Buying paint comes down to one ratio: paintable area times coats, divided by how many square feet a gallon covers. This calculator measures the walls and ceiling, deducts the doors and windows, applies your coats and coverage rate, and rounds up to the gallons and quarts you can actually buy.
How it works
Wall area is the perimeter times the height; the ceiling adds the floor footprint when painted. Openings are removed, then coats and coverage decide the gallons:
wall area = 2 × (length + width) × height
ceiling area = length × width (if painting ceiling)
paintable = wall + ceiling − openings
total area = paintable × coats
gallons raw = total area / coverage_per_gallonThe raw figure is then rounded up to the next quart (0.25 gal) so you buy a whole container with a little to spare for touch-ups.
Worked example
A 12 × 12 ft room with 8 ft walls, one door, and two windows, ceiling painted:
| Component | Calculation | Area |
|---|---|---|
| Walls | 2 × (12 + 12) × 8 | 384 sq ft |
| Ceiling | 12 × 12 | 144 sq ft |
| One door | — | −21 sq ft |
| Two windows | 2 × 15 | −30 sq ft |
| Paintable area | 384 + 144 − 51 | 477 sq ft |
At two coats: 477 × 2 = 954 sq ft total. At 400 sq ft per gallon: 954 / 400 = 2.4 gallons, rounded up to 2.5 gallons (two gallons and two quarts) to buy.
Choosing the right coverage rate
The 400 sq ft per gallon figure is a reasonable starting point for a smooth, primed surface with a light or similar color. Real coverage varies significantly:
| Surface condition | Typical coverage |
|---|---|
| Smooth, primed, similar color | 380–400 sq ft/gal |
| Slightly textured, primed | 300–360 sq ft/gal |
| Deep color over white (or vice versa) | 300–350 sq ft/gal |
| Bare drywall (unpainted) | 250–300 sq ft/gal |
| Heavily textured (orange peel, knockdown) | 200–280 sq ft/gal |
The coverage rate printed on your specific paint can is the most accurate figure to use — the actual product formulation, pigment density, and finish level all affect spread rate.
Primer vs. topcoat: run the calculator twice
Primer has its own coverage rate, often lower than topcoat paint because it is designed to penetrate and seal rather than build a decorative film. For a new drywall project, plan on one primer coat and two finish coats. Enter the room dimensions twice: once with the primer coverage rate and once with the topcoat rate. The primer total and finish coat total are separate purchases.
Why rounding up is intentional
The calculator rounds up to the nearest quart rather than the exact gallon needed for two reasons. First, you can only buy paint in whole containers. Second, touch-up paint made from the same can six months later matches the wall color far better than a separately mixed batch — most paint shops cannot guarantee a perfect match on a remixed can. The small overage is practical insurance.
Common mistakes
Using the wall color coverage rate for the ceiling. Ceiling paint often has different formulations and coverage rates from wall paint. If you are painting the ceiling in a dedicated ceiling white, check its own coverage figure.
Forgetting to count coats for primer. Primer coats use significant material. If you are priming and painting, budget for both separately.
Applying the same coverage rate to deeply textured walls. Heavy texture (stucco, knockdown, orange peel) increases the total surface area that paint must coat and can require 30–50% more paint than a smooth surface of the same geometric area.