Estimate paint for a room from its dimensions — wall area minus standard doors and windows, multiplied by coats and divided by the coverage rate. Runs entirely in your browser — your numbers never leave this device.
Standard openings are subtracted at 21 ft² per door and 15 ft² per window — measure openings that differ from those sizes and adjust the counts. Primer, trim paint and texture absorption are not included.
| Item | Imperial | Metric |
|---|---|---|
| Standard door | 21 ft² | 1.9 m² |
| Standard window | 15 ft² | 1.5 m² |
| Typical wall coverage | 350 ft²/gal | 10 m²/L |
Paint is estimated from wall area, not floor area. The four walls of a rectangular room total twice the length plus twice the width, multiplied by the ceiling height. Openings are then subtracted at standard sizes — 21 ft² for a door and 15 ft² for a window in imperial, 1.9 m² and 1.5 m² in metric — because standard openings are close enough to those figures that measuring each one rarely changes the purchase. Non-standard openings, such as a wide archway or a wall of glass, can be entered by adjusting the counts or the area directly.
The net area is multiplied by the number of coats and divided by the coverage rate. The default rate printed on this page is 350 ft² per gallon (10 m² per litre), a typical figure for interior wall paint on a prepared surface; the rate is editable because the tin in hand is the authority — manufacturers print their own coverage, rough or porous surfaces drink more, and a strong colour change can demand an extra coat regardless of area. The result is then read as a purchase: 1.97 gallons means buying 2, with the small remainder covering touch-ups.
Ceilings are a separate area — length times width — added only when they are being painted, as the second worked example shows. The estimate is a planning figure built on the stated standards; check it against the coverage printed on the chosen paint before ordering.
Because most interior doors and windows are close to the standards in the table — 21 ft² and 15 ft² — and at that accuracy the subtraction takes seconds. For a large opening, such as a double doorway or picture window, measure it directly and adjust the net area instead of using a count.
Coverage depends on the paint, the surface and the colour change. The 350 ft²/gal (10 m²/L) figure here is a typical planning rate for prepared interior walls and it is editable — the coverage printed on the chosen product is the figure to order against.
The calculator reports the exact quantity the arithmetic gives — 1.97 gallons in the first example — and the purchase is the next whole unit, 2 gallons. The small surplus is normal: it covers the roller, cutting in and later touch-ups.
No. This estimate covers finish paint for the coats entered. Primer is a separate product with its own coverage; on bare plaster, stained surfaces or a strong colour change, plan a primer coat as an additional quantity.
No. The calculation runs entirely in the browser and nothing entered is uploaded, stored or tracked.
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