Paint Calculator FAQ — Everything About Estimating Paint

Everything you might want to know about the paint calculator and how to estimate paint, gathered in one place. If your question is not here, send it to us — we add the most common new questions regularly.

Getting started

The paint calculator takes the dimensions of each room you want to paint, the openings (doors and windows) in each, the paint you plan to use and the number of coats, then returns the total paint quantity in litres and gallons, the number of cans to buy and an estimated cost. It works for interior and exterior projects, in metric and imperial units, and supports as many rooms as you like in a single calculation.

To start, measure each room's length, width and ceiling height with a tape measure, count the doors and windows, then type those numbers into the calculator. Pick your paint from the dropdown (or enter a custom coverage rate from the tin), set the number of coats, and the results appear instantly on the right. The room-by-room breakdown shows exactly where every litre goes.

How the maths works

For each room, the calculator works out the perimeter of the walls (2 × length + 2 × width), multiplies by the height to get gross wall area, subtracts the area of every door and window, and optionally adds the ceiling area (length × width). It applies your surface texture factor, multiplies by a realistic coat factor (full area for the first coat, 0.8× for each additional coat, because second coats absorb less), divides by the paint's coverage rate, and adds a 10% wastage allowance. The result is rounded up to whole cans so you never run short mid-project.

Accuracy and limits

On well-prepared walls the calculator is accurate to within about 5%. The factors that throw it off are the same ones that throw off any estimate: highly porous or textured surfaces (use the surface factor slider), dramatic colour changes (budget an extra coat), and very cheap paint with poor coverage (switch to a custom coverage rate from the tin). The 10% wastage allowance covers normal cutting-in, roller absorption and minor spillage.

Doors, windows and openings

Every door and window you enter is removed from the wall area before coats are calculated, so you only ever pay for paint on actual surface. The calculator applies standard door (1.9 m²) and window (1.5 m²) sizes by default, but each room has an "Advanced" section where you can override those sizes for unusual openings like French doors, picture windows or bi-folds.

Ceilings

Ticking "Include ceiling" adds the ceiling area to the paintable surface for each room, calculated separately so you can use a different (typically flatter, less spattered) ceiling paint if you wish. The number of ceiling coats is set independently of the wall coats, defaulting to one coat — usually enough for a same-colour ceiling refresh.

Interior vs. exterior

Switching to Exterior mode changes the available paint types to masonry, brick and cladding finishes with their much lower coverage rates, and the surface texture factor becomes essential for textured render, pebble-dash or bare brick. The maths is otherwise identical — the tool adapts to the realities of outdoor surfaces.

Units, currency and cans

Toggle between metric (metres, litres) and imperial (feet, gallons) at any time; results always show both units side by side. The cost section lets you enter your local price per litre, can size and currency symbol, and the total spend updates instantly. Cans are always rounded up — if you need 4.2 litres in 5-litre tins, you buy one tin and keep the rest for touch-ups.

Saving, sharing and printing

Click Save to history to keep up to twenty calculations in your browser's local storage — nothing is uploaded anywhere. The Share link button encodes your exact inputs into the page URL, so anyone you send it to sees the same numbers; great for handing to a decorator or getting a quote. The Print report button opens a clean, ink-friendly summary you can save as a PDF or hand to a supplier.