Exterior paint is not just a bigger version of a bedroom. Siding texture, gable ends, doors, windows, primer, sprayer loss, ladders, weather, and 5-gallon pails all change the buying decision. The goal is not to guess perfectly from the driveway. The goal is to get close enough that the store order and the job plan make sense.
The quick answer
A simple 30 x 40 ranch with 9 ft walls, two gable ends, two exterior doors, twelve windows, two coats, and a 10% extra allowance calculates to about 8.93 gallons of exterior paint in the MeasureTwice exterior paint calculator. That rounds to 9 one-gallon cans, or 2 five-gallon pails if you are buying larger containers.
| Example house | Calculated paint | Buy |
|---|---|---|
| 30 x 40 ranch, 9 ft walls, 2 coats | About 8.93 gallons | 9 gallons or 2 five-gal pails |
| 24 x 36 small house, 8 ft walls, 2 coats | About 6.73 gallons | 7 gallons or 2 five-gal pails |
| 40 x 50 two-story, 18 ft walls, 2 coats | About 23.37 gallons | 24 gallons or 5 five-gal pails |
Start with the house perimeter
For a rectangular house, the siding wall area starts with perimeter times wall height. A 30 x 40 footprint has a 140 ft perimeter. At 9 ft high, that is 1,260 square feet before openings and gables.
Add gables only when they are being painted
Gable ends are triangular wall areas. A practical planning formula is width times rise divided by two. If the same siding continues into the gable, include it. If the gable is brick, stone, shake you are not painting, or a different material, leave it out.
Subtract doors and windows, but stay realistic
Subtracting openings gives a tighter siding estimate. The calculator uses planning shortcuts for exterior doors and windows. Do not chase perfect precision for every pane; exterior paint has enough real-world loss from siding texture, masking, and touchups that a clean planning estimate is usually more useful than false precision.
Coverage depends heavily on siding
Smooth fiber cement or previously painted wood may cover differently than rough stucco, older lap siding, masonry, or patched surfaces. Manufacturer and retailer calculators commonly remind users to check the product label. Use the label coverage in the calculator when you know the exact paint you are buying.
When primer belongs in the estimate
Use primer for bare wood, patched siding, stains, chalky surfaces, masonry, raw repairs, and major color changes. Primer is not just another color coat. It may have a different coverage rate, and it may be needed only on problem areas rather than the entire house.
If the whole exterior needs primer, turn primer on in the calculator. If only patches or bare boards need primer, calculate the main paint normally and buy primer based on the repair area.
One-gallon cans or five-gallon pails?
Five-gallon pails make sense when the body color is consistent across the house. They reduce container changes and often fit larger exterior jobs better. One-gallon cans are useful for trim colors, doors, shutters, small sections, or when you are not fully sure the final color is locked.
Keep trim separate
House body paint, fascia, shutters, doors, and trim may use different sheens or colors. Estimate the siding body first, then use a trim-specific approach for the detailed work. Interior trim is handled by the trim paint calculator; exterior trim can use the same idea: linear feet, width, coats, and product coverage.
Before you buy
- Measure the footprint length and width.
- Enter wall height and gable rise.
- Count exterior doors and windows.
- Set coats, paint coverage, primer coverage, and extra allowance.
- Decide whether one-gallon cans or five-gallon pails fit the job.
- Check weather, prep time, ladders, masking, and safe access before scheduling the work.
Exterior paint quantity is a planning number, not a promise from the wall. Measure the house, use the calculator, then confirm the final coverage with the product label and the condition of your siding.