Exterior Paint Calculator

How this is calculated

A house is measured as PERIMETER times WALL HEIGHT. Walk around the outside, add the sides together, and multiply by the height from the ground to the eaves. That single rectangle is the bulk of the job, and measuring wall by wall only invites arithmetic mistakes at the corners. THE GABLES ARE THE PART PEOPLE FORGET. A gable is the triangle of wall above the eaves line at each end of a pitched roof, and its area is half the base times the rise. Two gables on a 30 ft wide house with a 6 ft rise are 180 square feet - which is a full gallon over two coats, on their own. They are also the hardest part to reach, so they are worth knowing about before the ladder goes up. THE ARITHMETIC: (perimeter x height) + gables - openings, times coats, divided by coverage. COVERAGE IS THE NUMBER THAT MOVES THE ANSWER. Sherwin-Williams' own data sheet for SuperPaint Exterior Latex Flat quotes 350-400 square feet per gallon at 4 mils wet, 1.4 mils dry - and that figure is for a SMOOTH, sound surface. The paint is not what changes; the substrate is. Smooth lap siding takes the quoted rate. Rough-sawn cedar, stucco, brick and split-face block have far more real surface than their flat measurement suggests, and they drink it. Dropping coverage from 350 to 200 takes this same house from 9.5 gallons to 16.6 - seven extra gallons, from one input. ⚠️ DEDUCT ONLY THE BIG OPENINGS. Windows and doors come off the total, but do not spend an afternoon measuring every one. A rough total is fine, and if you are painting the trim and fascia in the same colour you can reasonably skip the deduction entirely - the trim gives back roughly what the glass takes away. ⚠️ COLD WEATHER CHANGES THE JOB, NOT JUST THE COMFORT. The same data sheet puts recoat at 4 hours above 45F - but at 35-45F it is 24 to 48 HOURS. That is the difference between two coats in a day and two coats across a long weekend. Below 35F, or within 5 degrees of the dew point, it should not go on at all. Check the overnight low, not just the afternoon forecast. TWO COATS IS THE NORMAL ANSWER on exterior work, and it matters more outside than in: the second coat is what builds the film thickness the paint's weather resistance is rated at. One coat is a repaint in the same colour on a sound, previously painted surface, and nothing else. BUY IN 5-GALLON PAILS ONCE YOU ARE PAST ABOUT 4 GALLONS. It is cheaper per gallon, and - more usefully - it removes the batch-to-batch colour variation you can get across several separately tinted gallons. If you must use multiple containers, box them: pour them all into one larger vessel and mix before starting.

Common questions

How much paint do I need to paint a house?

For a typical single-storey house - 160 ft around, 10 ft to the eaves, two 30 ft gables with a 6 ft rise, minus 120 sq ft of windows and doors - the answer is 1,660 square feet of surface, which is 9.49 gallons for two coats at 350 square feet per gallon. You would buy two 5-gallon pails. The figure that moves that answer most is not the size of the house, it is the coverage: the same house on stucco at 200 sq ft per gallon needs 16.6 gallons.

Do I need to measure every wall separately?

No, and you are more likely to make a mistake if you do. Walk around the outside of the house and add the side lengths together into one perimeter, then multiply by the height from the ground to the eaves. That treats the whole house as one unrolled rectangle, which is exactly what it is as far as paint is concerned. Then add the gables separately, because they sit above the eaves line and the perimeter measurement does not include them.

What about the gable ends?

They are the part people leave out, and they are not small. A gable is the triangle of wall above the eaves at each end of a pitched roof, and its area is half the base times the rise. Two gables on a 30 ft wide house with a 6 ft rise come to 180 square feet - a full gallon over two coats. A steeper roof has considerably more. They are also the hardest area to reach, which is a good reason to know the number before you rent the ladder.

Why does exterior paint cover less than interior paint?

Often it does not - the paint is much the same. Sherwin-Williams' data sheet for SuperPaint Exterior Latex Flat quotes 350-400 square feet per gallon, the same range as interior paint. What changes is the SURFACE. That figure assumes something smooth and sound. Rough-sawn cedar, stucco, brick and split-face block have far more actual surface area than their flat measurement suggests, and coverage on them can fall to 200 or below. It is the substrate that costs you paint, not the fact that it is outdoors.

Should I deduct the windows and doors?

Deduct the big ones and do not agonise over the rest. A rough total is fine at this level of precision. There is also a reasonable argument for skipping the deduction entirely: if you are painting the trim, fascia and soffits in the same colour, that work gives back roughly what the glass takes away, and the leftover becomes your touch-up paint. Set the deduction to 0 if that is your situation.

How cold is too cold to paint outside?

The SuperPaint Exterior data sheet puts the floor at 35F for air, surface AND material temperature, and requires the surface to be at least 5 degrees above the dew point. But the number that actually changes your plans is recoat time: 4 hours above 45F, and 24 to 48 HOURS at 35-45F. That turns a two-coat weekend into a multi-day job. Check the overnight low as well as the afternoon forecast - a surface that drops near the dew point overnight can ruin a coat that went on fine at two in the afternoon.

Is it worth buying 5-gallon pails?

Past about 4 gallons, yes, for two reasons. It is cheaper per gallon, and it removes batch-to-batch colour variation - several separately tinted gallons of the same colour are not guaranteed to be identical, and on a large uninterrupted wall that shows. If you do end up with multiple containers, box them: pour them all into one larger vessel, mix, and work from that. It is five minutes that prevents a visible seam.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Exterior Paint Calculator at https://www.bfcbrilliance.com/tools/exterior-paint-calculator.

What I entered:
- Perimeter of the house (ft): ___
- Wall height to the eaves (ft): ___
- Number of gable ends: ___
- Gable width (ft): ___
- Gable rise (ft): ___
- Windows and doors to deduct (sq ft): ___
- Coats: ___
- Coverage (sq ft/gal): ___

What it calculated:
- Paint to buy: ___
- Exact gallons needed: ___
- Surface to paint: ___
- Walls alone: ___
- Gables alone: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Where the job touches structure, gas, or electrics, tell me to check local building code and whether a permit or a licensed trade is required.

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