Measure room length, width and wall height, then count doors and windows and enter their actual average excluded areas. The initial 20 ft² per door and 15 ft² per window are assumptions, not standard opening sizes. Enter the product’s coverage and coat count.
A ceiling and trim require separate areas.
Use product instructions and actual surface conditions for coverage and preparation. The area model does not determine primer, application method or extra coats. Mixed opening sizes can be represented by an area-weighted average or counted and calculated separately.
Zero coverage cannot produce a valid gallon quantity.
Paint: worked example
| Room length (ft) | 14 |
|---|---|
| Room width (ft) | 12 |
| Wall height (ft) | 8 |
| Number of doors | 2 |
| Number of windows | 3 |
| Average excluded door area (sq ft) | 20 |
| Average excluded window area (sq ft) | 15 |
| Coats of paint | 2 |
| Coverage per gallon (sq ft) | 350 |
| Gallons needed | 1.9 |
| Paintable wall area (sq ft) | 331 |
| Total area to cover (sq ft) | 662 |
Common error: Skipping the second coat in the math. Many colors need two for even coverage, and buying for one leaves the job unfinished.
Paint: formula
NIST Handbook 44: Units and Systems of Measurement · Rectangle: Wolfram MathWorld
Paint: changing Room length (ft)
| Room length (ft) | Gallons needed | Paintable wall area (sq ft) | Total area to cover (sq ft) |
|---|---|---|---|
| 11 | 1.6 | 283 | 566 |
| 13 | 1.8 | 315 | 630 |
| 14 (example) | 1.9 | 331 | 662 |
| 15 | 2 | 347 | 694 |
| 17 | 2.2 | 379 | 758 |
Paint: compare input ranges
| Input | Tested from | To | Gallons needed: low input | Gallons needed: high input | Swing |
|---|---|---|---|---|---|
| Coats of paint | 1 | 3 | 0.946 | 2.8 | 1.9 (100%) |
| Wall height (ft) | 7 | 9 | 1.6 | 2.2 | 0.594 (31%) |
| Coverage per gallon (sq ft) | 315 | 385 | 2.1 | 1.7 | 0.382 (20%) |
| Number of doors | 1 | 3 | 2 | 1.8 | 0.229 (12%) |
| Room length (ft) | 13 | 15 | 1.8 | 2 | 0.183 (9.7%) |
| Room width (ft) | 11 | 13 | 1.8 | 2 | 0.183 (9.7%) |
| Number of windows | 2 | 4 | 2 | 1.8 | 0.171 (9.1%) |
| Average excluded window area (sq ft) | 14 | 17 | 1.9 | 1.9 | 0.051 (2.7%) |
| Average excluded door area (sq ft) | 18 | 22 | 1.9 | 1.9 | 0.046 (2.4%) |
Paint: individual input comparisons
| Input | Value | Gallons needed | Paintable wall area (sq ft) | Total area to cover (sq ft) |
|---|---|---|---|---|
| Room length (ft) | 11 | 1.6 | 283 | 566 |
| 13 | 1.8 | 315 | 630 | |
| 14 | 1.9 | 331 | 662 | |
| 15 | 2 | 347 | 694 | |
| 17 | 2.2 | 379 | 758 | |
| Room width (ft) | 10 | 1.7 | 299 | 598 |
| 11 | 1.8 | 315 | 630 | |
| 12 | 1.9 | 331 | 662 | |
| 13 | 2 | 347 | 694 | |
| 14 | 2.1 | 363 | 726 | |
| Wall height (ft) | 6 | 1.3 | 227 | 454 |
| 7 | 1.6 | 279 | 558 | |
| 8 | 1.9 | 331 | 662 | |
| 9 | 2.2 | 383 | 766 | |
| 10 | 2.5 | 435 | 870 | |
| Number of doors | 0 | 2.1 | 371 | 742 |
| 1 | 2 | 351 | 702 | |
| 2 | 1.9 | 331 | 662 | |
| 3 | 1.8 | 311 | 622 | |
| 4 | 1.7 | 291 | 582 | |
| Number of windows | 1 | 2.1 | 361 | 722 |
| 2 | 2 | 346 | 692 | |
| 3 | 1.9 | 331 | 662 | |
| 4 | 1.8 | 316 | 632 | |
| 5 | 1.7 | 301 | 602 | |
| Average excluded door area (sq ft) | 16 | 1.9 | 339 | 678 |
