How Much Paint to Paint a House?

Measure the house as one rectangle

Not wall by wall. Walk around the outside, add the sides together, and multiply by the height from the ground to the eaves.

A 50 × 30 house is 160 ft of perimeter. At 10 ft to the eaves that's 1,600 sq ft — the walls alone figure.

Measuring each elevation separately gets you the same answer with four more chances to make a mistake at the corners.

Then add the gables, because that's the part people leave out

A gable is the triangle of wall above the eaves line at each end of a pitched roof. The perimeter measurement doesn't include it.

Area of a gable = base × rise ÷ 2.

Two gables on a 30 ft wide house with a 6 ft rise:

2 × 30 × 6 ÷ 2 = 180 sq ft.

That's a full gallon over two coats on its own — and it's the hardest area on the house to reach. Much better to know that before you rent the ladder than after.

A steeper roof has considerably more. Double the rise to 12 ft and the gables become 360 sq ft, which is two gallons.

The whole job

Walls 1,600 + gables 180 − openings 120 = 1,660 sq ft of surface to paint.

At 350 sq ft per gallon, two coats:

1,660 × 2 ÷ 350 = 9.49 gallons.

So you buy two 5-gallon pails and have half a gallon left for touch-ups. The calculator shows the exact figure alongside the rounded one, because the gap between 9.49 and 10 is exactly the kind of thing worth seeing before you're at the till.

Coverage is the input that actually moves the answer

Here's the part that surprises people: exterior paint doesn't cover less than interior paint.

Sherwin-Williams' own data sheet for SuperPaint Exterior Latex Flat quotes 350–400 sq ft per gallon at 4 mils wet, 1.4 mils dry. That's 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 real surface area than their flat measurement suggests, and they drink it.

Change coverage from 350 to 200 and this same house goes from 9.49 gallons to 16.6 — seven extra gallons from one input, without changing a single measurement.

That's the substrate costing you paint. Not the brand.

Should you deduct the windows and doors?

Deduct the big ones. Don't spend an afternoon measuring every one — a rough total is fine at this precision.

There's also a decent argument for skipping the deduction entirely: if you're 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 windows and doors to deduct to 0 if that's your situation.

Cold weather changes the job, not just the comfort

This is the one that catches people out on a spring or autumn repaint.

The same data sheet puts recoat at 4 hours above 45°F.

At 35–45°F it is 24 to 48 hours.

That is the difference between two coats in a day and two coats spread across a long weekend. And below 35°F — air, surface and material temperature — or within 5°F of the dew point, it shouldn't go on at all.

Check the overnight low, not just the afternoon forecast. A surface that drops near the dew point overnight can ruin a coat that went on perfectly at two in the afternoon.

Two coats, and why it matters more outside

Inside, the second coat is mostly about colour depth. Outside it's structural to the product: the second coat is what builds the film thickness the paint's weather resistance is rated at.

The data sheet's 350–400 figure is per coat, at 1.4 mils dry. One coat is a thinner film than the product was tested as.

One coat is defensible only for a repaint in the same colour over a sound, previously painted surface. The one coat would need output is there so you can see the difference — 4.74 gallons against 9.49 — rather than guess at it.

Buy pails, and box them

Past about 4 gallons, 5-gallon pails are cheaper per gallon. But the better reason is colour.

Several separately tinted gallons of "the same" colour are not guaranteed to be identical, and on a large uninterrupted wall that difference shows as a soft vertical seam where you switched tins.

If you do end up with multiple containers, box them: pour them all into one larger vessel, mix, and work from that. Five minutes that prevents a permanent stripe.

Before the first coat

• Wash it, and kill mildew rather than paint over it — it's a fungus and it grows back through new paint
• Scrape loose and flaking paint back to sound material
• Spot-prime bare wood, especially cedar and redwood: their tannins bleed through and stain the finish
• Caulk gaps after priming, not before
• Buy all the paint in one go, in one batch

The measuring is ten minutes. The prep is the job.