Quick answer
Paint (gallons) = (2 × (length + width) × height − openings) × coats ÷ coverage. A 12 × 10 ft room with 8 ft walls has 352 ft² of wall; two coats at 350 ft² per gallon need 2.01 gallons.
Enter the room size, wall height, coats and coverage. The calculator returns gallons and area.
Related tools: Square Footage Calculator: Area of a rectangular space in square feet, square metres and square yards. Concrete Calculator: Cubic yards of concrete for a slab, plus bag counts. Gravel Calculator: Cubic yards of gravel for an area.
How to use the Paint Calculator
- Enter room length (ft) (above 0).
- Enter room width (ft) (above 0).
- Enter wall height (ft) (above 0). It starts at 8.
- Optionally, enter doors and windows total area (ft²) (0 or more).
- Enter coats (a whole number, between 1 and 5). It starts at 2.
- Enter coverage per gallon (ft²) (above 0). It starts at 350.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Room length | A number, in ft | > 0 |
| Room width | A number, in ft | > 0 |
| Wall height | A number, in ft | > 0, starts at 8 |
| Doors and windows total area (optional) | A number, in ft² | ≥ 0 |
| Coats | A whole number | ≥ 1, ≤ 5, starts at 2 |
| Coverage per gallon | A number, in ft² | > 0, starts at 350 |
Formula and method
Formula
gallons = (2(l + w) × h − openings) × coats ÷ coverage
IEEE-754 double precision; shown to at most 6 decimal places.
Typical interior latex paint covers about 350 ft² per gallon; check the can label for your product.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Room length = 12; Room width = 10; Wall height = 8; Coats = 2; Coverage per gallon = 350.
| Result | Value |
|---|---|
| Paint needed | 2.0114 gallons |
| Wall area | 352 |
| Paintable area | 352 |
Example 2
Inputs: Room length = 12; Room width = 10; Wall height = 8; Doors and windows total area = 52; Coats = 1; Coverage per gallon = 400.
| Result | Value |
|---|---|
| Paint needed | 0.75 gallons |
| Wall area | 352 |
| Paintable area | 300 |
The first line is the headline result (paint needed). Below it the paint Calculator lists wall area and paintable area, which describe the same case from other angles. IEEE-754 double precision; shown to at most 6 decimal places.
Reading the result
352 × 2 = 704 ft² of coverage ÷ 350 = 2.01 gallons. Buy 2 gallons plus a quart, or 3 gallons if you want leftovers. Ceilings are not included.
Adjustments
Subtract windows and doors if you wish, and add extra for dark colours, porous walls or textured surfaces. Primer is a separate product with its own coverage.
How the result changes with room length
Holding every other input at the first worked example, this table changes only room length. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Room length | Paint needed | Wall area | Paintable area |
|---|---|---|---|
| 6 | 1.4629 gallons | 256 | 256 |
| 12 | 2.0114 gallons | 352 | 352 |
| 24 | 3.1086 gallons | 544 | 544 |
| 48 | 5.3029 gallons | 928 | 928 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Doors and windows total area: entering "400" shows "Openings must be smaller than the wall area." beside that field and no result.
Choosing between the Paint Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Square Footage Calculator](/construction/square-footage-calculator/) | length, width and unit | Area |
| [Concrete Calculator](/construction/concrete-calculator/) | length, width and depth / thickness | Cubic yards |
| [Gravel Calculator](/construction/gravel-calculator/) | length, width and depth / thickness | Cubic yards |
| [Board Foot Calculator](/construction/board-foot/) | thickness, width and length | Board feet |
| [Mulch Calculator](/construction/mulch-calculator/) | length, width and depth / thickness | Cubic yards |
Background: construction estimates
Material quantities are geometry plus a margin. Calculate the exact volume or area, add an allowance for waste, spillage, cuts and uneven ground (calculator pages commonly mention 5 to 10 percent, a rule of thumb that varies by material and job) and round up to whole units. One cubic yard is exactly 27 cubic feet.
Suppliers often have minimum order sizes and may charge for small loads, so confirm quantities, product yields and local code with your supplier or a qualified estimator before ordering.
Assumptions and limitations
- Openings must be smaller than the wall area
- Ceilings are not included
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for doors and windows total area to confirm this.
Frequently asked questions
How much paint for one room?
A 12 × 10 room usually needs about 2 gallons for two coats.
Do I paint the ceiling too?
Calculate it separately.
What coverage should I use?
Use 350 ft² per gallon unless the label says otherwise.
What does the Paint Calculator calculate?
Gallons of paint for the walls of a room. It returns Paint needed, Wall area and Paintable area.
What formula does the Paint Calculator use?
gallons = (2(l + w) × h − openings) × coats ÷ coverage. Typical interior latex paint covers about 350 ft² per gallon; check the can label for your product.
What do I need to enter in the paint Calculator?
You enter Room length (ft), Room width (ft), Wall height (ft), Coats and Coverage per gallon (ft²). Optional: Doors and windows total area (ft²).
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Openings must be smaller than the wall area.
What are the limits of this calculator?
Openings must be smaller than the wall area. Ceilings are not included.
What is an example of a paint result?
With room length 12, room width 10, wall height 8, coats 2 and coverage per gallon 350, the paint needed is 2.0114 gallons.
Can I rely on this calculator for a decision?
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work. For this calculator in particular: Openings must be smaller than the wall area.
Related calculators
- Square Footage Calculator — Area of a rectangular space in square feet, square metres and square yards.
- Concrete Calculator — Cubic yards of concrete for a slab, plus bag counts.
- Gravel Calculator — Cubic yards of gravel for an area.
- Board Foot Calculator — Board feet of lumber from thickness, width and length.
- Mulch Calculator — Cubic yards of mulch for a bed.
- Soil Calculator — Cubic yards of soil for an area.
- Wood Weight Calculator — Weight of a volume of wood by species (air-dried, 12% moisture).
Sources
- OpenStax, University Physics Volume 1 (Rice University): mechanics
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.