How to Calculate How Much Paint You Need for a London Property

Why Getting the Quantity Right Matters

Ordering too little paint mid-way through a job is more than an inconvenience. Batch-to-batch variation — even from the same brand and the same colour — can be visible, particularly in dark colours or where the mixing ratio has been adjusted. Having to blend the end of one tin with the start of another to minimise the join is a professional technique that requires experience. For a homeowner repainting a hallway on a Saturday, running out at 4pm is simply a disaster.

Ordering too much is a waste of money and creates a disposal problem: unused paint, particularly oil-based products, cannot simply be poured down the drain and must be disposed of correctly.

Getting the quantity close — erring slightly on the side of too much rather than too little — is a skill that professional London decorators develop through experience. The calculation is straightforward once you understand the variables.

The Coverage Rate: Where It All Starts

Every paint product has a theoretical coverage rate, usually expressed as square metres per litre. Standard emulsion typically covers 12 to 16 square metres per litre. Premium, heavily pigmented products such as Farrow & Ball Estate Emulsion may cover as little as 10 to 13 square metres per litre, partly because of their chalk content and partly because of their rich pigment load. High-build masonry paint may cover only 5 to 8 square metres per litre.

These are theoretical figures based on application to a smooth, non-porous surface. In practice, coverage is almost always lower because:

• Porous surfaces (new plaster, bare wood, unlined masonry) absorb more paint
• Textured surfaces have greater actual surface area than their flat dimensions suggest
• Cutting in around edges, switches, and skirtings produces more wastage than rolling open wall areas
• A second coat on a surface with colour variation may go on thinner as the applicator is more confident

A practical planning figure to use for most London interior walls is 12 square metres per litre per coat. This provides a reasonable safety margin.

Calculating Wall Area: Room by Room

The basic calculation for a rectangular room is:

(Width + Length) x 2 x Height = Total wall area

Then subtract the area of windows and doors (a standard internal door is approximately 1.8 square metres; a standard single-sash window is approximately 1.2 to 1.5 square metres). For most rooms, deducting one square metre per door and window opening from the total gives a workable figure.

Example: A standard London terraced house bedroom

• Room dimensions: 4.5m x 3.8m, ceiling height 2.7m
• Total wall perimeter: (4.5 + 3.8) x 2 = 16.6m
• Total wall area: 16.6 x 2.7 = 44.82 square metres
• Deduct one door (1.8m2) and one window (1.3m2) = 41.72 square metres
• At 12m2 per litre per coat, this room requires 3.5 litres per coat
• For two coats: 7 litres
• With 10% wastage allowance: 7.7 litres, so order 2 x 2.5L and 1 x 5L (total 10L) to be safe

Ceilings: The Often-Underestimated Surface

A ceiling is simply the floor plan area of the room. For the example above (4.5m x 3.8m), the ceiling area is 17.1 square metres. At 12m2 per litre, this requires 1.4 litres per coat. For two coats, 2.8 litres — a 2.5L tin should cover it with care, but a 5L tin gives comfortable coverage and a reserve for touch-up.

Woodwork: Calculating Skirtings, Architraves, and Doors

Woodwork coverage rates are generally higher than walls — a gloss or eggshell typically covers 15 to 18 square metres per litre. But calculating woodwork area is less intuitive.

A practical rule of thumb for a standard room:

• Skirtings: Measure the room perimeter in metres, multiply by 0.15 (for a 150mm skirting height) — this gives the skirting area in square metres
• Architraves: Each door has approximately 5 to 6 metres of architrave run; multiply by 0.07m (70mm wide) for the area
• Doors: Each face of a standard door is approximately 1.8 to 2.0 square metres; a door is typically painted on both faces plus the four edges

For a full house, an experienced London decorator will calculate room by room and then aggregate. A typical two-bedroom London flat requires approximately 2.5 to 3.5 litres of satinwood or gloss for all woodwork in two coats.

Exterior Painting: Different Rules Apply

Exterior surfaces are more complex because texture varies enormously. Smooth rendered walls behave similarly to interior walls. Rough brick or pebbledash can require two to three times as much paint per square metre as a smooth surface. Masonry paint products should be applied generously.

A practical planning figure for exterior brickwork or render is:

• Smooth render: 6 to 8m2 per litre
• Medium texture: 4 to 6m2 per litre
• Heavy texture or pebbledash: 3 to 5m2 per litre

Ordering Tips: Minimising Batch Variation Risk

If your calculation says you need 4.2 litres, do not order one 5L tin and hope for the best. Order all the paint you need at the same time, from the same batch where possible. When buying from a decorating merchant or tinted from a mixing machine, ask for all tins to be mixed from the same batch of base.

For large projects, it is worth decanting all the tins into a larger container and mixing them together — a process called boxing — before application. This eliminates any visible variation between tins and ensures the colour is perfectly consistent from start to finish.

Keep at least 10% of your total quantity in reserve for touch-up work. After two or three months, even a perfectly colour-matched new tin may look slightly different from the aged paint on the walls.

Belgravia Painters provides fully itemised material specifications with every quote, including paint quantities calculated room by room. Contact us to arrange a free survey of your London property.