To calculate paint needed for an exterior, measure the length of every outside wall and multiply by its height, add those wall areas together, subtract the area of doors, windows and garage doors you are not painting, multiply the result by the number of coats (usually two), and divide by the coverage printed on the tin — then round up to whole gallons. The full formula reads: paint to buy = ⌈(total wall area − openings) × coats ÷ coverage⌉, where coverage is the square feet one gallon covers in a single coat on a smooth primed surface. A two-story home with roughly 2,000 square feet of siding, minus 150 square feet of openings, painted in two coats at 350 square feet per gallon, needs about 11 gallons of finish. Doing that same arithmetic by hand for a full exterior is error-prone because openings and coverage vary by surface, so the Paint Calculator runs the formula live as you type and keeps the unit pair consistent — square feet with gallons, or square metres with litres.
Why exterior paint jobs need their own estimate
Exterior and interior paint jobs share the same formula, but the inputs drift in opposite directions. Outside, surfaces are rarely the smooth, primed drywall that tin labels assume — siding is often textured or rough-sawn, stucco drinks paint, and bare wood or weathered fibre-cement soaks the first coat faster than the suggested rate. Wind, sun and temperature swings also push painters to lay paint on thicker than they would indoors, which uses more per square foot. At the same time, exterior openings are larger: a single garage door can be 80 to 120 square feet, picture windows run 25 to 40 square feet each, and a sliding patio door lands in the same range. Those numbers matter because the paint formula subtracts every opening you are not painting, so overestimating openings under-buys paint and underestimating them leaves a half-empty tin at the end. The Paint Calculator handles both ends: it rounds the final answer up to whole gallons so you never fall short on the day, and it refuses to let the openings equal or exceed the wall area, because there has to be something left to paint.
Measuring each exterior wall
Measuring exterior walls means walking the perimeter with a tape measure, not reading from a floor plan. Walls are not always rectangular — gable ends are triangles, dormers add odd shapes, and bay windows break a flat elevation into multiple flat pieces. The cleanest method is to break the home into rectangles plus triangles, measure each piece, and add their areas together.
For a rectangular wall, multiply length by height. Measure length along the base from corner to corner, and height from the top of the foundation (or grade) to the eave or roofline. For a typical two-story home with a 10-foot first floor, an 8-foot second floor and a gable on top, the average wall height runs around 12 to 15 feet, not a flat 8. For a single-story ranch, 8 to 9 feet is closer to reality.
For gable triangles, multiply the base of the wall (width) by the height of the triangle (from the top of the rectangle below to the peak) and divide by 2. Add that triangular area to the rectangular wall beneath it.
Enter the sum — length of every wall times eave height, all added — into the wall area field of the Paint Calculator. Keep every measurement in the same unit; mixing feet and metres silently produces wrong gallons because the coverage and output pair automatically.
Subtracting doors, windows and garage openings
You do not paint glass, door faces or the panels of a garage door, so knock their area off the total before multiplying by coats. The calculator adds every opening you enter into the deductions field and refuses to let the total equal or exceed the wall area, because then there is nothing left to paint. Reasonable opening sizes to use as defaults:
| Opening | Rough size | Area to subtract |
|---|---|---|
| Entry door | 3 ft × 7 ft | ~20 sq ft |
| Standard window | 2.5 ft × 4 ft | ~10 sq ft |
| Large picture window | 5 ft × 6 ft | ~30 sq ft |
| Sliding patio door | 6 ft × 7 ft | ~40 sq ft |
| Single garage door | 8 ft × 7 ft | ~55 sq ft |
| Double garage door | 16 ft × 7 ft | ~110 sq ft |
If you would rather skip the deduction step entirely, leave the openings field blank — the result rounds up slightly and that small over-estimate becomes spare paint for future touch-ups.
How coverage changes outside
Coverage is the area one tin covers in a single coat, and the figure on the label assumes smooth, primed, ideal-condition walls. On exterior siding it almost always lands lower than the number printed on the can.
As a starting point, one US gallon covers about 350 to 400 square feet per coat and one litre of exterior emulsion covers about 10 to 12 square metres per coat, on smooth primed surfaces. Rough-sawn timber, stucco, porous fibre-cement and bare masonry drink more — real-world coverage commonly runs 10 to 20 percent below the tin number on those substrates. Gloss and semi-gloss finishes also spread less per coat than flat or satin finishes, because they contain less solids by volume and lay down thinner.
The calculator multiplies your net wall area by the number of coats, divides by the coverage you entered, and rounds the result up to the next whole gallon or litre. If your siding is rough or the paint is glossy, lower the coverage figure you type in (for example, use 300 instead of 350 sq ft per gallon) and the buy recommendation grows accordingly. The exact numbers for your surface and product come straight from the tool — adjust the coverage field and read the new answer.
Worked example: a small single-story home
Walk through the math once with real numbers so the formula sticks. A single-story home has 220 linear feet of exterior wall and an 8-foot eave height.
- Total wall area: 220 × 8 = 1,760 sq ft.
- Subtract openings: one entry door (~20 sq ft), six standard windows (~10 sq ft each), and one 8 × 7 ft garage door (~55 sq ft). Total openings = 20 + 60 + 55 = 135 sq ft. Net wall area = 1,760 − 135 = 1,625 sq ft.
- Apply coats: most repaints need two coats for an even, durable finish, so 1,625 × 2 = 3,250 sq ft of paint coverage to lay down.
- Divide by coverage: at 350 sq ft per gallon, 3,250 ÷ 350 ≈ 9.29 gallons. Rounded up to whole gallons: 10 gallons of finish.
That 10-gallon figure assumes smooth primed siding and standard 350 sq ft/gal coverage. On rough-sawn timber at the lower end of real-world coverage (about 290 sq ft/gal), the same job needs 3,250 ÷ 290 ≈ 11.21, rounded up to 12 gallons — a two-tin jump that vanishes if you keep the label number and ignore the surface. For a step-by-step sanity check on any home, the Paint Calculator returns this same total live as you adjust the inputs.
Tips for buying exterior paint with confidence
A few habits keep the estimate honest once you are at the counter.
- Buy from a single batch. Colours drift slightly between production runs, so an extra tin from the same batch is the only reliable touch-up supply. The calculator already rounds up to whole gallons, but a dedicated extra tin is still worth it on a full exterior.
- Decide on coats before you buy, not after. Two coats is the standard for an even, durable finish, so the calculator defaults to two. A fresh colour over matching primer may manage one; a light colour over a dark wall, or bare and porous siding, frequently needs three. Each extra coat adds one full net wall area to the paint volume.
- Mute the surfaces the calculator does not cover. Soffits, fascia, trim, shutters and a separate primer coat are separate calculations. If the primer is a different product, count it on the label of the primer tin, not the topcoat.
- Mind the weather. Exterior paint goes on when the surface is dry and the temperature sits inside the range printed on the tin. Painting outside the range wastes coverage and ruins the finish, even if your gallon estimate was perfect.
Once you know the gallons, you can translate them into dollars with the How to Calculate Paint Cost Per Square Foot for Any Room guide, which uses the same wall-area inputs to turn a coverage number into a budget.
Putting it all together
Measuring twice and rounding up is the whole job. Total wall area, minus openings, times coats, divided by coverage, rounded to whole tins. For most single-story exteriors that math lands between 8 and 12 gallons of finish; for two-story homes with more siding it commonly rises into the high teens. The Paint Calculator does the arithmetic in your browser as the numbers change, so the only thing left is the measuring tape and the trip to the store.