How Much Paint Do I Need? The Complete Calculator Guide
Few DIY moments are as deflating as running out of paint with one wall left to go — or, almost as bad, staring at three half-empty tins you will never open again. That is exactly the problem a good how much paint do I need calculator solves. Instead of guessing, you measure your walls, subtract the doors and windows, account for the number of coats, and get a precise litre figure — then buy the cheapest combination of tin sizes to cover it. CoatCount does all of that in seconds, and this guide explains the maths behind it so you can buy paint like a professional decorator.
How this paint coverage calculator works
Every paint coverage calculator follows the same four steps. First, it measures the paintable wall area: for a rectangular room that is twice the length-plus-width multiplied by the ceiling height, which gives you all four walls. Second, it subtracts anything you will not paint — a standard interior door is about 1.9 square metres and a typical window around 1.4 square metres, and in a room with two doors and two windows those openings can easily remove 6 square metres from your total. Third, it multiplies by the number of coats. Fourth, it divides by the paint's coverage rate, usually printed on the tin as square metres per litre. The result is then rounded up, because paint is sold in whole tins and you would much rather have a little left over than run short mid-wall.
CoatCount adds two refinements most free calculators skip. The colour-change multiplier adjusts the coat count automatically: repainting a dark wall in a light colour needs three coats, not two, because the old colour keeps "grinning through". And the tin-size optimiser does not just tell you the litres — it searches every combination of 1, 2.5, 5 and 10 litre tins and picks the cheapest one that covers your total, so you never overpay for convenience sizes.
Paint coverage per litre: what the tin really means
The single biggest input to any paint coverage calculator is the coverage rate, and it varies more than most people expect. A typical interior matt emulsion covers around 10 to 12 square metres per litre on a previously painted wall in good condition. Silk and satin finishes sit around 9 to 11 square metres per litre because their slightly thicker film goes on a touch heavier. Eggshell — popular for kitchens and bathrooms — lands in the same 10 square metres per litre neighbourhood. Primer and undercoat are thirstier: 8 to 10 square metres per litre is normal, and on bare, porous plaster the first coat can disappear at an alarming rate, which is why decorators thin the first coat into a "mist coat" on new plaster. Exterior masonry paint is the hungriest of all at roughly 6 to 8 square metres per litre, because rough render drinks paint.
These figures assume a smooth, sealed surface applied with a roller. Textured walls, heavy stipple or rough plaster can easily cut coverage by 15 to 20 percent, and spraying typically uses more paint than rolling. When in doubt, use the conservative end of the range — or bump the waste buffer in the calculator up to 15 percent. The default of 10 square metres per litre in CoatCount suits standard interior emulsion on previously painted walls, which covers the most common repainting job.
How many litres of paint for a room? Worked examples
Let us run the numbers for three typical rooms so you can sanity-check your own results. Take a compact bedroom, 3 by 3.5 metres with a 2.4 metre ceiling: the four walls total 31.2 square metres. Subtract one door (1.9 m²) and one window (1.4 m²) and you have about 27.9 square metres of paintable wall. Two coats at 10 m² per litre means 5.6 litres — round up and buy a 5 litre tin plus a 1 litre tin, or a single 10 litre tin if it is cheaper per litre.
Now a standard living room, 4 by 5 metres with 2.7 metre ceilings: walls total 48.6 square metres. Minus two doors and two windows (about 6.6 m²) leaves roughly 42 square metres. Two coats at 10 m²/L gives 8.4 litres — the optimiser would likely pick a 5 litre plus a 2.5 litre plus a 1 litre tin, or check whether two 5 litre tins beat that on price. Finally, repainting that same living room from a dark charcoal to a light warm white changes the coat count to three, pushing the need to 12.6 litres — a full extra tin compared with a like-for-like repaint. That jump is exactly why the colour-change question matters so much.
A quick rule of thumb if you are in the paint aisle without your phone: one litre covers about one 3-by-3 metre wall in a single coat. Most rooms need one 5 litre tin per average bedroom wall-set per coat, and you almost always need two coats. It is a rough estimate, but it will keep you from the classic mistake of buying a single 2.5 litre tin for a whole room.
