Why the tin's spread rate is a best case, not a guarantee
Every tin of paint carries a coverage figure, usually stated as square metres per litre. That number comes from the manufacturer's own testing, done on a smooth, sealed, properly primed surface with a roller of the type they used in the lab. It is real, but it is a best case, not an average. A previously unpainted plasterboard wall, a wall that has been patched and sanded in a few spots, or anything with texture, will drink up more paint than the tin promises, sometimes a lot more. That is why this calculator asks you to enter the spread rate yourself rather than assuming one for you. Check the actual tin sitting in your shed, not a number remembered from the last job or copied from a website. Different products, different sheens, and different brands all carry different figures, and a sealer or primer in particular covers a much smaller area per litre than a topcoat does.
Porous surfaces are the biggest variable. Bare plaster, new render, and anything that has never been painted before will soak up the first coat noticeably faster than a wall that already has a coat or two on it. If you are painting a brand new build or a wall that has just been stripped back to bare plaster, budget for the coverage to run short of the tin's stated figure on that first pass. Painters often use a diluted first coat, or a dedicated sealer, specifically to even this out and stop the topcoat from disappearing into a thirsty surface. If in doubt, ask whoever sold you the paint what that particular product does on an unsealed surface, because they will know their own range better than a generic guide can.
How to measure a room properly
For walls, the simplest approach is to measure the room's length and width, then its wall height, and let the calculator work out the total wall area from there using the standard room-perimeter method. Measure to the nearest ten millimetres if you can, at floor level and again near the ceiling, because older houses in particular are rarely perfectly square, and a bow in a wall or a sloped ceiling will throw a straight tape measurement out a little. If a room has an unusual shape, an alcove, or a section with a different wall height, it is often easier to skip the room-dimensions option and add up the wall area directly instead, breaking the room into simple rectangles on paper first and summing them by hand before you type in a single total.
Ceilings are measured the same way as a floor: length times width. A sloped ceiling, like the underside of a pitched roof in an attic conversion, has more surface area than its flat floor plan suggests, in the same way a pitched roof needs more material than its footprint implies. If your ceiling slopes, measure along the slope itself rather than the flat plan distance, or you will come up short.
Why you subtract doors and windows, and when it isn't worth bothering
Every door and window in a wall is an area you are not painting, so subtracting it from the total keeps you from buying paint you will never use. A standard internal door opening is a reasonable few square metres, and an average window somewhere similar, so on a room with two or three openings the saving is real but not enormous. It matters more on a room with a lot of glazing, a sliding door, or several windows along one wall, where the openings might account for a sizeable share of that wall's total area.
Here's the thing though: on a small job it is often not worth the fuss. If you are painting one bedroom with a single door and one modest window, the difference between subtracting the openings and just buying an extra litre "to be safe" is trivial, and rounding up to the next tin size covers the gap anyway because tins come in fixed sizes, not exact litres. Where it is worth doing properly is a larger area, a whole house repaint, or a commercial space with a lot of glazing, where an unsubtracted opening total can add up to a genuine extra tin or two across the whole job.
Coats, and why the second one uses less paint than the first
Two coats is the standard for a proper finish on most interior walls, and this calculator lets you set the number of coats because some jobs need more and some need fewer. A single coat over an existing colour in good condition might be enough for a touch-up. A colour change, especially a light colour going over a dark one, often needs three coats to fully cover, and that is worth knowing before you buy rather than after you have run short on the second weekend.
What most people notice once they are actually rolling a wall is that the second coat goes on faster and seems to use less paint than the first. That is a real effect, not just a feeling. The first coat is doing the hard work of sealing a bare or previously different-coloured surface and evening out the base colour, so it soaks in more and covers less area per litre. The second coat is going over a surface that is already primed and largely the same colour, so it glides on thinner and covers more of the tin's stated area. This calculator applies your spread rate evenly across every coat for simplicity, which means it will estimate slightly generously on later coats. Treat that as a small buffer in your favour rather than a reason to distrust the total, since a few hundred millilitres of spare paint left in the tin is a far better outcome than being one wall short with the shops closed.
Buying enough the first time, and why a leftover tin beats running out
Running out of paint mid-wall is worse than it sounds, and worse than having a bit left over in the tin when the job is done. If you finish a job and have some paint spare, you have a useful touch-up supply for scuffs and marks down the track, at no real cost beyond the tin sitting in the shed. If you run short partway through a wall and have to buy more, you are relying on the shop having the exact same product in stock, in the exact same batch and colour code as what you already used.
Paint is mixed in batches, and even the same colour name from the same brand can shift very slightly from one batch to the next, particularly with a custom-tinted colour rather than a standard off-the-shelf shade. Two tins from different batches, applied side by side on the same wall, can show a faint difference under certain light, especially in a large open room with a lot of natural light moving across it during the day. Buying enough in one trip, from one batch if the shop can manage it, is the safer approach for any wall that needs to look uniform. If you do need to go back for more, ask the shop to check the batch number against your original tin, not just the colour name and code, and if they cannot match it, consider repainting the whole wall rather than just the unfinished section.
Rounding up to the next tin size, rather than trying to buy the exact litre count this calculator gives you, is the practical way to build in that buffer. Tins come in fixed sizes such as 1, 4, 10 and 15 litres depending on the brand, and this calculator rounds your total litres up to a whole number of tins at whatever size you enter, so you are never left short by a design flaw in the maths.
A note on sheens, primers and undercoats
The spread rate on a tin varies not just by brand but by the finish inside it. A flat or matte finish typically covers a touch more area per litre than a gloss or semi-gloss, because the higher-sheen finishes usually carry more resin. A dedicated primer or sealer, as mentioned above, covers noticeably less than either, because its job is to bind to and even out the substrate rather than build an even topcoat. If your job involves a primer coat followed by two topcoats, run this calculator twice, once with the primer's own spread rate and coat count, and once with the topcoat's, then add the two litre totals together rather than trying to average one spread rate across the whole job. It only takes a minute and it is far more accurate than guessing a blended figure.
Where this fits with the rest of the job
A paint calculation is one part of costing a repaint, alongside labour, prep work like filling and sanding, and any trim or ceiling work done separately from the main walls. If you are quoting a paint job for someone else, whether as a favour or as a tradie pricing the work, our markup and margin calculator is worth running once you have your materials total from here, so the price you quote actually reflects what the paint and your time are worth, not just a guess rounded to a nice number. And if painting is part of a business you are running rather than a one-off job around the house, our painters page covers what a painter's own site needs to actually bring in enquiries, not just look tidy.