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Paint Calculator
Get an exact paint estimate for any room, ceiling or exterior wall. Enter your dimensions, openings and coats — see litres, gallons, cans and cost instantly.
How to use this paint calculator
Start by measuring the room you want to paint. You need three numbers per room: the length, the width and the ceiling height. A standard 5 m tape measure is plenty for most rooms — for tall stairwells, a laser measure saves time. Round to the nearest centimetre; the calculator does the rest.
Next, count the openings. Every standard door removes about 1.9 m² of paintable wall and every standard window around 1.5 m². The calculator applies these defaults automatically, but you can override the size of any door or window if yours are unusual — French doors, picture windows and bi-folds all behave differently. Openings are always removed before coats are applied, so you only ever pay for paint on actual surface.
Then choose your paint. The dropdown covers the most common interior and exterior finishes, each pre-loaded with a realistic coverage rate in square metres per litre. If you already know your paint's spread rate from the tin, pick Custom coverage and type it in directly — this is the single most accurate setting you can use. Finally, set the number of coats. Two coats is the norm for a colour change; one is sometimes enough for a like-for-like refresh.
How the calculation works, step by step
Under the hood the calculator walks through every room you have entered and builds a true surface model:
- Wall area — for each room, perimeter = 2 × (length + width). Wall area = perimeter × height.
- Openings — the area of every door and window is subtracted from the wall area.
- Ceiling area — if you have ticked "Include ceiling", length × width is added for each room.
- Texture factor — the combined area is multiplied by your surface factor (1.0 for smooth, up to 2.0 for pebble-dash).
- Coats — the first coat uses the full paintable area; each additional coat uses a reduced 0.8× factor to reflect real-world absorption.
- Volume — paintable area ÷ coverage rate = litres. A 10% wastage allowance is added for the roller, tray and cutting-in.
- Cans — litres ÷ can size, rounded up to the next whole can so you never run short.
Every one of those steps is visible in the room-by-room breakdown, so you can see exactly where each litre goes. If a number looks off, the breakdown tells you which input to adjust.
Quick reference
Paint it right — the essentials at a glance
🧮 Coverage per litre
Exactly how many m² or ft² a litre and a gallon really cover — and what throws the number off.
✨ Which finish where
Matte, eggshell, satin, semi-gloss and high-gloss — matched to the right surface in every room.
🧰 Prep checklist
The 8 preparation steps that decide whether your paint lasts two years or ten.
🖌️ Pro techniques
Cutting in, rolling without lap marks and keeping a wet edge — the tricks that hide the DIY.
Common questions about estimating paint
How much extra paint should I buy?
The calculator already includes a 10% wastage allowance for the roller, tray, cutting-in and minor spills, which is enough for almost all interior jobs. For exterior work on very rough render, consider bumping the surface factor to 1.2–1.4 to add a little more headroom. We do not recommend buying more than that — leftover paint usually goes to waste.
Why does my second coat need less paint?
The first coat soaks into the surface and primes it. By the time you apply the second coat the wall is already sealed, so less paint is absorbed and more stays on the surface where you want it. Our model uses a 0.8× factor for additional coats, which closely matches real-world consumption on prepared walls.
Do I need primer as well?
If you are painting bare plaster, repairs, a drastic colour change or a glossy existing surface, yes — run the calculator twice, once with a primer coverage rate and once with your topcoat, and add the totals. Painting over sound, same-type paint usually does not need primer.
How do I handle textured or rough walls?
Slide the Surface texture factor above 1.0. A value of 1.2 suits lightly textured render, 1.5 is right for stucco, and pebble-dash or bare brick may need 1.7–2.0 because the real surface area is far larger than the flat measurement suggests.
Can I save and revisit my calculation?
Yes. Click "Save to history" and the calculation is stored in your browser's local storage — nothing leaves your device. Your last twenty estimates are kept and can be reloaded with a single click. Use the Share button to generate a link that contains your exact inputs.