Paint Calculator
How many gallons of paint you need
Painting a room or exterior? Enter the total wall length and height, deduct the doors and windows, choose your coats, and get the gallons to buy plus a cost estimate. Below: coverage per gallon by surface, a room-size chart, what painting a room costs, and the ceiling and trim most calculators forget.
How much paint do I need?
Add the length of every wall, multiply by ceiling height, subtract the doors and windows, then multiply by the number of coats. Divide by the coverage on the can. Used as a paint estimator, an interior paint calculator square feet tool, or a paint by square foot calculator, it is the same arithmetic; add a price to turn it into a cost to paint calculator.
Rule of thumb: a gallon covers about 350 ft² per coat on a smooth primed wall. Most rooms take two coats, so a typical bedroom is 2 gallons, not 1.
Paint coverage per gallon
| Surface | Coverage per gallon | Note |
|---|---|---|
| Smooth, primed drywall | 350 to 400 ft² | The figure on the can |
| Lightly textured | 300 to 350 ft² | Orange peel |
| Heavy texture | 250 to 300 ft² | Knockdown, popcorn |
| Unprimed drywall | 200 to 250 ft² | Prime it instead |
| Exterior siding | 250 to 350 ft² | Rough sawn at the low end |
| Quart (trim) | ~90 ft² | A quarter of a gallon |
The printable material coverage chart carries paint spread rates alongside coverage figures for the other materials on a renovation, which is handy when you are pricing a whole room rather than one wall.
Gallons by room size
| Room | Wall area | Less openings | Two coats |
|---|---|---|---|
| 10 × 12 ft | 352 ft² | 301 ft² | 1.7 gal |
| 12 × 12 ft | 384 ft² | 333 ft² | 1.9 gal |
| 12 × 15 ft | 432 ft² | 381 ft² | 2.2 gal |
| 15 × 20 ft | 560 ft² | 509 ft² | 2.9 gal |
| 20 × 20 ft | 640 ft² | 589 ft² | 3.4 gal |
What painting a room costs
Interior paint runs about $25 to $70 per gallon depending on quality, so a typical bedroom in two coats is $50 to $140 in paint. Professional painting adds roughly $2 to $6 per square foot of floor area, or about $400 to $900 for an average bedroom.
Cheap paint is usually false economy: a $25 gallon that needs three coats costs more in paint and far more in labour than a $55 gallon that covers in two.
One coat or two?
Plan on two coats for new drywall, any colour change, and dark over light. One coat is realistic only when refreshing the same colour over a sound, clean wall in a good-quality paint.
Don't forget ceilings and trim
This calculator covers walls. A ceiling adds roughly the room's floor area, so a 12 × 12 ceiling is about 144 ft² — under half a gallon a coat, though most people buy a full gallon of flat ceiling paint. Trim is usually a different sheen bought by the quart, and a quart covers around 90 ft². Budget about one quart per average room.
Buy from one batch
Custom-mixed colours vary slightly between cans, so buy it all at once and box it — pour the gallons into one bucket and stir — for a seamless colour across every wall. If you run out mid-job and buy another can, the difference can be visible on a long wall.
Painting mistakes that cost gallons
- Using floor area instead of wall area. A 12 × 12 room has 144 ft² of floor and 384 ft² of wall.
- Assuming 350 ft² on textured walls. Knockdown can drop you to 250.
- Buying for one coat. Almost every job needs two.
- Skipping primer on new drywall and trying to fix flashing with extra finish coats.
- Topping up mid-job from a new can without boxing it.
- Forgetting closets. They are small but they are walls.
How this paint calculator works out its numbers
Area is total wall length × height, minus openings, multiplied by coats. Openings deduct a standard 21 ft² per door and 15 ft² per window.
- Coverage defaults to 350 ft² per gallon, the figure most manufacturers publish for interior latex on a smooth, primed surface.
- Texture reduces it sharply — knockdown and popcorn can drop real coverage to 250 ft² or less, because the surface area is larger than the flat measurement suggests.
- This calculator covers walls only. Ceilings and trim are separate and are described on the page rather than silently folded in.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Paint Calculator: frequently asked questions
How much does a gallon of paint cover?
About 350 ft² per coat on smooth, primed drywall. Lightly textured walls drop to 300 to 350, heavy knockdown or popcorn to 250 to 300, and unprimed drywall to 200 to 250. A quart covers roughly 90 ft².
How many gallons to paint a 12x12 room?
A 12 by 12 ft room with 8 ft ceilings is 384 ft² of wall, or 333 ft² after one door and two windows. At two coats that is 1.9 gallons, so buy 2.
How much paint do I need for a 10x10 room?
320 ft² of wall before openings, or 284 ft² after one door and one window. Two coats needs 1.6 gallons, so buy 2.
How much does it cost to paint a room?
Interior paint is about $25 to $70 per gallon, so a typical bedroom in two coats is $50 to $140 in materials. Professional painting adds roughly $2 to $6 per square foot of floor area.
How much paint for a ceiling?
A ceiling is about the same area as the floor, so a 12x12 ceiling is around 144 ft² and needs under half a gallon per coat. Most people buy a full gallon of flat ceiling paint and keep the remainder for touch-ups.
Do I need one coat or two?
Two for new drywall, any colour change, and dark over light. One coat is realistic only when refreshing the same colour over a sound wall using good-quality paint.
Is expensive paint worth it?
Usually yes. A cheap gallon that needs three coats costs more in paint and far more in time than a better one that covers in two, and the finish holds up longer to washing.