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HOME · INTERIOR + EXTERIORPaint Calculator
How much paint do you actually need — and what will it cost? This paint calculator works out interior rooms, whole exteriors and fences from your measurements, subtracts the doors and windows, and rounds up to the whole gallons you will buy at the store. One trip, no half-empty cans left over for a decade.
- 100% free, no sign-up
- Interior, exterior + fence modes
- Trim + primer calculated separately
- Cost estimate included
Paintable area, minus doors and windows, times the number of coats, divided by the coverage on the can. That is the whole calculation — the rest is measuring carefully.
A standard interior door is about 21 square feet and a standard window about 15. Exterior doors run about 24 and exterior windows about 20 — the page uses the right pair per mode.
Stores do not sell 1.72 gallons. The page shows the exact figure for honesty and the whole gallons to buy for the shopping list.
Smooth interior walls take about 350 square feet per gallon; rough stucco takes about 250 and brick as little as 175. Pick the preset that matches your wall.
Your project
Interior mode is one room. Exterior mode is a whole house with siding, gables, trim and primer. Fence mode is a length of fence or deck boards.
The longer wall of the room, in feet.
The shorter wall of the room, in feet.
Floor to ceiling — 8 ft is standard.
Two coats is standard, especially over a new colour.
Each standard interior door is about 21 sq ft, subtracted automatically.
Each standard window is about 15 sq ft, subtracted automatically.
The long side of the house footprint.
The short side of the house footprint.
Ground to eaves — about 9 ft per storey.
Two coats for a colour change or bare wood.
Triangular wall sections under the roof peaks — usually 2.
Peak height above the eaves for one gable.
Each about 24 sq ft, subtracted automatically.
Each about 20 sq ft, subtracted automatically.
Fascia, soffits and window trim in linear feet. Trim paint is calculated separately at 250 sq ft per gallon.
One primer coat over the body area, at 300 sq ft per gallon. Primer is for bare or patched surfaces.
Run of fence, or total board length for a deck.
Fence height, or board width for a deck laid flat.
A fence seen from both sides needs both sides painted.
Two coats for weather protection outdoors.
Paint & cost
Coverage comes from your paint can — the presets below are starting points you can adjust.
Pick the closest surface, then fine-tune the coverage number below if your can says otherwise.
From the can. Rough surfaces drink more paint, so they cover fewer square feet per gallon.
Your paint's shelf price. The cost estimate multiplies it by the gallons to buy.
Please note: coverage varies by brand, colour and surface — dark colours over light walls and rough surfaces both drink more paint. The can's own coverage figure beats any preset here. Buy an extra quart for touch-ups if the store is far away.
How this paint calculator works
Three steps, in the order the math runs — measure first, subtract the openings second, divide by coverage last.
Measure the area
For a room, enter length, width and wall height — the page works out the wall area from the perimeter. For a house, enter the footprint, the height to the eaves and the gables. For a fence, enter length, height and how many sides.
Subtract what you won't paint
Doors and windows are subtracted at standard sizes: 21 and 15 square feet indoors, 24 and 20 outdoors. If your doors or windows are unusually large, the coverage number below is where you absorb the difference.
Divide by coverage, round up
Multiply the net area by the coats, divide by the coverage from your paint can, and round up to whole gallons. The cost estimate multiplies those gallons by your price per gallon.
Coverage: the number that decides everything
Most paint estimates go wrong on one number: coverage. The can's figure is a laboratory number for smooth walls — your walls are not laboratory walls.
Read the can, then adjust
Start from the coverage printed on your paint can, then adjust for reality: rough or textured surfaces drink more paint, dark colours over light walls usually need the extra coat, and bare drywall or fresh plaster drinks primer like a sponge.
The presets on this page — 350 for interior walls, 300 for exteriors, 250 for rough surfaces, 175 for brick — are honest starting points. If your can disagrees, type the can's number in; the page recalculates instantly.
Buy a little extra, on purpose
Two reasons to round up beyond the math: touch-ups and batch matching. A quart left over from the same batch touches up invisibly; a new can from a different batch can show a faint line. Keep the leftover, write the colour code on the lid, and store it somewhere it will not freeze.
Unopened cans can usually go back to the store, so the cost of one extra gallon is mostly the cost of a second trip you did not have to make.
Worked examples
Three real projects, worked through the same formula. The first is the example pre-filled in the calculator.
| Project | Paintable area | Exact gallons | Buy | Cost |
|---|---|---|---|---|
| Bedroom 12 × 10 ft, 8 ft ceiling, 1 door, 2 windows, 2 coats, 350 sq ft/gal | 301 sq ft | 1.72 gal | 2 gal | $100 at $50/gal |
| House 40 × 30 ft, 18 ft to eaves, 2 gables, smooth siding, 2 coats, 300 sq ft/gal | 2,552 sq ft | 17.01 gal body | 18 gal body + 1 trim | $1,045 at $55/gal |
| Fence 100 ft × 6 ft, both sides, 2 coats, 250 sq ft/gal | 1,200 sq ft | 9.60 gal | 10 gal | $450 at $45/gal |
Example 1: the bedroom walls are 2 × (12 + 10) × 8 = 352 square feet, minus 21 for the door and 30 for two windows, which leaves 301. Times 2 coats is 602, divided by 350 is 1.72 gallons exact — buy 2, which costs $100 at $50 a gallon. Example 2: the house walls are a 140-foot perimeter times 18 feet, plus two gables of 120 square feet each, which is 2,760, minus 48 for two doors and 160 for eight windows, leaving 2,552. Times 2 coats divided by 300 is 17.01 gallons of body paint — buy 18 — plus 1 gallon of trim from 200 linear feet, which is $1,045 at $55 a gallon for the 19 gallons. Example 3: the fence is 100 × 6 × 2 sides = 1,200 square feet, times 2 coats divided by 250 is 9.60 gallons — buy 10, which is $450 at $45 a gallon.
Paint questions, answered
How much paint do I need?
Measure the walls, subtract the doors and windows, multiply by the coats and divide by the coverage on the can. This paint calculator does exactly that: a 12 × 10 ft bedroom with an 8 ft ceiling, one door and two windows needs 2 gallons at two coats and 350 square feet per gallon. For a house or a fence, switch modes — the method is the same, the measurements change.
What does an exterior paint calculator do differently?
It measures the house instead of a room: footprint perimeter times height to the eaves, plus the triangular gable ends, minus exterior doors and windows — and it calculates trim separately, because trim is usually a different paint. This page's exterior mode also offers a primer coat for bare or patched surfaces.
Can I use this as a painting cost calculator?
Yes. Enter your price per gallon and the page multiplies it by the whole gallons to buy, so you get an estimated paint cost next to the quantities. It covers paint only — brushes, rollers, tape and drop cloths are extra, and a pro's labour is a different budget entirely.
What about a fence painting cost calculator?
Switch to the fence mode: enter the fence length, height and how many sides, plus coats and coverage, and the page gives you gallons and cost. Rough outdoor wood drinks paint, so the 250 square feet per gallon preset is the honest starting point for an old fence.
Should I count primer separately?
Yes, when the surface is bare, patched or a big colour change. Primer has its own coverage — about 300 square feet per gallon — and its own price, so this page calculates it as its own line in exterior mode rather than folding it into the body paint.
What if my walls are textured or dark?
Drop the coverage. Rough stucco or heavy texture takes about 250 square feet per gallon instead of 350, and brick as little as 175. Going dark over light usually costs you the second coat you were hoping to skip. When in doubt, buy the extra gallon — unopened cans go back.