Paint Calculator
Work out exactly how much paint you need for any room, in gallons or litres, with doors, windows, ceilings, primer, extra coats and cost all included. Add as many rooms as you like and buy the right number of cans the first time.
- ✓ Multi-room projects
- ✓ Imperial & metric
- ✓ Doors & windows deducted
- ✓ Can-size breakdown
- ✓ Live cost estimate
Your project
Coats & coverage
Primer & cost
Why this paint calculator beats the store estimators
Most paint calculators online handle one blank rectangle, ignore your openings and hand you a decimal you still have to translate into cans. This one finishes the job.
Whole house, not one wall
Add every room in the project and get one combined shopping figure, plus a per-room breakdown so you know where the paint is going.
Real doors and windows
Deduct each opening instead of guessing. Default sizes match a standard 3 x 7 ft door and 3 x 5 ft window, and both are editable.
Can breakdown, not decimals
3.4 gallons is useless at the counter. You get the cheapest sensible mix of 5-gallon buckets, gallons and quarts to actually buy.
Primer handled separately
Primer covers less than topcoat, so it is calculated at its own rate and its own price rather than being folded into one number.
Imperial and metric
Switch between feet and square feet per gallon or metres and square metres per litre. Every field, default and result converts with you.
Instant and private
Everything runs in your browser as you type. No account, no email wall, no upsell to a paint brand, nothing stored anywhere.
How to use the paint calculator
Four inputs get you a reliable number in under a minute.
- Measure the room
Take the length and width of the floor and the wall height. For odd shapes, split the space into rectangles and add each one as a separate room.
- Count your openings
Enter how many doors and windows are on the walls you are painting. These are subtracted from the wall area automatically.
- Set coats and coverage
Two coats is standard. Check the can for its real coverage rate: most interior latex covers 350–400 sq ft per gallon, or roughly 10 m² per litre.
- Read the can breakdown
The panel converts your exact requirement into buckets, gallons and quarts, adds primer if you toggled it on, and prices the whole job.
How much paint do I need? Quick reference
Walls only, 8 ft ceilings, one door and two windows deducted, two coats at 350 sq ft per gallon, before any touch-up allowance.
| Room size | Wall area | Paintable | Two coats | Buy |
|---|---|---|---|---|
| 10 x 10 ft | 320 sq ft | 269 sq ft | 538 sq ft | 1 gal + 3 qt |
| 12 x 12 ft | 384 sq ft | 333 sq ft | 666 sq ft | 2 gallons |
| 12 x 16 ft | 448 sq ft | 397 sq ft | 794 sq ft | 2 gal + 2 qt |
| 15 x 20 ft | 560 sq ft | 509 sq ft | 1,018 sq ft | 3 gallons |
| 20 x 24 ft | 704 sq ft | 653 sq ft | 1,306 sq ft | 3 gal + 3 qt |
What you get out of estimating properly
- ✓No second trip. Running out mid-wall means a lap mark you can see forever, plus a batch that may not match.
- ✓No wasted money. Over-buying by two gallons on a small job is roughly the cost of the roller, tray, tape and drop cloths combined.
- ✓Accurate quotes. Painters and landlords can price materials per room instead of eyeballing the job.
- ✓Better colour changes. Going dark to light shows you immediately whether a third coat or a tinted primer is the cheaper route.
- ✓Bucket maths done. A 5-gallon bucket is usually 10–20% cheaper per gallon, and the breakdown shows when you cross that line.
- ✓Less leftover paint. Fewer half-used cans hardening in the garage and less hazardous waste to dispose of.
Paint calculator FAQs
The questions people ask at the paint counter, answered with the numbers.
How much paint do I need for a 12x12 room?
A 12 x 12 ft room with 8 ft ceilings has 384 sq ft of wall. Take off one door and two windows and you are painting about 333 sq ft. Two coats is 666 sq ft, which at 350 sq ft per gallon is 1.9 gallons, so you buy 2 gallons. Add roughly another gallon if you are painting the 144 sq ft ceiling as well.
How many square feet does a gallon of paint cover?
One gallon covers about 350 to 400 sq ft in a single coat on smooth, previously painted drywall. On textured walls, bare drywall, masonry or raw wood the same gallon may only reach 250 to 300 sq ft. In metric, one litre covers roughly 10 to 12 m² per coat. Always check the figure printed on the can and enter it in the coverage field.
Should I subtract windows and doors when calculating paint?
Yes, once you have more than one or two openings. A standard door is about 21 sq ft and a standard window about 15 sq ft, so a room with one door and three windows loses 66 sq ft of wall, which is around a fifth of a gallon per coat. On a whole-house job those deductions add up to a full can or more.
How many coats of paint do I need?
Two coats is the standard for a durable, even finish and is what manufacturers assume in their coverage claims. One coat only works for a refresh in the same colour with a premium paint. Go to three coats when you are covering a dark colour with a light one, painting a saturated red or yellow, or working over patchy repairs, or use a tinted primer first to save a topcoat.
Do I need primer, and how much?
Prime bare drywall, raw wood, patched areas, stained surfaces and any drastic colour change. Primer is thinner and soaks in, so it covers less: budget 200 to 300 sq ft per gallon for one coat rather than 350. Toggle primer on in the calculator and it is worked out at its own rate and priced separately.
How much paint do I need for a ceiling?
Ceiling area is simply length times width, so a 12 x 12 ft ceiling is 144 sq ft. Two coats is 288 sq ft, which is under one gallon. Textured or popcorn ceilings can eat 20 to 30% more paint than the flat figure suggests, so raise the waste percentage or lower the coverage rate before you buy.
Is it cheaper to buy a 5-gallon bucket of paint?
Usually yes. A 5-gallon bucket typically works out 10 to 20% cheaper per gallon than five separate gallons, and it guarantees a single colour batch across the job. It only pays off once you genuinely need four or more gallons, otherwise you are storing leftovers you will never open again.
How do I calculate paint for an irregular or L-shaped room?
Split the space into rectangles and add each one as its own room in the calculator, then ignore the shared internal wall if there is no wall there. For a sloped or vaulted ceiling, treat the triangular gable as half of base times height and add it as an extra room with the ceiling option turned off.
Get the number right before you get to the till
Enter your room dimensions, deduct the doors and windows, pick your coats and walk into the paint aisle knowing exactly which cans to put in the trolley.
Calculate my paint →