From measured space to purchasable material
Exterior walls and trim often use different coatings, coverage rates and numbers of coats. This calculator separates the two purchases instead of applying one paint rate to the whole house. Wall area includes rectangular elevations and optional triangular gables, with opening deductions. Trim is entered as a measured paintable area. Coverage values should come from the selected coating for the actual surface, since rough masonry and smooth siding can absorb very different quantities. The estimate excludes access equipment, repairs, preparation materials and any primer that the coating system requires.
Measure, select yield and round the order
- Measure exterior wall and gable areas without counting openings twice.
- Enter product-specific wall and trim coverage separately.
- Round each coating to the container size you will buy.
Net requirement and purchase calculation
A worked example
1,200 ft² of rectangular walls plus 120 ft² gables minus 200 ft² openings gives 1,120 ft². At two coats, 300 ft²/gal and 10% allowance, wall paint is 8.21 gal, rounded to 9 one-gallon containers.
Exterior Paint: what changes the result?
Keep measured geometry separate from coverage, laps and purchase units. 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 |
|---|---|---|---|
| Gross rectangular wall area | 1,200 sq ft | 1,440 sq ft | Net paintable wall area: 1,120 → 1,360 sq ft Calculated wall paint: 8.2133 → 9.9733 US gal |
| Total triangular gable area | 120 sq ft | 144 sq ft | Net paintable wall area: 1,120 → 1,144 sq ft Calculated wall paint: 8.2133 → 8.3893 US gal |
| Unpainted wall openings | 200 sq ft | 240 sq ft | Net paintable wall area: 1,120 → 1,080 sq ft Calculated wall paint: 8.2133 → 7.92 US gal |
| Wall coats | 2 | 3 | Calculated wall paint: 8.2133 → 12.32 US gal Wall paint containers: 9 → 13 |
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
- Porous or textured surfaces may use more coating than a smooth-surface example.
- Primer is not automatically included in finish-coat coverage.
- Work-at-height, existing coating condition and preparation must be planned separately.
Frequently asked questions
Why calculate exterior trim separately?
Trim can use a different product and coverage rate, and its leftover material cannot necessarily substitute for wall coating.
Does this include primer?
No. Calculate primer separately when the coating system or surface requires it.
Can coverage be lower for rough stucco?
Yes. Use the relevant product’s coverage range for the actual surface rather than assuming the example applies.
Calculation method: JICWH methodology. Examples are planning assumptions, not product recommendations.