How Much Paint Do I Need?Complete Guide
Everything you need to know to calculate the right amount of paint for walls, ceilings, and trim — avoid buying too much or running out mid-project.
Quick Answer
One litre of interior emulsion covers approximately 10 to 12 square metres with one coat. For a standard room (3.6×3.6 m, 2.4 m ceilings), you need about 5 litres for two coats on the walls. Add 2.5 litres for the ceiling and 1 litre of gloss or satinwood for trim and doors. Always round up — leftover paint is essential for future touch-ups.
How to Measure Your Walls
Accurate wall measurements are the foundation of any paint estimate. Here is how to measure like a professional decorator:
- Measure the length of each wall in metres and multiply by the ceiling height. A 4.2-metre wall with 2.4-metre ceilings = 10.1 square metres. Repeat for all four walls and add them together.
- For quick whole-room calculations, measure the room perimeter (sum of all wall lengths) and multiply by ceiling height. A 3.6×4.2 m room has a 15.6-metre perimeter, times 2.4-metre ceilings = 37.4 square metres of wall area.
- Subtract openings: a standard door is approximately 1.9 square metres (0.8×2.1 m) and a standard window is approximately 1.4 square metres (1.2×1.2 m). For a room with one door and two windows, subtract about 4.7 square metres.
- For rooms with vaulted or cathedral ceilings, measure the wall height at the tallest point and the shortest point, average them, and multiply by the wall length.
- Include cupboard interiors if you plan to paint them — a standard built-in wardrobe adds about 5 square metres of wall area.
- Record all measurements before going to the paint supplier. Take photos of the rooms for reference when choosing colours.
A typical 140 m² house has 370 to 460 square metres of paintable wall area, depending on ceiling height, number of rooms, and window/door count.
Coverage Rates by Paint Type
Not all paints cover the same area. Coverage rates vary by finish, quality, and the surface you are painting.
Matt emulsion has the highest coverage rates at 12 to 14 square metres per litre because it has lower binder content and spreads easily. It is ideal for ceilings and low-traffic rooms but shows marks and is difficult to clean. Silk emulsion covers 10 to 12 square metres per litre and is the most popular choice for living rooms, bedrooms, and hallways — it has a subtle sheen that is easy to clean without being too shiny.
Eggshell finish covers 10 to 12 square metres per litre and is excellent for kitchens, bathrooms, and high-traffic areas due to its moisture resistance and washability. Satinwood covers 10 to 12 square metres per litre and is the standard choice for trim, skirting boards, doors, and window frames — its durable surface is easy to wipe clean. Gloss covers 10 to 12 square metres per litre and is used for doors, window frames, and any surface where maximum durability and reflectivity are desired.
Premium paints from brands like Dulux, Farrow & Ball, and Little Greene typically achieve the higher end of these coverage ranges due to higher pigment concentration and better levelling properties. Economy paints often fall at the lower end and may require an additional coat to achieve full coverage. Textured, porous, or unpainted surfaces reduce coverage by 20 to 30 per cent regardless of paint quality — always use an appropriate primer on these surfaces first.
How Many Coats of Paint Do You Need?
The number of coats directly affects how much paint you need to buy. Here is when to apply one, two, or three coats:
- One coat is sufficient only for touch-ups using the exact same paint colour and sheen on a previously painted surface in good condition. Some premium paints advertise one-coat coverage, but professional decorators still recommend two coats for best results.
- Two coats is the standard recommendation for virtually all painting projects. The first coat seals and primes the surface, while the second coat provides the final colour depth, uniform sheen, and maximum durability. Two coats of quality paint can last 7 to 10 years on interior walls.
- Three coats are necessary when making a dramatic colour change — especially going from a dark colour (red, navy, forest green) to a light colour (white, cream, pastel). The first coat of tinted primer neutralises the old colour, followed by two topcoats for full coverage.
- New, unpainted plasterboard or fresh plaster always requires a mist coat (watered-down emulsion) plus two topcoats (three coats total). The mist coat seals the porous surface and provides a uniform base for the paint. Skipping the mist coat on new plaster causes flashing — visible differences in sheen where the filler and bare plaster absorb paint at different rates.
- Textured surfaces like Artex and stippled ceilings may require an extra coat because the texture creates peaks and valleys that are difficult to cover evenly with a single pass.
For budgeting purposes, always calculate paint quantity based on two coats. If you need primer or a mist coat, calculate that separately at the primer's coverage rate.