Dark to light: why you need that extra coat
Paint is translucent. Every coat lets a little of what is underneath show through, and the stronger the contrast between the old and new colour, the more coats it takes to kill it completely. Going from a deep navy, forest green or charcoal to an off-white is the hardest transition in decorating: two coats will almost always leave a ghost of the old colour visible in daylight, especially on the wall opposite the window. Three full coats — or a tinted primer followed by two topcoats — is the professional standard, and CoatCount applies it automatically when you select a dark current colour and a light new one.
The reverse trip, light to dark, is far kinder: rich pigments cover pale walls easily, and two coats are plenty. Repainting in a similar tone, or refreshing the exact same colour, can even be done in one careful coat if the existing paint is sound — though most decorators still recommend two for an even sheen. New, unpainted plaster is a special case: it needs a mist coat (emulsion thinned with water) or a dedicated primer plus two topcoats, which is why the calculator counts it as three coats of paint volume. Skipping the primer on bare plaster is a false economy — the topcoat soaks in unevenly and you will end up buying the extra tin anyway.
Tin sizes: buying smart at the paint store
Paint tins come in 1, 2.5, 5 and 10 litre sizes, and the price per litre drops steeply as tins get bigger — a 10 litre tin is often 30 to 40 percent cheaper per litre than buying the same paint in 1 litre tins. That tempts people to just grab the biggest tin, but a 10 litre tin for a 7 litre job leaves 3 litres gathering dust, and opened paint has a shelf life of only a couple of years. The sweet spot is the cheapest combination that covers your litres with minimal leftover, which is a small optimisation puzzle: for example, 7.3 litres might be cheapest as a 5 litre plus a 2.5 litre tin rather than a 10 litre tin, depending on the store's pricing.
CoatCount solves this with a proper search over every combination, not a greedy guess. Enter the shelf prices from your local store and it will tell you exactly which tins to put in the basket — and it will also tell you how much will be left over, so you can decide whether to size up for touch-up paint. One pro tip: always keep a small amount of each colour for future scuff repairs. A dab from the original tin, applied with a small brush and "feathered" at the edges, is invisible; a new tin mixed months later almost never matches perfectly.
Estimating your total project cost
Once you know the litres and the tin combination, costing the project is simple multiplication — but most people forget the extras. Beyond the tins themselves, budget for primer if any surface is new or the colour change is extreme, plus sundries: rollers and sleeves, a decent brush for cutting in, masking tape, dust sheets and filler for cracks and nail holes. As a rough guide, sundries add 15 to 25 percent on top of the paint cost for a DIY job. If you are hiring a decorator, labour typically costs two to four times the materials, which makes accurate paint buying even more valuable — every wasted tin is money twice over.
The multi-room feature in CoatCount exists for exactly this kind of whole-house planning. Add each room with its own dimensions, doors, windows and colour change, and the calculator aggregates everything into one shopping list and one total. Buying all your paint in a single trip — and ideally from the same batch — avoids the subtle shade variations that can occur between production batches, which matter more than you would think on large light-coloured walls.
Seven mistakes that waste paint (and money)
First, forgetting the ceiling: a 4 by 5 metre ceiling is 20 square metres, nearly half a wall-set, and ceiling paint is usually bought separately in flat white. Second, measuring in feet and entering the numbers as metres — a 12 foot wall is 3.66 metres, and mixing the units roughly triples your estimate. Third, counting coats but not the primer on new plaster. Fourth, ignoring the waste buffer: rollers, trays, brush cleaning and the paint that stays in the tin add up to about 10 percent. Fifth, buying tester pots in the final colour and then not counting them — they do not cover meaningfully, but people sometimes subtract them mentally. Sixth, assuming one coat will do over a strong colour. And seventh, the most expensive mistake: buying paint before measuring at all. Five minutes with a tape measure and this calculator beats every guess.
Painting is one of the highest-impact, lowest-cost home improvements there is — a weekend and a few tins can completely transform a room. The difference between a frustrating project and a satisfying one is rarely skill; it is preparation. Measure carefully, subtract the openings, respect the colour change, buy the right tins, and keep a little back for touch-ups. CoatCount handles the arithmetic — you handle the brush strokes.