Calculating the area to paint and the quantity of paint in Geneva

Floor area, developed surface: do not confuse the two

The first mistake is to think in terms of floor area. A flat described as “3.5 rooms, 75 m²” states its floor area, the one on the lease. But you do not paint the floor: you paint the walls and the ceiling. It is the developed surface that counts — and it is always far larger than people expect.

In a single room, the walls often represent two to three times the floor area. A bedroom of 12 m² on the floor can exceed 30 m² of walls. Across a whole flat, you quickly add up several hundred square metres of developed surface. That is why a proper measurement never starts from the lease area, but measures room by room.

The calculation is done in two separate blocks, because walls and ceiling do not always call for the same finish:

• Walls: room perimeter × ceiling height.
• Ceiling: room length × width.

Calculating the wall area: perimeter × height

The perimeter is the full way round the room: you add up the length of the four walls (or of every face, in a non-rectangular room). You multiply this perimeter by the ceiling height.

Take a room 4 m by 3. Its perimeter is 4 + 3 + 4 + 3 = 14 metres. At a height of 2.7 m, the gross wall area is 14 × 2.7 ≈ 38 m².

To refine, you subtract the large openings — doors, glazed bays, large windows — which are not painted. A standard door is about 1.8 m², a common window 1.5 to 2 m². You do not deduct small areas (sockets, switches): the margin absorbs them. This level of detail is useful for buying the right quantity; for an order of magnitude, the gross area is enough.

A Geneva particularity: the older housing stock — buildings in the Eaux-Vives, in Plainpalais, period flats in Champel — has generous ceiling heights, often from 2.8 to over 3.2 m, sometimes more with cornices and mouldings. For an equal floor area, you paint markedly more wall there than in a recent building at 2.5 m. The calculation shows it at once: height is a multiplier.

From coverage to quantity: how many litres

Every tin of paint states a coverage: the number of square metres one litre covers for one coat, for example “10 to 12 m²/l”. This is the central figure in the quantity calculation.

The formula fits on one line:

Quantity ≈ developed surface × number of coats ÷ coverage

Take our 38 m² of walls again, in two finish coats, with a coverage of 10 m²/l: 38 × 2 ÷ 10 = 7.6 litres. You round up to the next tin size and keep a margin for future touch-ups (a part-used tin kept aside lets you fix a scratch without redoing the wall). Allowing about 5 to 10 % extra is reasonable.

Part of the calculation	How to get it
Wall area	Perimeter × ceiling height
Ceiling area	Length × width
Developed surface	Walls + ceiling, per room
Number of coats	Primer + 2 finishes (common case)
Coverage	Read on the tin, in m²/l and per coat
Quantity	Area × coats ÷ coverage + margin

Two factors make the actual coverage differ from the stated one:

• The surface. A porous or new surface — new plaster, an open render, tired old matt paint — “drinks” the paint: the first coat is absorbed and the coverage drops. Hence the primer, which evens out the absorption before the finish.
• The number of coats. Generally a primer + two finish coats. A vivid shade or a move from dark to light adds passes. The detail of coats and drying is explained on our page coats, drying and smell.

Why the measurement does not replace the quote

You might think that once the area is known, the price follows on its own, per square metre. This is the most misleading shortcut in the trade — and it leads straight to unpleasant surprises.

It is not the area that determines the work, it is the condition of the surfaces. Two rooms identical in measurement can call for very different days of work. The first has sound walls: a coat of primer, two coats, done. The second needs holes filled, cracks bridged, an old glossy oil-based paint sanded back, a damp mark treated, an uneven render smoothed — all before the first finish coat. Same area, incomparable work.

This is exactly why a proper quote is drawn up after an on-site visit, never over the phone or from a plan. The measurement gives the area and the quantity of material; the visit reveals the actual preparation, which often weighs as much as the painting itself. Our pricing method sets out how these two dimensions — area and condition — combine, and the page on the cost of a painter in Geneva explains what goes into the price without displaying a rate per square metre, on principle.

The method in practice, room by room

To estimate a room yourself, the steps are always the same:

1. Measure the length, the width and the ceiling height (a laser measure is precise; a tape measure will do).
2. Calculate the walls: (length + width) × 2 × height, then subtract the large openings if you want to refine.
3. Calculate the ceiling if it is involved: length × width.
4. Add up to get the developed surface of the room.
5. Apply the quantity formula: area × coats ÷ coverage, plus the margin.

Repeated room by room, this calculation gives a reliable estimate of the area and the litres — useful for a purchase, for understanding a quote, or simply to avoid being caught out by the scale of a flat with high ceilings. The terms used (opacity, primer, coverage) are defined in the painting glossary.

But the estimate stops there: it does not replace the diagnosis of the surfaces. To turn an area into an accurate quote, you have to see the real condition of the walls. That is the whole purpose of the quote visit, free and without obligation, which can be requested directly on our quote page. The work is covered by the 2-year legal warranty (Art. 371 CO).