← All calculators

Free project calculator

Paint calculator

This paint calculator is a coverage estimator that converts room dimensions into wall area, subtracts standard door and window openings, multiplies by coats, and returns the litres and cans to buy. It works for interior latex on walls and ceilings.

Your rooms

Enter every room getting the same colour, then count doors and windows across all rooms in the fields below. Measurements in decimal — 6 inches is 0.5 ft.

Deducts 21 sqft per door — a standard estimator constant, not a measurement.
Deducts 15 sqft per window — a standard estimator constant, not a measurement.
Ceilings, built-ins, or a feature wall: enter the flat square footage to add.
On the paint can's spec label. Most interior latex covers about 10 m² per litre per coat.
CAD. For a materials estimate only — delivery not included.

How the math works

Paint coverage is wall area minus standard openings, multiplied by coats, divided by the coverage per litre on the can. The calculator takes each room's perimeter — 2 × (length + width) — times the ceiling height to get wall area, subtracts 21 sqft per door and 15 sqft per window, adds any flat area you enter, multiplies by the number of coats, and divides by the coverage per litre for the litres needed. Cans round up to full 3.78 L gallons — paint is sold by the can, not the litre.

Two coats is the standard for a colour change. One coat only holds up when you are repainting the same colour.

The door (21 sqft) and window (15 sqft) deductions are standard estimator constants, not measurements. For a wall of glass, measure it and reduce the extra-area field accordingly.

Primer is a separate product and a separate quantity. And coverage drops on porous or textured surfaces — first coats on new drywall drink more paint.

Paint calculator FAQ

How much does a litre of paint cover?
Most interior latex covers about 10 m² (roughly 107 sqft) per litre per coat on a smooth, primed surface. The exact figure is printed on the can's spec label — use that number. Porous or textured surfaces cover less, so treat the label figure as a best case.
Do I need two coats?
For any colour change, yes — two coats is the standard for even coverage and true colour. One coat only holds up when repainting with the same colour in the same sheen. Deep or bright colours over a light base can need a tinted primer plus two coats.
How are doors and windows deducted?
The calculator subtracts 21 sqft per door and 15 sqft per window — standard estimator constants, not measurements. If a room has unusually large glazing, measure the glass area yourself and reduce the additional-area field to match.
Does this include primer?
No. Primer is a separate product with its own coverage rate, needed on new drywall, patched areas, stain-prone surfaces, and drastic colour changes. Run the calculator once for primer at one coat if the whole surface needs it.
How do I add ceilings?
Enter the ceiling as additional flat area: its length times width in the extra-area field. Ceilings are usually painted separately in a different sheen, so consider running the calculator twice — once for walls, once for ceilings.

Got your number? Order online and pick up same-day at the Scarborough yard.

Shop paint & supplies