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 totally transform a space, making it feel bigger, brighter, or a whole lot less beige. But before you grab the biggest you find and hope for the best, let us 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 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 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 sizes, you'd buy one 4L and two 1L cans. Sorted.
Don't Forget the Ceiling and Trim
The ceiling is length × width. For skirting boards and door frames, estimate the total running length and multiply by average width (usually 10–15cm). Or buy a smaller 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 -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've 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!