My paint purchases have consistently been slightly off — either running out at a critical moment or ending up with cans I had no use for — until I started calculating coverage properly.
There's a particular DIY frustration that arrives when you open your last tin of paint and stare at three-quarters of an unpainted wall. Or the opposite: stacking four leftover tins in the shed where they'll slowly turn unusable. Both are caused by one thing — not calculating paint quantities properly before starting.
Coverage Rate: The Key Number
Every paint tin states its coverage rate in m² per litre. Common rates: matt/silk emulsion 10-12 m²/litre | gloss/satinwood 13-17 m²/litre | primer 10-14 m²/litre | masonry paint 4-8 m²/litre. Our paint calculator applies these rates automatically. Use our area calculator for room dimensions first.
Step 1: Calculate Wall Area
Perimeter × ceiling height, minus doors (≈3.6 m²) and windows (≈1.44 m² each). Example: 5m × 4m room, 2.4m ceiling, one door, one window: (5+4+5+4) × 2.4 − 3.6 − 1.44 = 38.16 m².
Step 2: Number of Coats
- Bare/new plaster: mist coat + 2 full coats
- Previously painted, similar colour: 2 coats
- Major colour change: 2-3 coats
Multiply wall area by coats: 38.16 m² × 2 = 76.32 m² total coverage required.
Step 3: Litres Needed
76.32 ÷ 12 m²/litre = 6.36 litres. Round up to available tin sizes (1L, 2.5L, 5L, 10L). A 5L + 2.5L = 7.5L is practical here. Add 10% for wastage and touch-ups.
Don't Forget the Ceiling
Ceiling area = length × width. 5 × 4 = 20 m². At 12 m²/litre over 2 coats: 40 ÷ 12 = 3.33 litres. One 5L tin of white emulsion covers this with overage for touch-ups.
Keep a Touch-Up Tin
Always keep a small sealed tin for future touch-ups. Properly stored paint lasts 2-5 years and makes scuffed skirting disappear in seconds. It's worth far more than its cost when you need it two years after the job is done.
Further reading: Dulux's website has detailed per-product coverage guidance. Use Dulux's paint quantity guidance.
Coverage Rates and Why They Vary
Paint coverage is stated on the tin as a rate such as 12 square metres per litre. This assumes a single coat on a smooth, primed surface. In practice, your coverage will be lower for several reasons: textured or rough walls absorb more paint, dark colours need additional coats to achieve opacity, and brush or roller application gives different results than spray. For budgeting, apply the manufacturer's stated coverage only as an upper estimate. Use 10 square metres per litre as a working figure if painting onto new plaster or a porous surface, and plan for two coats as standard on any colour change.
Calculating Paint for Your Room
Calculate the total paintable area by measuring each wall width multiplied by its height, then add the ceiling area. Subtract door and window openings — a standard door is roughly 1.6 square metres and a standard window 1.2 square metres. Divide the total area by your adjusted coverage rate to get litres needed per coat, then multiply by the number of coats. Add 10-15% for touch-ups, trim around switches and sockets, and variation in application. Round up to the nearest available tin size.
Finish Types and Their Practical Differences
Matt finishes absorb light and hide surface imperfections, making them suitable for ceilings and low-traffic walls. Eggshell and satin finishes have a slight sheen, are more wipeable, and are typically used in kitchens, bathrooms, and hallways where walls need occasional cleaning. Gloss finishes are durable and reflective, most often used on woodwork and trim. Using the wrong finish in the wrong location shows quickly — a matt paint in a kitchen gets marked easily, while a high-gloss wall in a living room shows every imperfection in the surface. Choosing finish alongside colour is part of calculating the right paint for the job.
Leftover Paint and Storage
A small amount of leftover paint from each room is worth keeping for touch-ups. Store paint in the original tin with the lid pressed firmly down, kept in a cool, frost-free location. Write the room name and the date on the tin. Properly stored, most paint remains usable for 2-5 years. Dispose of hardened or unusable paint at a household waste site — do not pour liquid paint down the drain. If you have significantly more leftover than expected, the calculation was off: review your coverage assumptions and wall area measurements to understand where the difference came from before the next project.