Painting a Room: How to Calculate Exactly How Much Paint to Buy
You've decided to give that tired old room a spruce-up. New paint can totally transform a space, making it feel bigger, brighter, or just a whole lot less beige. But before you grab the biggest can you can find and hope for the best — let's talk maths.
Nothing kills the DIY buzz faster than running out of paint halfway through the second coat, or ending up with enough leftover to paint a small country. Getting the quantity right is surprisingly simple.
Why Getting It Right Matters
Too little and you're stuck with a half-finished wall and the annoying task of trying to match the colour later (spoiler: it's harder than it looks). Too much and you've wasted money. Getting it just right means a smoother job, a happier wallet, and a professional-looking finish.
Step 1: Measure Your Walls
Total Wall Area = (Wall Length × Wall Height) × Number of Walls
Say you've got a room with four walls — two that are 4m long and two that are 3m, with 2.5m ceiling height:
- Long walls: (4m × 2.5m) × 2 = 20 sqm
- Short walls: (3m × 2.5m) × 2 = 15 sqm
- Total: 35 sqm
Step 2: Subtract Doors and Windows
You don't paint over glass (usually). Measure each door and window, calculate their area, and subtract from your total.
- Standard door: 2m × 0.8m = 1.6 sqm
- Large window: 1.5m × 1.2m = 1.8 sqm
35 sqm minus 3.4 sqm = 31.6 sqm of actual paintable area.
Step 3: Know Your Paint's Coverage
Every can states its coverage rate. A common rate is 10–12 sqm per litre, but always check the label.
Paint Needed = Paintable Area ÷ Coverage Rate
31.6 sqm ÷ 10 sqm/litre = 3.16 litres
Step 4: Multiply for Coats
You'll almost always need two coats. Going from dark to light? You might need three.
3.16 litres × 2 coats = 6.32 litres
With 1L and 4L can sizes, you'd buy one 4L and two 1L cans. Sorted.
Don't Forget the Ceiling and Trim
The ceiling is just length × width. For skirting boards and door frames, estimate the total running length and multiply by average width (usually 10–15cm). Or just buy a smaller can of trim colour and top up if needed.
Rather not do the maths? Use the Paint Calculator to get a precise estimate for your room in about 30 seconds.
The Just-In-Case Buffer
Always buy a little extra for touch-ups down the track. When a chair scrapes the wall or the dog decides to redecorate, you'll be glad you have matching paint sitting in the shed. An extra litre is usually enough for a standard room.
Quick Summary
- Measure total wall area (length × height × number of walls)
- Subtract door and window areas
- Divide by coverage rate per litre
- Multiply by number of coats
- Add a little buffer
Happy painting!