| 18 | 1.9 | 335 | 670 | |
| 20 | 1.9 | 331 | 662 | |
| 22 | 1.9 | 327 | 654 | |
| 24 | 1.8 | 323 | 646 | |
| Average excluded window area (sq ft) | 12 | 1.9 | 340 | 680 |
| 14 | 1.9 | 334 | 668 | |
| 15 | 1.9 | 331 | 662 | |
| 17 | 1.9 | 325 | 650 | |
| 18 | 1.8 | 322 | 644 | |
| Coats of paint | 0 | 0 | 331 | 0 |
| 1 | 0.946 | 331 | 331 | |
| 2 | 1.9 | 331 | 662 | |
| 3 | 2.8 | 331 | 993 | |
| 4 | 3.8 | 331 | 1,324 | |
| Coverage per gallon (sq ft) | 280 | 2.4 | 331 | 662 |
| 315 | 2.1 | 331 | 662 | |
| 350 | 1.9 | 331 | 662 | |
| 385 | 1.7 | 331 | 662 | |
| 420 | 1.6 | 331 | 662 |
Paint: combined input changes
| Room length (ft) / Room width (ft) | 10 | 12 | 14 |
|---|---|---|---|
| 11 | 1.4 | 1.6 | 1.8 |
| 13 | 1.6 | 1.8 | 2 |
| 14 | 1.7 | 1.9 | 2.1 |
| 15 | 1.8 | 2 | 2.2 |
| 17 | 2 | 2.2 | 2.3 |
Paint: which assumptions move the result?
Paint: values and definitions
| Quantity and definition | Value used |
|---|---|
| Room length (ft) | 14 |
| Room width (ft) | 12 |
| Wall height (ft) | 8 |
| Number of doors — Count doors separately from their entered average area. | 2 |
| Number of windows — Count windows separately from their entered average area. | 3 |
| Average excluded door area (sq ft) — Assumed 20 square feet; replace with measured excluded area per door. | 20 |
| Average excluded window area (sq ft) — Assumed 15 square feet; replace with measured excluded area per window. | 15 |
| Coats of paint | 2 |
| Coverage per gallon (sq ft) — Use the actual product coverage and surface conditions; 350 is an assumed example. | 350 |
| Gallons needed | 1.9 |
| Paintable wall area (sq ft) | 331 |
| Total area to cover (sq ft) | 662 |
Paint: half and double Coats of paint
| Coats of paint | Gallons needed |
|---|---|
| 1 | 0.946 |
| 2 | 1.9 |
| 4 | 3.8 |
Home & Construction calculators: Fence Calculator, Drywall Calculator, Paver Calculator, Wallpaper Calculator, Carpet Calculator, Rainwater Harvesting Calculator.
Paint is bought by the gallon but covers by the square foot, so the job is turning a room into wall area. Four walls of a 14 by 12 room at 8 feet give 416 square feet, less about 85 for two doors and three windows. Two coats over 331 square feet needs about 1.9 gallons at 350 square feet per gallon, so two cans cover it with little left.
The paint can’s stated coverage is the product-specific area-per-coat input. Rough or dark surfaces take more, and coverage is quoted for smooth, primed walls.
Calculate the wall area on the square footage and rectangle calculators, wallpaper needs on the wallpaper calculator, and drywall on the drywall page. The cost of materials can be compared using the unit price comparison calculator. The material volume and tile pages cover floors.
How much paint do I need for a room?
Divide paintable area times coat count by the coverage stated for the selected paint and surface. Enter that coverage rather than treating the example value as a product specification.
Do I need primer?
For new drywall, stains, dramatic color changes or glossy surfaces, yes. Otherwise a paint-and-primer product may be enough.
Should I paint the ceiling first?
Yes. Paint the ceiling first, then the walls, then the trim, to avoid drips on finished surfaces.