How much does a measurement error of a couple of centimetres shift a home's floor area?
Roughly twice as much as you think — an area error is about twice the length error, in percent. Here is the calculation you can do yourself in two minutes.
Short answer: An area error is roughly double the percentage of the length error that causes it, because an area is two lengths multiplied together. If your measurements are 1% too big, the area is roughly 2% too big. Concretely: if a room is measured 4 cm too wide and 4 cm too long in a room of 400 x 500 cm, the area becomes 20.4 m² instead of 20.0 m² — 2% too much. Across a whole home of 120 m², a 1% length error corresponds to around 2.4 m². That's irrelevant for paint and flooring, but it matters if the area is to be compared with the official building register or a sales listing.
The maths behind "twice as much"
A rectangular room's area is width times length. If both measurements are 1% too big, the area becomes 1.01 × 1.01 = 1.0201 — that is, 2.01% too big. That's the whole explanation: the error hits twice, once in each direction.
The rule of thumb holds as long as the error is small (up to a few percent): area error in percent ≈ 2 × length error in percent.
The same applies the other way for volume, if you're calculating on volume for, say, heating or ventilation: there the factor is three, because three lengths are multiplied together.
Three examples in numbers
A single room
A room that is actually 400 x 500 cm, i.e. 20.00 m². The measurements are each 4 cm too big: 404 x 504 cm = 20.36 m². The difference is 0.36 m², or 1.8%. The length error was 4 ÷ 400 = 1.0% and 4 ÷ 500 = 0.8% — together the expected roughly 1.8%.
A whole home
A flat of 120 m². If every room is measured consistently 1% too big, the area becomes roughly 122.4 m² — 2.4 m² too much. 2.4 m² is not nothing when comparing against a sales listing. But it's also not enough to explain a difference of 8-10 m², which almost always comes down to something else entirely: official housing areas are measured externally, while you measure internally.
The skirting board alone
If you measure to the skirting board instead of to the wall face, the room typically ends up 2-4 cm too narrow in each direction. In a room of 300 x 400 cm, that's 3 cm in each direction: 297 x 397 = 11.79 m² against 12.00 m². That's 1.8% — from an error that's purely about where you placed the tape measure.
Random and systematic errors are not the same thing
This decides whether the error accumulates or cancels out when you add the rooms together:
- Random errors swing both ways. One room is measured slightly too big, the next slightly too small. Across ten rooms they largely cancel each other out, and the home's total area comes out better than any single room.
- Systematic errors point the same way every time — for example measuring to the skirting board in every room, or including wall covering everywhere. They do not cancel out. Ten rooms each 2% too small give a home that is 2% too small.
So the key question is not "how big is the error" but "does it point the same way in every room". You can settle that yourself: measure two or three rooms with a tape measure afterwards. If the scan sits on the same side of the tape measure every time, the error is systematic, and you can subtract it across the whole home.
When it matters — and when it doesn't
- Paint: makes no difference. Wastage and two coats are allowed for anyway, and paint is sold in whole litres and tins.
- Flooring: almost no difference. Typically 5-10% extra is bought for wastage and cutting, and that comfortably covers a 2% area error.
- Tiles: the same — the wastage is bigger than the error. But the room's actual width matters for how the tiles should be laid out, and that measurement needs to be taken on site.
- Comparison against the official building register or a sales listing: matters. Here you're talking about individual square metres, so you need to know how big your own uncertainty is before you can claim the register is wrong. (The register goes by a different name in every country.)
- Ordering something made to measure: here percentages are irrelevant — that's measured on site with a tape measure, every time.
How to find your own percentage
- Pick the home's longest unbroken wall. A long wall matters: 2 cm is a lot over 150 cm and little over 600 cm, and it's the ratio you're after.
- Measure it with a tape measure from wall face to wall face, over the skirting board, keeping the tape horizontal.
- Look up the same measurement in your floor plan and subtract.
- Calculate: difference ÷ measured length × 100 = your length error in percent.
- Multiply by 2. That's your expected area error in percent.
Example: 618 cm with a tape measure, 623 cm in the drawing. 5 ÷ 618 × 100 = 0.8% length error, so roughly 1.6% area error. On a home of 95 m², that's around 1.5 m². Now you know how wide your figure's margin is — and that's worth more than a claimed precision from a manufacturer.
What HouseSense does
HouseSense calculates in internal measurements — from wall face to wall face — and in whole centimetres. That's the same kind of measurement you'd take yourself with a tape measure, and so every single figure in the app can be checked. That's the entire premise of the maths above.
The app doesn't state a claimed precision in centimetres, because such a figure would differ from room to room and so couldn't be relied on. The principle is that the drawing may be dressed up, the figures may not: a floor plan may be straightened out so it's readable, but a measurement must never be changed to make the sum come out right.