Calculator · Exterior area
Estimate your paintable exterior area
From a footprint, a wall height and a roof pitch. It treats your house as a simple box with a simple roof, which is exactly why it comes with a stated error margin instead of a confident number. The geometry is shown line by line, so you can see what it counted and what it could not.
The calculator
Four measurements and a roof form
Footprint from the ground, wall height per storey, and how many storeys. If the roof has gable ends, the pitch lets the model add the triangles.
The geometry
The same calculation, on paper
A 14 m × 9 m single-storey bungalow, 2.55 m wall height, gable roof at 25°, 12% openings.
- Perimeter: 2 × (14 + 9) = 46.00 m
- Total wall height: 1 storey × 2.55 = 2.55 m
- Wall area: 46.00 × 2.55 = 117.30 m²
- Ridge rise: (9 ÷ 2) × tan 25° = 4.5 × 0.4663 = 2.10 m
- One gable triangle: ½ × 9 × 2.0984 = 9.44 m²; two gables = 18.89 m²
- Subtotal: 117.30 + 18.89 = 136.19 m²
- Openings at 12%: −16.34 m²
- Estimate: 119.84 m², so call it 120 m², and the honest range is 96 – 144 m²
The gable width used for the triangles is the narrower footprint dimension, because a gable normally sits across the span rather than along the ridge. If your gable faces the other way, swap the two footprint figures.
Why the estimate reads low
Every simplification in the model removes wall rather than adding it. A recessed entry has three walls where the footprint has one line. A projecting bay adds three faces and removes one. An attached garage adds a return the perimeter does not see. A dormer is pure addition. A split-level house has a wall on the downhill side that is taller than the storey height you entered.
So a footprint estimate is a floor, not a midpoint. The Extra walls field is there to be used — pace out the bits the rectangle misses and add them.
What a real measure-up adds that no calculator can
- Condition. How much preparation, how much timber repair, how much stripping. This is most of the labour and none of the geometry.
- Access. Scaffolding, edge protection, a wall you can only reach off a neighbour’s property, a two-storey elevation over a steep bank.
- Substrate. Which changes the specification entirely — see the cladding pages.
Which is the same reason this site will not turn your square metres into a price: there is no official New Zealand cost dataset to do it with.
Questions people actually ask
How accurate is an estimate from a footprint?
Within about 20% on an ordinary house, and worse than that on a complicated one. The model treats your house as a rectangular box with a simple roof, and real houses have bays, returns, recessed entries, projecting garages, dormers, split levels and decks. Every one of those adds wall the model does not know about, which is why the error is asymmetric — a footprint estimate almost always comes out low. Use it to sanity-check a quote, not to order the paint for a large job.
Why does a real measure-up come out different?
Because someone with a laser measure walks the building and counts what is actually there: each elevation separately, the gable ends measured rather than derived from a pitch, the soffit width, the fascia and barge run in linear metres, the joinery counted by unit, the balustrades, the garage door, the retaining wall you wanted done at the same time. They also record access — whether a wall needs scaffolding, whether a ladder will reach, whether there is room for a spray setup — and access moves the labour far more than the extra square metres do.
What roof pitch should I use if I do not know it?
Most New Zealand houses sit between 15° and 30°. A 1900s to 1930s villa or bungalow is usually steeper, around 25° to 35°; a 1960s to 1980s house is often shallower, 15° to 22°; a modern low-slope roof can be under 10°. The pitch only matters here for the gable-end triangles, so an error of a few degrees changes the total by very little. If your roof is hipped on all sides, select hip and the pitch stops mattering altogether.
Does this include the roof area?
No. This estimates paintable wall area, plus gable ends where they exist. Roof area is a separate calculation — it is the footprint divided by the cosine of the pitch, plus overhangs — and roof coating is priced and specified quite differently from walls. The roof page covers that.
What about the soffits, fascia and joinery?
Not included, deliberately, because they are usually a different product in a different colour and are measured differently — fascia and barge in linear metres, joinery by the unit, soffit as width × the run of eave. Lumping them into a wall square-metre figure is how estimates end up wrong in both directions. Count them separately, then add them.
Why is the opening deduction only 12% by default?
Because glazing is typically a modest share of an exterior wall, and because deducting too much is a common estimating error. Cutting in around an opening consumes more paint and far more time per square metre than the flat wall it displaced, and the reveals and sills around it are usually being painted too. A large amount of glazing — a modern house with a lot of full-height joinery — warrants pushing the figure up, and an older house with small double-hung windows warrants pulling it down.