimeter of 2 × (12 + 12) = 48 ft, so the wall area is 48 × 8 = 384 ft². At the standard 350 ft² per gallon and two coats that is 384 × 2 ÷ 350 = 2.19 gallons — so you buy 3 gallons (paint is always rounded up). Subtract your doors and windows and the calculator updates the math instantly.
Paint coverage by surface type
| Surface | Coverage per gallon | Coverage per litre |
|---|---|---|
| Smooth / sealed (primed drywall) | 400 ft² | 9.8 m² |
| Standard drywall | 350 ft² | 8.6 m² |
| Textured / porous (stucco, rough wood) | 250 ft² | 6.1 m² |
| Primer / sealer | 200–300 ft² | 4.9–7.4 m² |
Paint needed by room size (8 ft walls, 2 coats, no openings)
| Room size | Wall area | Gallons (2 coats) | Buy |
|---|---|---|---|
| 10 × 10 ft | 320 ft² | 1.83 | 2 gal |
| 12 × 12 ft | 384 ft² | 2.19 | 3 gal |
| 12 × 16 ft | 448 ft² | 2.56 | 3 gal |
| 15 × 20 ft | 560 ft² | 3.20 | 4 gal |
| 20 × 24 ft | 704 ft² | 4.02 | 5 gal |
One coat or two?
Two coats are the standard for an even, durable finish and are assumed by default. A single coat may be enough when you are repainting the same color over a clean, sealed surface. Always use two coats — plus primer — for new drywall, bare wood, a drastic color change, or covering a stain.
When you need primer
- •New, unprimed drywall — the paper and compound soak up the first coat unevenly.
- •Bare or patched wood, and any raw repair.
- •A drastic color change — primer blocks the old color so you need fewer finish coats.
- •Water stains, smoke or marker — use a stain-blocking primer so they do not bleed through.
Common paint estimating mistakes
- •Buying short. Running out mid-job means a second trip and risks a batch color mismatch — always round up, never down.
- •Ignoring surface porosity. Textured, stucco or bare surfaces cover far less than smooth drywall — pick the right coverage preset.
- •Forgetting the second coat. One coat rarely hides — most jobs need two.
- •Skipping primer on new drywall. The first coat is wasted soaking in; primer is cheaper and seals the surface.
Working in metric? Switch the toggle and the calculator presents litres and square metres natively. As a guide, 350 ft²/gal is about 8.6 m²/L, and one US gallon is 3.79 litres.
Frequently asked questions
How much paint do I need for a 12×12 room?+
A 12×12 room with 8 ft walls has 384 ft² of wall. At 350 ft²/gal and two coats that is 768 ÷ 350 = 2.19 gallons, so you buy 3 gallons. Subtract doors and windows to fine-tune.
Do I need primer on interior walls?+
Prime new drywall, bare wood, stained spots or a drastic color change. Over an existing painted wall in a similar color you can usually skip it and add a coat instead.
How much area does a gallon of paint cover?+
A gallon covers about 350–400 ft² (8.6–9.8 m²/L) in one coat on a smooth, primed surface. Textured, porous or bare surfaces drop to around 250 ft²/gal.
Do I need one coat or two?+
Two coats are standard for an even, durable finish. One coat can work when repainting the same color over a clean, sealed wall. New drywall and color changes always need two coats plus primer.
When do I need primer?+
Prime new drywall, bare or patched wood, drastic color changes, and stained surfaces. Primer seals the surface so the finish coats cover evenly and you use less paint overall.
How do I convert the estimate to litres?+
Switch the calculator to metric and it shows litres and square metres natively. One US gallon is 3.79 litres, and 350 ft²/gal is about 8.6 m²/L.
Methodology & sources
Paintable area is wall perimeter × height (with openings subtracted) plus the ceiling where selected. We divide by the coverage rate, multiply by coats, carry full precision and round up to whole containers. The default 350 ft²/gal is the published single-coat spread rate for interior latex on a primed, smooth surface; porous and textured surfaces cover less. Paint is never rounded down.
- Sherwin-Williams published spread-rate guidance (one gallon covers ~350–400 ft² of smooth, primed surface) — verified June 28, 2026
Last reviewed June 28, 2026. Estimates are indicative — verify against current product specs and local requirements before ordering.
We're committed to keeping these tools accurate and improving them over time. If you'd like to contribute to their accuracy, or you run into any issues or errors, please email us at info@tradesppl.com.