All articles · 7 min read

Which measurements does a contractor need to give a firm, fixed-price quote?

Five figures decide it: net wall area, floor area, ceiling area, the size and position of every opening, and the ceiling height. Known, and a fixed price can be worked out — miss one, and it becomes an estimate.

Why exactly these five figures

Contractors don't price by room. They price by quantity times unit rate: so many square metres of wall, so many linear metres of skirting, so many pieces of window reveal. A firm quote is a fixed price the contractor is on the hook for, and it can only be set once the quantities are known. Net wall area, floor area, ceiling area, the openings' dimensions and the ceiling height are the five figures almost every trade prices from — the painter, the flooring fitter, the carpenter, the plasterer and the insulation fitter all use the same five figures, just with a different unit rate each.

Miss one of the figures, and it gets guessed. And a guess baked into a fixed price is always expensive for one of the two parties — so either a safety margin gets added on top, or the quote gets swapped for an estimate that can be adjusted afterwards.

1. Net wall area — the wall area minus doors and windows

The net wall area is the area that actually has to be painted, filled, plastered or skimmed. It's calculated as the wall's gross area (length times ceiling height) minus the area of the openings sitting in the wall.

An example you can follow. A room 4.20 m by 3.60 m with 2.50 m to the ceiling:

• Perimeter: (4.20 + 3.60) × 2 = 15.60 m
• Gross wall area: 15.60 × 2.50 = 39.00 m²
• One window 1.20 × 1.30 m = 1.56 m². Two doors 0.90 × 2.04 m = 2 × 1.84 = 3.68 m². Openings in total: 5.24 m²
• Net wall area: 39.00 − 5.24 = 33.76 m²

The difference between 39 and 33.8 m² is 5.24 m² — a good 13% of the gross area. On a two-coat paint job that's around 10 m² of paint and the labour that goes with it. That's exactly where the gap between a quote and an invoice lives.

Be aware that trades deduct openings differently. A painter will rarely deduct small openings, because the edge around them costs more work than the area that disappears. So both figures should be provided — gross and net — so the contractor can choose. Give only one figure without saying which it is, and the misunderstanding ends up in the quote instead of in your text.

2. Floor area — and why it isn't the registered figure

The floor area is the room's internal area: length times width, minus fixed items that don't need flooring under them (a masonry chimney breast, a built-in wardrobe with no base). It's the figure flooring is ordered by, and underfloor heating loops are calculated by.

The registered floor area can't be used for this. The public building register works out the floor area around the outside of the outer walls and counts both outer and inner walls in. The sum of the rooms' floor areas therefore typically comes in 10-20% below the registered floor area, and most in small homes and houses with thick outer walls, because the walls' share grows the smaller the house is. Order flooring by the registered area and you buy too much; price hours by it and you pay for hours that don't exist. As a rule of thumb: the register is for authorities and property listings, internal measurements are for contractors.

Remember to add wastage on top for materials. For plank flooring laid straight in a rectangular room, allow for roughly 5-10% wastage; for herringbone, diagonal laying, or plenty of niches and corners, closer to 10-15%. Wastage is a material allowance, not an area — put the measured area in the enquiry, and let the contractor add their own wastage on top.

3. Ceiling area — almost, but not quite, the same as the floor

In a room with a flat ceiling and vertical walls, the ceiling area is the same as the floor area, and that's the figure used for ceiling paint and for plasterboard.

In a room with sloped ceilings, a ridge or collar ties, the two figures differ. An attic room of 4.5 × 4.0 m has 18 m² of floor — but with a 45-degree roof pitch and a 1.5 m flat section of ceiling in the middle, the ceiling surface itself is roughly 23 m², because each sloped surface is 1.77 m long where it only covers 1.25 m horizontally. If the room has sloped ceilings, that needs to go in the enquiry along with the knee wall height (how tall the wall is where the slope begins) and the ridge height. Without them, no fixed price can be set for boarding, painting or insulation.

4. Every opening's size and position

A window isn't just a deduction from the wall area. It's also reveals that need filling, sills that need painting, and a frame that decides whether the skirting can run all the way round. So every opening needs to come with width, height, sill height (the distance from the floor up to the underside of the window frame) and distance to the nearest corner.

The position isn't pedantry. It decides:

• how many linear metres of skirting are needed, and how many mitre cuts are in them;
• whether a door leaf can swing open, or whether it needs to be a sliding door;
• whether there's room for a radiator, a kitchen worktop or a bed under the window;
• how many reveals need lining — an opening typically has three or four sides, each of which costs labour.

Also count the openings as a tally. Many unit rates are per item, not per m²: fitting an internal door, a window reveal, a skirting corner. "Four windows and three doors" is a figure that can be multiplied.

5. Ceiling height — measured in several places

Ceiling height multiplies into every wall figure, so an error here propagates everywhere. Measure 2.50 m where there's actually 2.42 m, and the error is 8 cm — and over 15.6 linear metres of wall that's 1.25 m² too much in the quote.

Measure the height in at least three places in the room: two opposite corners and a spot in the middle of a long wall. In older houses floors are rarely level, and a 2-4 cm difference from corner to corner is entirely normal. If there is a difference, write both the lowest and the highest into the enquiry rather than an average — that way the adjustment is priced in, and no one is surprised on the day.

What isn't a measurement, but still decides the price

The five figures give the quantities. Price also depends on condition and access, and that information costs only a couple of lines in the enquiry:

• Condition of the substrate: is the wall lining paper, felt, raw plaster, three layers of old wallpaper, or cracked render?
• Floor and access: a third-floor flat with no lift changes material handling noticeably.
• Furnished or empty room: covering and moving things is time.
• Occupied during the work: yes or no — it changes how the work can be scheduled.
• Photos: two or three pictures per room often save a site visit.

And one thing no measurement can answer: whether a wall is load-bearing. Neither a tape measure, a scan nor an app can determine that. If a wall needs to be removed or broken through, that requires a building surveyor or a structural engineer — and the work usually needs approval before it starts. Write in the enquiry that the question is unresolved, rather than guessing.

How to get the figures without measuring everything by hand

All of it can easily be measured with a tape measure and a notepad — it typically takes 10-20 minutes per room, once the ceiling height has to be measured in three places and every opening needs four figures.

HouseSense is built to save that round trip. Whether the room is scanned with LiDAR on an iPhone Pro, or drawn on the app's graph paper, the takeoff works out the net wall areas with doors and windows deducted, totals the floor and ceiling areas, and lists every opening with width, height and position in a window and door schedule. The whole thing can be sent on as a PDF — and the PDF can be written in a different language from the app's, if the contractor doesn't read your language.

However the figures are obtained, the rule is the same: give the contractor quantities, not rooms. "The living room, kitchen and one bedroom need painting" is a request for an estimate. "33.8 m² net wall, 15.1 m² ceiling, 4 openings, 2.47-2.51 m ceiling height, substrate lining paper" is a request for a quote.