How the paint calculator works
A room measurement can support a useful buying estimate, but it cannot predict exactly how paint will behave on your wall. The calculator shows a low, likely, and high amount so that uncertainty stays visible.
1. Measure the surfaces
Wall area: 2 × (length + width) × height
Ceiling area: length × width
Door and window allowances are subtracted from wall area. The result never drops below zero.
2. Apply coats and coverage
Paintable area is multiplied by the number of coats, then divided by an adjusted coverage rate. Until a label rate is supplied, the calculator uses these planning ranges:
| Selection | High coverage | Likely | Low coverage |
|---|---|---|---|
| Budget | 400 | 350 | 300 |
| Standard | 450 | 400 | 350 |
| Premium | 500 | 450 | 400 |
Rates are square feet per gallon. These are planning assumptions, not promises about a product. Use the exact label or technical data sheet when available.
3. Adjust for the surface
The selected texture and condition reduce the working coverage rate. Smooth, previously painted walls use the base rate. Heavy texture, new drywall, stains, and glossy surfaces widen the practical uncertainty.
These adjustments are intentionally visible in the detailed breakdown. They are approximations. A small test area is the best way to learn how a specific coating and surface work together.
4. Add a buffer and round for purchase
When selected, the calculator adds 15 percent after estimating the raw gallons. It then rounds each point in the range up to a whole gallon because that is the common purchase unit. The unrounded likely amount remains visible, so you can see when a rounded result is close to the next can.
Worked example
A 20 × 20 foot room with 8 foot walls has 640 square feet of wall area.
Two coats require 1,280 square feet of coverage.
At the likely standard rate of 400 square feet per gallon, that is 3.20 gallons before the buffer.
With a 15 percent buffer, it becomes 3.68 gallons, which rounds to a four-gallon purchase. The conservative end rounds to five gallons.
What this calculator does not know
- The exact coating, color change, tint base, roller nap, spray loss, or painter technique unless you account for it.
- Whether a wall needs repair, sealing, or a separate primer system.
- Whether two surfaces in the same estimate will use different products.
- Package sizes, return rules, or current prices at your store.
Calculation version 1. Last reviewed July 11, 2026.