Paint Calculator
The paint calculator estimates how many gallons and quarts of paint to buy for one interior room's walls, a ceiling, a whole exterior, or a primer coat. Enter the room dimensions and it computes the paintable area, applies your coats and coverage rate, and rounds the honest raw gallons up to purchasable cans so you buy enough in one trip. It defaults to a conservative, manufacturer-sourced 350 square feet per gallon and two coats, and lets you edit the coverage rate for textured or new-drywall surfaces.
From square feet to cans you can buy
Every mode here runs one formula: paintable area times coats, divided by the paint's coverage rate, then rounded up to cans you can actually buy. A 12 by 12 foot room with 8 foot walls has 384 square feet of wall; at two coats and a conservative 350 square feet per gallon that is 384 x 2 / 350 = 2.19 gallons — about 8.3 litres over 35.7 square metres — which rounds up to 2 gallons plus 1 quart. The tables and sections below are that same formula applied to real rooms, containers, and surfaces.
A real repaint needs two coats; the label's one-coat number is a smooth-wall, same-color best case. Finish modes default to two coats and primer to one.
The 350 square feet per gallon default is the conservative end of the manufacturer band. Drop it to about 275 for textured or new drywall, or raise it toward 400 for premium paint on smooth walls.
Raw gallons round up to whole gallons plus at most one quart, so you never run short mid-coat and the leftover becomes your touch-up supply.
A ceiling (length times width) and primer are their own quantities — estimate and buy them apart from the wall paint.
How much paint by room size
Every figure below comes straight from this calculator at two coats, 8-foot walls, and the conservative 350 square feet per gallon default, so the chart and the tool always agree. Coverage is the lower bound of the 350 to 400 square-foot-per-gallon manufacturer band; raise it for premium paint or lower it for texture and the gallons change.
| Room (length x width, 8-ft walls) | Paintable wall area | Two-coat paint to buy |
|---|---|---|
| 10 x 10 ft | 320 sq ft | 2 gallons |
| 10 x 11 ft (small bedroom) | 336 sq ft | 2 gallons |
| 12 x 12 ft | 384 sq ft | 2 gallons + 1 quart |
| 12 x 14 ft | 416 sq ft | 3 gallons |
| 1,000 sq ft of wall | 1,000 sq ft | 6 gallons |
These assume no openings deducted. Subtracting a door and a couple of windows rarely changes the whole-gallon answer, which is one reason many painters skip the deduction entirely.
How far a gallon, quart, or 5-gallon bucket goes
This lookup answers the reverse question — not how much a room needs, but how far a container goes on a single coat. The two columns bracket the manufacturer band: 350 square feet per gallon (conservative) and 400 (premium / smooth). Retail can sizes are the standard 8-ounce sample, quart, gallon, and 5-gallon bucket (NIST liquid-volume definitions: a quart is 0.25 gallon).
| Container | One-coat coverage at 350 sq ft/gal | One-coat coverage at 400 sq ft/gal |
|---|---|---|
| 8 oz sample (0.0625 gal) | about 21.9 sq ft | 25 sq ft |
| Quart (0.25 gal) | 87.5 sq ft | 100 sq ft |
| 1 gallon | 350 sq ft | 400 sq ft |
| 5-gallon bucket | 1,750 sq ft | 2,000 sq ft |
Halve any figure for a realistic two-coat job. A quart is the right call for a small ceiling, an accent wall, or trim; once you need more than about three quarts, a full gallon costs less per square foot.
Why you buy more than the can's number suggests
Coverage is the single biggest driver of how much paint you buy, and it is where the label is most optimistic. Manufacturer data sheets put a standard interior finish at 350 to 400 square feet per gallon (Sherwin-Williams SuperPaint Interior, PDS 101.11B), and a premium paint a little higher at 400 to 450 (Benjamin Moore Regal Select, N547). Those are one-coat, smooth-wall, same-color figures. Two things pull real coverage down: a repaint almost always needs two coats, and texture or porosity soaks up paint. Benjamin Moore's own sheet cautions that coverage varies "depending on surface texture and porosity — be sure to estimate the right amount of paint for the job." On rough or new-drywall surfaces, painters commonly plan closer to 250 to 300 square feet per gallon — a field convention, not a number printed on any data sheet. That is why the calculator defaults to the conservative 350, offers a 275 textured preset, and lets you type a custom rate: the coverage number is honest, sourced, and yours to adjust.
The difference is real money on the same room. Keep the 12 by 12 foot room (384 square feet of wall, two coats) but switch the surface to the textured / new-drywall preset of 275 square feet per gallon: the honest requirement rises to about 2.79 gallons and the purchase jumps from 2 gallons plus a quart to a full 3 gallons. That is the editable-coverage difference no hardcoded brand tool exposes — the same room can be 2 gallons and a quart or a full 3 gallons depending on the wall.
Rounding raw gallons up to cans you can buy
The tool never leaves you with an un-buyable fraction. It takes the honest raw gallons and rounds up to whole gallons plus at most one quart: a leftover of a quarter gallon or less tops up with a quart, and anything more rounds up to the next full gallon — which costs less per unit than three or four quarts and leaves you touch-up supply. Beyond a single room the same formula covers four jobs. Interior mode uses perimeter times wall height; ceiling mode uses length times width and drops the height and openings fields, because a ceiling is a separate quantity you should buy apart from the walls. Exterior mode adds triangular gable ends: a 40 by 30 foot house with 10 foot walls and two 30-by-8-foot gables works out to 1,640 square feet of siding (1,400 of wall plus 240 of gable), about 9.37 gallons at two coats, which rounds up to 10 gallons — two 5-gallon buckets. Primer mode is its own quantity again: the same 12 by 12 room at a PVA drywall primer's 400 square feet per gallon and a single coat needs about a gallon, rounding to 1 gallon bought in addition to the finish paint.
Openings, trim, and the limits of an allowance
Two adjustments and one honesty note round out the estimate. Openings are optional and off by default, because subtracting a door and a couple of windows rarely changes the whole-gallon answer and many painters skip it: turn the deduction on and the tool removes 20 square feet per door and 15 per window — 50 square feet for one door and two windows — so the 12 by 12 room drops from 384 to 334 square feet, still 2 gallons at two coats. Trim, doors, and baseboards are a separate small quantity, usually a quart or two, that this tool does not itemize. To size it by hand, multiply the run of trim in linear feet by its paintable face width in feet, then divide by the coverage rate for two coats: roughly 120 linear feet of baseboard and casing at about half a foot of face is 120 x 0.5 = 60 square feet, which at two coats is 60 x 2 / 350 = 0.34 gallons — about a quart and a half. Finally, treat the result as a buy-right allowance, not a professional takeoff: it assumes simple rectangular rooms and gable triangles, a single coverage rate per job, and standard can sizes. Non-symmetric gables, dormers, oversized picture windows, and very porous or rough surfaces all need a look with a tape measure.
What this calculator is not: cost, color, and spray-can paint
This tool answers one question — how much paint to buy — and deliberately skips three adjacent ones so it stays accurate.
It is not a price quote. It estimates gallons and quarts, not what a painter charges or what the paint costs; labor and material prices vary too much by region and season to state responsibly, so use a local cost guide or contractor quotes for dollars.
It is not a color preview. It tells you how much paint, not how a color will look on your wall. For that, use your paint brand's online color visualizer, which every major manufacturer offers, or buy a sample pot and test a patch in your own light.
It is not for automotive or aerosol paint. It covers wall and house paint sold in gallons and quarts, not spray-can or automotive coatings, which are sold in ounces and specified by the can's own coverage rating.