Skip to main content
Redmoon Converters
🔨 Construction & DIY

Paint Coverage Calculator (Litres / Gallons)

Calculate exact paint needed: wall area minus doors and windows, multiplied by coats and primer, with a surface-texture coverage factor.

Built and maintained by Paul Clark, Redmoon Software · Checked against what this tool computes on

NameLWHeightDoorsWindows
Paint needed
0 gal
Wall area — · Primer —
RoomWall areaNet area
Quick reference

Wall area per room: 2 × (L + W) × height. Optionally add ceiling = L × W.

Subtract openings: 21 sqft per door, 15 sqft per window (≈ 1.95 m² and 1.4 m²).

Paint needed: total_net_area × coats / coverage_per_gal. Coverage depends on surface texture: smooth 350, light 300, heavy 250 sqft/gal.

Primer: one coat at ~200 sqft/gal across all surfaces. Always round up to whole containers.

How it works

The calculator works room by room. For each row it takes wall area as 2 × (length + width) × height, optionally adds length × width for the ceiling, then subtracts a flat 21 square feet per door and 15 square feet per window to get net paintable area. Totals across all rooms are multiplied by your coat count and divided by the coverage rate, and the answer is rounded up to whole containers, because you cannot buy two-thirds of a tin.

Surface texture is the coverage rate, and it is the biggest single lever: Smooth gives 350 sqft/gal, Light texture 300 and Heavy texture 250, so a heavily textured wall needs 40 percent more paint than a smooth one for the same area. Coats defaults to 2 and accepts 1 to 5. Turning Primer on adds a separate single coat costed at 200 sqft/gal, reported alongside the topcoat figure rather than mixed into it.

The opening deductions are fixed averages, not measurements: every door counts as 21 sqft and every window as 15 sqft regardless of the real size or the unit system you selected. Trim, doors themselves, closets and soffits are not counted. Metric mode computes in square feet and gallons internally and converts at 3.78541, then rounds to whole litres — which will not line up with the 2.5 and 5 litre tins actually sold, so round to real tin sizes yourself.

Frequently asked questions

How much paint do I need for a 12 by 12 foot room?

With 8 foot ceilings that is 384 sqft of wall. Deduct one door and one window and you get 348 sqft net; two coats at the smooth 350 sqft/gal rate works out to 1.99 gallons, which the tool rounds up to 2 gallons.

Do I have to subtract windows and doors myself?

No. Each room row has Doors and Windows count fields. Enter how many of each and the tool removes 21 sqft per door and 15 sqft per window automatically. If a room has an oversized picture window, enter it as two windows to approximate the area.

Does the estimate cover the ceiling as well as the walls?

Only if you set Include ceiling to Yes, which adds length × width to every room's area. It is off by default, and it uses the same texture coverage rate as the walls, so adjust the texture setting if your ceiling is a different finish.

How many coats should I plan for a big colour change?

Keep coats at 2 and switch Primer on. Primer is costed separately as one coat at 200 sqft/gal, which is the standard approach for going dark to light, over patched drywall, or onto bare or glossy surfaces where topcoat alone will not sit evenly.

Why did the total jump a whole gallon after a small edit?

Because the result is rounded up to whole containers. Adding a few square feet can push 1.98 gallons to 2.01 and force a third tin. The per-room table below the result shows the underlying gross and net areas so you can see what actually moved.

Coverage rates are for a smooth, sealed surface

The coverage options — around 350, 300 and 250 square feet per gallon — describe a smooth previously painted wall. Every departure from that costs coverage, and the losses are larger than people expect.

Texture is the biggest: heavily textured plaster, render or textured wallpaper can consume 30–50% more than a flat wall of the same measured area, because the actual surface area is far greater than its footprint. Bare porous substrates — new plaster, bare timber, filler patches — absorb the first coat rather than being coated by it, which is what a primer or a thinned mist coat is for.

Two coats is the default for a reason

The two-coat default is not conservatism. A single coat rarely achieves uniform film build or opacity, and the second coat is what gives an even sheen — patchiness in a finished wall is usually a film-thickness variation catching the light rather than a colour variation.

Colour change dictates whether two is enough. Going darker over light is usually fine; light over a strong dark colour, or anything over red or yellow, commonly needs three coats or a tinted primer.

Deducting windows and doors is optional and often not worth it. On a normal room the openings are a small share of the area and the deduction is smaller than the waste allowance you should be carrying anyway — and buying one tin short mid-job risks a batch mismatch.

Related tools