Surface Preparation
Proper surface preparation is the single most important factor in a long-lasting paint job. No amount of expensive paint can compensate for poor prep work.
- Clean all surfaces before painting. Dust, grease, and cobwebs prevent paint adhesion. Wipe walls with a damp cloth or use sugar soap solution for greasy kitchen walls. Allow surfaces to dry completely before painting.
- Fill nail holes, dents, and cracks with lightweight filler. Apply with a filling knife, let it dry, and sand smooth with 150-grit sandpaper. For larger holes (over 12mm), use scrim tape and filler, applied in two thin coats.
- Sand glossy surfaces lightly with 150-grit sandpaper to create a key for the new paint to grip. This is especially important when painting over gloss or satinwood finishes.
- Remove or sand any loose, peeling, or flaking paint down to a sound surface. Feather the edges of remaining paint with sandpaper to create a smooth transition.
- Apply masking tape to trim, skirting boards, ceiling lines, and any areas you want to protect. Press the tape firmly with a scraper or card to prevent paint from bleeding underneath.
- Protect floors and furniture with dust sheets. Cotton dust sheets grip the floor better than plastic and absorb drips. Move furniture to the centre of the room and cover with plastic sheeting.
Calculating Ceiling Paint
Ceilings require their own calculation since they often use a different paint formulation than walls.
Ceiling paint (matt emulsion) is specifically formulated with higher solids content for better spatter resistance when rolling overhead, and it dries to a perfectly matt finish to minimise the appearance of imperfections. Standard ceiling paint covers 12 to 14 square metres per litre. To calculate the amount needed, multiply the room length by the room width — a 3.6×4.5 m room has 16.2 square metres of ceiling. At 13 m² per litre, that is about 1.25 litres — buy 2.5 litres for that room.
Most ceilings only need one coat if they are being painted the same white colour. If you are changing the ceiling colour or painting over a stain, apply a stain blocker first and then two topcoats. For Artex or textured ceilings, reduce the coverage rate to 8 to 10 square metres per litre because the texture absorbs more paint. A whole-house ceiling (140 m²) typically requires 10 to 12 litres of ceiling paint.
Calculating Trim and Door Paint
Trim, skirting boards, doors, and window frames are typically painted with a satinwood or gloss finish for durability and easy cleaning.
Skirting boards: Measure the total linear metres of skirting in the room. Standard skirting is 100 to 150mm tall. A room with 15.6 linear metres of skirting has approximately 2.3 square metres of paintable surface. One litre of satinwood covers about 10 to 12 square metres — enough for a large room's skirting boards.
Doors: A standard interior door (both sides) has approximately 3.6 square metres of paintable surface. Window trim (frame and architrave) averages 1 square metre per window. For a whole house, plan on 2 to 5 litres of gloss or satinwood for all skirting boards, door frames, window trim, and any dado or picture rails. Paint doors with a foam roller for the smoothest finish — brush marks are very visible on gloss paint on large flat surfaces.
Paint Cost Breakdown
Understanding the full cost of a painting project helps you budget accurately and choose the right products for your needs.
Wall paint is the largest cost component. Budget £15 to £50 per 2.5 litres depending on quality tier. For a typical room requiring 5 litres at 2 coats, wall paint costs £30 to £100. For a whole 140 m² house (approximately 30–40 litres needed for walls), paint costs £180 to £800. Buying 5-litre or 10-litre tins saves 15 to 20 per cent per litre compared to 2.5-litre tins.
Primer costs £8 to £20 per litre. New plaster requires a mist coat (watered-down emulsion at no extra material cost) or a dedicated primer at about 1 litre per 10–12 square metres. For a 140 m² house with all new plaster, that is 30 to 40 litres of mist coat. Self-priming paints (available in premium lines) eliminate this step but are more expensive per litre. For repaints over similar colours, primer is usually unnecessary.
Supplies for a whole-house interior paint job cost £80 to £160 total: roller frames and sleeves (£15 to £25), extension pole for ceilings (£10 to £20), angled brushes in 50mm and 65mm widths (£15 to £25), masking tape (£20 to £35 for multiple rolls), dust sheets (£10 to £25), paint trays and liners (£8 to £12), sandpaper and filler (£8 to £12), and rags and cleanup supplies (£4 to £8). Quality tools make a significant difference in the final result — do not skimp on roller sleeves and brushes. Available from Dulux, B&Q, Screwfix, and Toolstation.
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