How the exterior paint calculator works
An outside wall is not a box room. The rectangular part runs all the way round the building up to the eaves, and on a pitched roof there are triangular gable ends above that line which a room-by-room method misses. The calculator multiplies the perimeter by the eaves height, adds half the base times the rise for each gable, then removes the openings. The result is divided by a spreading rate that depends on how rough the facade is.
Paint = (perimeter × eaves height + gables × ½ × gable width × gable rise − openings) × coats ÷ spreading rate × 1.10An exterior door is counted as 2.0 m² (21 ft²) and a window as 1.5 m² (16 ft²). The final 10% covers spatter, paint lost in the tray and the extra that soaks into cracks and hairline crazing.
Worked example
A detached house with a 10 m × 8 m footprint has a perimeter of 36 m. Up to the gutters it stands 5.5 m, giving 36 × 5.5 = 198 m². The two short sides carry gables 8 m wide rising 3 m to the ridge: 2 × ½ × 8 × 3 = 24 m². That makes 222 m² gross. Two doors (4 m²) and ten windows (15 m²) take off 19 m², leaving 203 m². On textured render spreading 6 m² per litre, two coats need 203 × 2 ÷ 6 = 67.7 L, or 74.4 L with the margin: eight 10 L tubs. The same house in smooth, already painted masonry would need only 45 L, which shows why the surface matters more than a few square metres of measuring error.
Spreading rates for facades
| Facade | Metric | Imperial | Remarks |
|---|---|---|---|
| Smooth painted masonry | 10 m²/L | 400 ft²/gal | Sound, chalk-free older coating |
| Timber, vinyl or fibre-cement siding | 9 m²/L | 350 ft²/gal | Laps and grooves add area |
| Textured render, facing brick | 6 m²/L | 250 ft²/gal | Mortar joints drink paint |
| Pebbledash, roughcast, split-face block | 4 m²/L | 150 ft²/gal | Work it in with a long-pile roller |
Masonry paint is usually sold in 5 L and 10 L tubs in Europe; exterior acrylic in North America comes in 1-gallon cans and 5-gallon pails. Buying the largest size is cheaper and keeps the colour consistent across a whole elevation.
Tips before you start
- Check the weather window. Most exterior coatings need air and wall above roughly 5–10 °C (40–50 °F), no rain for several hours after application and no direct midday sun on the side you are painting.
- Test for chalking. Rub a dark cloth on the old coating. If it comes away white, wash and apply a stabilising primer, otherwise the new paint will peel.
- Fascias, soffits and window frames are separate. They take gloss or satin trim paint; measure their length and allow roughly a litre per 10–12 m of fascia board per coat.
- Work at height safely. Two-storey walls and gables are a job for a scaffold tower or hired scaffolding, not a stretched ladder.
- Paint one wall from corner to corner. Stopping mid-wall leaves a visible lap line once dry.
A reproducible calculation with complete inputs
For a reproducible exterior paint calculator example, use the metric input system and enter House perimeter: 36 m; Height to the eaves: 5.5 m; Number of gable ends: 2; Gable width (base): 8 m; Gable height (eaves to ridge): 3 m; Exterior doors: 2; Windows: 10; Wall surface: Textured render or brick; Coats: 2 coats (recommended). These settings describe one specific scenario, not a new default for every job. The result follows the method and allowances already explained above. Keep the entire set of values when comparing it with a hand calculation; omitting a selected material or an allowance can change the result even when the dimensions match. The following rows show the unrounded calculation values alongside the precision used by the interface.
| Result | Calculation value | Displayed value | Unit |
|---|---|---|---|
| Paint needed | 74.4333333333 | 74.4 | L |
| 10 L tubs | 8 | 8 | |
| Area to paint | 203 | 203.0 | m² |
| Gable area | 24 | 24.0 | m² |
| Coverage used | 6 | 6 | m²/L |
How should I prepare these entries?
House perimeter: Walk around the building: add up the length of every wall you will paint. Height to the eaves: enter the value in m. Number of gable ends: The triangles under a pitched roof. A hipped roof has none. Gable width (base): enter the value in m. Gable height (eaves to ridge): enter the value in m. Exterior doors: enter the value in the field’s stated scale. Windows: enter the value in the field’s stated scale. Wall surface: choose among Textured render or brick, Smooth, previously painted masonry, Wood, vinyl or fibre-cement siding, Pebbledash, roughcast or split-face block. Coats: choose among 2 coats (recommended), 1 coat (same colour, sound surface). Choose the unit system before transferring your measurements. A depth, length, area and coverage can have different field units even within one system. Read each label individually rather than entering the same unit convention in every box. Optional openings or allowances should describe your actual scenario instead of being silently left at a sample value. The worked inputs above identify exactly which selections produced the table.
Frequently asked questions
How much paint do I need for the outside of a house?
A typical two-storey detached house has 180–250 m² (1,900–2,700 ft²) of wall. Two coats on textured render usually take 65–95 L, or 17–25 US gallons.
How do I calculate the area of a gable end?
Multiply the width at the base by the height from eaves to ridge and halve it. An 8 m wide gable rising 3 m is 12 m².
How many square feet does a gallon of exterior paint cover?
About 350–400 ft² per coat on smooth siding, 250 ft² on brick or stucco and as little as 150 ft² on roughcast or split-face block.
Do I need two coats of masonry paint?
Yes in most cases. The first coat fills the texture and the second gives full colour and weather protection; one coat only works over the same colour on a sound surface.
Can I paint the outside of my house in cold weather?
Only above the minimum on the label, usually 5–10 °C (40–50 °F), and early enough for the surface to dry before dew or frost forms in the evening.