Free Contractor Tool
Paint Calculator
Enter your wall dimensions and the calculator subtracts doors and windows, adjusts coverage for your surface, and tells you exactly how many gallons to buy — and what it will cost.
Built by SubcontractorHub — the software contractors use to quote, finance, and run every job.
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How Much Paint Do I Need?
Add each wall, set the number of doors and windows, pick your surface and coat count, and get gallons instantly. Results update as you type.
Sets the coverage rate. Rougher surfaces hold more paint.
Walls to paint
21 sq ft each
15 sq ft each
Paint needed
Gallons needed
0.3
Cans to buy
1
Paintable area
60
Est. cost
Enter a price above
Gross wall area
96 sq ft
Openings removed
36 sq ft
Coverage rate
400 sq ft/gal
Coats
2
Paint Coverage by Surface
The “400 square feet per gallon” on the can assumes an ideal surface. Real coverage depends on how much texture the paint has to fill.
How many coats do you actually need?
Two coats is the working default, and most manufacturers require it for their warranty to hold. One coat is only realistic for a same-colour refresh over a clean, sealed wall.
Budget three coats when going from dark to light, when covering saturated reds, oranges, or yellows, or when painting over patched drywall that has not been primed. Quoting two and needing three is one of the fastest ways to lose margin on a repaint.
Do you subtract doors and windows?
Subtract any opening bigger than a standard door. A door is about 21 square feet and an average window about 15. On one bedroom that is barely half a gallon, so many painters ignore it.
On a full exterior with twenty-plus openings it adds up to several gallons — enough to matter on the material line. This calculator subtracts them for you so the estimate stays honest either way.
Worked Examples
12 × 12 bedroom
Four 12-foot walls at 8 feet is 384 sq ft. Take out one door and one window and you have 348 sq ft. At 400 sq ft/gal that is 0.87 gallons per coat — call it 2 gallons for two coats. Add 1.4 gallons if the ceiling is included.
Open-plan living area
A 24 × 16 room with 9-foot ceilings gives 720 sq ft of wall. Remove two doors and four windows (102 sq ft) for 618 sq ft. On light orange-peel texture at 350 sq ft/gal, two coats needs about 3.5 gallons — buy 4.
Stucco exterior
1,800 sq ft of stucco wall less 250 sq ft of openings is 1,550 sq ft. At 200 sq ft/gal, two coats is 15.5 gallons — roughly three 5-gallon buckets. The same area on smooth siding would take half that.
From Material Takeoffs to a Signed Proposal
Knowing the gallon count is the easy part. SubcontractorHub turns your measurements into a branded proposal you can present on a tablet before you leave the driveway — materials, labor, and markup already built in.
Offer good-better-best options, show monthly financing, take a digital signature on the spot, and push the signed job straight to your schedule.
See How It WorksTurn takeoffs into proposals in minutes
Materials, labor, and markup calculated automatically
Good-better-best options in one presentation
Financing and monthly payments shown on site
Signature captured and job sent straight to ops
Paint quantities are estimates based on the dimensions, surface type, and coat count entered. Actual coverage varies with paint quality, application method, colour change, and how porous the substrate is. Check the coverage rate printed on your product and buy a little extra for touch-ups.
Frequently Asked Questions
- How much paint do I need?
- Add up the wall area by multiplying each wall's width by the ceiling height, subtract the area of doors and windows, then divide by the paint's coverage rate — usually about 350 to 400 square feet per gallon on smooth walls. Multiply by the number of coats. A standard 12 by 12 foot room with 8-foot ceilings needs roughly 1.2 gallons for two coats.
- How much does a gallon of paint cover?
- One gallon covers about 350 to 400 square feet on smooth, previously painted drywall. Lightly textured walls drop to around 350, heavy knockdown texture to about 300, wood siding to roughly 300, and porous surfaces like stucco or brick to as little as 150 to 200 square feet per gallon.
- How many coats of paint do I need?
- Two coats is standard and what most manufacturers require for their warranty. One coat can work for a same-colour refresh over a clean, sealed surface. Plan on three coats when going from a dark colour to a light one, when covering strong reds or yellows, or when painting over bare patched drywall without primer.
- Do I subtract windows and doors when calculating paint?
- Yes, for anything larger than a standard door. A typical door is about 21 square feet and an average window about 15. On a small room the difference is under half a gallon, but on a whole-house exterior with many openings it can be several gallons. This calculator subtracts them automatically.
- How much paint do I need for a 12x12 room?
- A 12 by 12 foot room with 8-foot ceilings has 384 square feet of wall. Subtracting one door and one window leaves about 348 square feet. At 400 square feet per gallon that is roughly 0.9 gallons per coat, so about 1.8 gallons — two gallons — for two coats. Add another 1.4 gallons if you are painting the ceiling as well.
- Should I include primer in my paint estimate?
- Price primer separately whenever you are covering bare drywall, raw wood, patched repairs, or making a drastic colour change. Primer covers at a similar rate to paint, so a single primer coat needs about the same volume as one finish coat. Skipping primer on porous surfaces usually costs more in extra finish coats than the primer would have.