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How do I convert my internal measurement to the external area a register uses?

Add the wall thickness times the house perimeter to your internal measurement. Here is the sum with real numbers, why it works, and how to take the two measurements it needs.

Why a conversion is needed at all

The area a home is registered with is, as a rule, measured externally: the line runs along the outside face of the external wall all the way around the house, and the internal partition walls count towards the figure. That's how most public building registers work.

When you walk round yourself with a tape measure, you measure internally from wall to wall, and then every wall is out of the sum. The two figures can therefore never come out the same, and they're not meant to. But they can be translated into each other if you know the walls. That's the whole conversion.

What you need to measure

1. The external wall thickness

Measure the thickness in a window reveal or an outside door: from the inner wall face to the outer one. That is the depth of the reveal plus the frame. Do it at three different windows — on different sides of the house — because the thickness is rarely the same all the way round on a house that has been extended or had insulation added later.

For a sanity check: an older house with a solid wall or an uninsulated cavity wall is often 25-30 cm, a post-war house with an insulated cavity wall typically 30-38 cm, and new build often 40-50 cm, because insulation requirements have tightened over the decades. Always use your own measured figure. The range here is only there to catch a mismeasurement.

2. The house's perimeter

Walk around the house with a measuring wheel or a long tape and add the sides together. If the house is a rectangle, the perimeter is 2 × (length + width). If it has a wing, a bay or an extension, you have to follow every kink in the façade — that's exactly where the walls take up extra room.

If you can't get all the way around the house, you can work out the perimeter from the inside: add 2 × the external wall thickness to each of the internal outer measurements, and add the sides together.

3. The internal walls

Count the running metres of internal walls on a floor plan, or by walking through the rooms, and note their thickness. Light partition walls are typically 10-15 cm; a masonry internal wall can be 20-25 cm. A shared wall between two rooms is only counted once here — you're after the wall's own footprint, not its two sides.

The sum, step by step

A house where the rooms' floor areas add up to 66.0 m², with a 36 m perimeter, 35 cm external walls and 22 running metres of 12 cm internal wall:

• External walls: perimeter 36 m × thickness 0.35 m = 12.6 m²
• Internal walls: 22 running metres × 0.12 m = 2.6 m²
• Total: 66.0 + 12.6 + 2.6 = 81.2 m²

If the register says 80 m², you're within a metre's uncertainty on the perimeter — and actually closer than the sum shows, because the rule of thumb counts the corners twice and so overestimates slightly. That's as close as a hand measurement gets, and there's nothing wrong with the registration.

The other way round — from the registered figure to your floor: 80 − 12.6 − 2.6 = about 65 m² of floor. That's the figure you should order flooring and paint by, not the 80. Buy flooring for 80 m² and you're paying for 15 m² that sits inside the walls.

Why the perimeter rule works

A rectangular house that measures a × b externally has internal dimensions of (a − 2t) × (b − 2t), where t is the external wall thickness. Multiply that out, and 2t × (a + b) disappears — and 2 × (a + b) is exactly the house's perimeter. Hence the rule: the loss is the thickness times the perimeter.

The rule counts the four corners twice, which gives a small overestimate of 4t². With 35 cm walls that's 4 × 0.35² = 0.49 m² — under half a square metre, and smaller than the uncertainty in your own measuring. Ignore it.

The important consequence is that the difference follows the house's shape, not its size. Two houses of 80 m² externally lose different amounts to their walls. A square house measures 8.94 × 8.94 m and has 8.24 × 8.24 = 67.9 m² internally — a loss of 12.0 m². A long, narrow house of 5 × 16 m has 4.3 × 15.3 = 65.8 m² internally — a loss of 14.2 m². Same external area, over two square metres apart. That's why there's no percentage that fits every house, and why you need to measure your own perimeter.

The three places the conversion breaks down

Sloped ceilings. In one-and-a-half storey houses there is floor you can't stand upright on, and special rules apply for how area under a sloped roof counts. The perimeter-and-thickness conversion covers flat geometry — it can't translate a loft storey. Measure the headroom, and get the rule confirmed by the authority that keeps the register, rather than guessing.

Multiple storeys. A registered floor area is the sum of the storeys' areas. Work out each storey separately with its own perimeter and add them up — a first floor that's set back from the ground floor doesn't have the same perimeter as the ground floor. Staircases and stairwells also have their own rules, which you shouldn't guess at.

Basement, conservatory and garage. These normally have their own fields in the register and don't usually count towards the living area. Keep them out of the conversion, or you're comparing two different things.

When the conversion doesn't add up

If your converted figure lands within 2-3 m² of the registered area, everything is as it should be. That's within the uncertainty of a hand measurement: on a house with a 36 m perimeter, a 5 cm error in the wall thickness alone shifts the figure by 36 × 0.05 = 1.8 m², and it's entirely possible to be out by 5 cm in a window reveal with trim.

If, on the other hand, there's 10-20 m² left over that the walls can't explain, something else is going on: an extension that was never registered, area under a sloped ceiling, a basement or conservatory filed under the wrong field — or a registered figure that comes from an old drawing rather than an actual measurement. At that point it's no longer a sum, but a question of registration, and it belongs with the authority that keeps the register.

If you'd rather let the app keep track

HouseSense measures internally from wall to wall and gives you the area per room and in total — scan the home with the iPhone's LiDAR, or draw it on graph paper with your finger. Walls that meet each other become a single shared wall, so two rooms never count the same square metres twice.

So the app gives you the left-hand side of the sum — the measured internal area — and a floor plan where you can read off the house's shape and its running metres of wall. You still have to measure the external wall thickness yourself in a window reveal, and you still have to do the conversion to the external figure yourself. HouseSense doesn't decide what counts as living area in a public register. The authority does.

Quick reminder

• External ≈ internal + (thickness × perimeter) + internal walls.
• Measure the thickness at three window reveals, not just one.
• Measure the perimeter outside, and follow every kink in the façade.
• Work out each storey separately, with that storey's own perimeter.
• Keep basement, conservatory and garage out of it.
• A gap of over 10 m² isn't maths — it's a registration that needs checking.