From measured space to purchasable material
Work out how much paint to buy before opening a tin. This room paint calculator separates the four walls from the ceiling, deducts the actual area of openings, and applies your chosen number of coats to both surfaces. Enter the coverage printed on your paint label rather than relying on a universal coverage claim. Porous plaster, textured surfaces, major color changes, and application method can all change the amount you use. The result shows the unrounded liquid requirement alongside whole cans, so you can distinguish the mathematical estimate from the amount available to purchase.
Measure, select yield and round the order
- Measure the room length, width, and full painted wall height.
- Add together the areas of doors, windows, and other unpainted sections.
- Enter label coverage for one coat, choose coats and ceiling inclusion, then compare liquid need with whole-can purchasing.
Net requirement and purchase calculation
A worked example
A 12 × 10 × 8 ft room has 352 sq ft of gross wall area. Deducting 40 sq ft leaves 312 sq ft; two coats at 350 sq ft per gallon with 10% extra need 1.961 gallons, rounded to two 1-gallon cans.
Paint: what changes the result?
Check how the chosen product changes the order. The comparisons below are calculated from this page’s default example. Each row changes only the named input; all other inputs stay at their displayed example values. These are mathematical scenarios, not measurements from a completed project.
| Changed assumption | Default input | Comparison input | Result: default → comparison |
|---|---|---|---|
| Room length | 12 ft | 14.4 ft | Paint needed with allowance: 1.9611 → 2.2025 US gal Whole cans to buy: 2 → 3 |
| Room width | 10 ft | 12 ft | Paint needed with allowance: 1.9611 → 2.1623 US gal Whole cans to buy: 2 → 3 |
| Wall height | 8 ft | 9.6 ft | Paint needed with allowance: 1.9611 → 2.4037 US gal Whole cans to buy: 2 → 3 |
| Total door and window area | 40 sq ft | 48 sq ft | Paint needed with allowance: 1.9611 → 1.9109 US gal Net wall area: 312 → 304 sq ft |
Displayed values are rounded. Whole-pack results change in steps; continuous quantities may change before a new pack is needed. To compare your actual project, enter your dimensions above, save a scenario, then change one assumption.
Coverage, allowance and purchasing limits
- Opening deductions are areas, not counts; measure each opening if an accurate deduction matters.
- One coverage figure is used for walls and ceiling. Calculate them separately when using paints with different yields.
- The allowance is additional liquid, whereas the whole-can rounding is a purchasing constraint; they are shown separately.
Frequently asked questions
How much paint do I need for a 12 by 10 room with two coats?
With 8 ft walls, 40 sq ft of openings, 350 sq ft per gallon coverage, and 10% extra, the walls need about 1.96 gallons; buy two 1-gallon cans.
Should I include doors and windows in my paint calculation?
Deduct the parts that will not receive the same paint. If painting trim or a door, estimate that paint separately.
Does this include primer?
No. Primer has its own coverage and coat requirements; use the primer calculator for that product.
Read the project guides
- How Much Paint for a Room with Doors, Windows, and Two Coats?
Measure net wall area, separate ceiling and trim paint, apply real product coverage, and round a two-coat room estimate to cans.
- How to Plan Even Board and Batten Spacing Around Outlets
Measure a board and batten accent wall, calculate equal clear gaps, check outlets, and turn the layout into a practical trim order.
Calculation method: JICWH methodology. Examples are planning assumptions, not product recommendations.