The Paint Calculator estimates how much paint a room needs before you buy a tin: enter the room's length, width, and wall height, the number of doors and windows, the coats planned, and your paint's coverage rate, and it returns the wall area, the paintable area after openings, the litres required, the amount to purchase with a waste allowance, and the tin counts in 1 L, 2.5 L, and 5 L sizes.
How It Works
Openings are deducted at standard allowances, 1.8 square metres per door and 1.4 per window, rather than measured individually. A waste percentage (default 10%) inflates the requirement, and the purchase figure rounds up to the nearest tenth of a litre. Dimensions may be entered in metres or feet; feet are converted before the arithmetic.
Parameters
| Input | Required | Meaning | |---|---|---| | Length / width / height | Yes | Room dimensions; height is wall height | | Doors / windows | No | Deducted at 1.8 and 1.4 m2 each | | Coats | No | Default 2 | | Coverage per litre | No | From the tin; default 14 m2/L | | Surface factor | No | Adjusts coverage for surface condition (default 1) | | Waste % | No | Default 10% |
Worked example
A 5 m by 4 m room with 2.4 m walls, one door, and two windows: wall area is 2 x (5 + 4) x 2.4 = 43.2 m2; openings deduct 1.8 + 2 x 1.4 = 4.6 m2, leaving a net 38.6 m2; two coats at 14 m2/L need 38.6 x 2 / 14 = 5.51 litres; with 10% waste the purchase figure is 6.1 litres, which the tin counts express as seven 1 L tins, three 2.5 L tins, or two 5 L tins if you buy a single size. In practice you would buy a 5 L plus a 1 L (or a 2.5 L) and keep the remainder for touch-ups, and the example shows why the calculator reports litres first and tins second: the litre figure is the truth, and the packaging decision is yours.
Coverage and the surface factor, honestly
The coverage printed on the tin (commonly 12 to 16 m2/L) is measured on smooth, sealed, primed surfaces in laboratory application; real walls tax it. The surface factor is where that reality enters: a factor below 1 models a thirstier surface (fresh plaster and bare drywall can drink a first coat at half the rated coverage, textured and roughcast surfaces present more area than their flat measurement, and a strong colour change can demand a third coat the coats field should carry instead). Primer is a separate product with its own coverage and is not inside this calculator's litres; on new plaster a mist coat or dedicated primer is not optional, and its cost belongs in the project budget beside the topcoat the calculator sizes. When in doubt between two coverage assumptions, run both: the difference on the example room is about a litre, and discovering you needed the pessimistic figure after the shops close is the classic DIY evening disaster.
What is not in the room
The calculator paints four walls. The ceiling is a separate area (length x width, usually in a dedicated ceiling paint with its own coverage), and skirting, architraves, doors themselves, and window frames are trim work measured by length and painted with trim paint in quantities this wall calculation does not pretend to estimate. Non-rectangular rooms decompose into rectangular wall runs measured individually; sloped ceilings, stairwells (measure the wall as its full height rectangle and accept over-ordering, or in stepped sections), and bay windows all defeat the single-room model in the safe direction if you measure generously and keep the waste at 10%. Paint left over is an asset (labelled, it is your future touch-up supply, colour-matched forever); paint short is a crisis, which is the asymmetry the waste allowance prices.
Frequently Asked Questions
How many coats do I actually need?
Two topcoats over a primed or previously painted surface in a similar colour is the standard the default encodes. One coat suffices only for same-colour refreshes with high-hide paints; three may be needed over strong colours or stains (with a stain-blocking primer as the real fix). Each coat multiplies the litres linearly in the calculator, as it does on the wall.
Should I deduct wardrobes and large furniture areas?
Only if they are permanent and floor-to-ceiling, like fitted wardrobes: the paint behind them will never be seen, but it also never needs repainting when furniture moves. Most painters do not deduct movable furniture footprints, and the fixed door and window allowances already simplify generously in your favour.
Why does the calculator ask for a surface factor instead of just coverage?
Because the two vary independently: coverage is the product's property from its data sheet, while the factor is your surface's property (porous, textured, glossy, previously painted). Keeping them separate lets you carry one honest product figure across rooms whose surfaces differ, instead of inventing a new coverage number per room.