Wall & Ceiling
Paint Calculator
Calculates litres or gallons of paint for walls, ceilings, and trim
Updated 13 May 2026 · Live
What this tool does
Calculates litres or gallons of paint for walls, ceilings, and trim.
Formula Used
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How the paint calculator works
This calculator estimates how much paint a job needs for walls, a ceiling or trim. It multiplies the area to cover by the number of coats, divides by the paint’s coverage per litre, and converts the litres into whole tins of the size you enter, with an indicative cost. Coverage is the manufacturer’s figure — commonly around 12 m² per litre for emulsion on a smooth, sealed surface, and rather less for gloss or masonry paints.
Coats and coverage
Two coats is the usual default over an existing similar colour. Bare plaster takes a thinned mist coat first; a strong colour change, a dark-to-light switch, or a porous surface can need an extra coat. Coverage varies widely by product — woodwork primers, undercoats and gloss cover far less per litre than wall emulsion — so the coverage from the specific tin gives the truest result.
Working out the area
Wall area is the height multiplied by the perimeter of the walls being painted; a ceiling is its length times width; trim is estimated from its run and an assumed band width. Door and window openings can be deducted for a tighter figure, though many decorators leave small openings in and rely on the part-tin left over.
What this tool does not do
It does not size primer or undercoat separately from the topcoat, split a job across different paint types, or confirm whether a stain block is needed. It does not replace a professional quote or factor regional pricing. The product data sheet gives coverage, recoat times and any priming required.
On-site considerations
Paint is sold in fixed tin sizes, so the calculator rounds litres up to whole tins; the surplus is handy for touch-ups. Mixing from a single batch keeps large walls even in colour, and cutting in by hand around edges and sockets uses slightly more than the flat coverage figure implies.
Adjusting the defaults
Every input here is editable. Enter your own area, number of coats, coverage, tin size and price, and the litres, tins and cost recalculate instantly. If a default does not match your paint or surface, your own numbers always take precedence.
Using this paint calculator alongside other BuildMetricLab tools
This calculator works best as part of a wider plan. Pair the paint quantity with our project contingency and labour-cost calculators to build a complete estimate before contacting a supplier. All BuildMetricLab tools run entirely in your browser — no sign-up, no data sent anywhere, and every formula is shown on-page so you can audit the maths.
Sources & methodology
Calculates litres or gallons of paint for walls, ceilings, and trim. Every result is calculated from the values you enter, and all inputs are editable.
Frequently asked questions
Are paint calculator results accurate enough to order materials?
Use them as a starting estimate only. Verifying the final quantity with your supplier or contractor before ordering is good practice — site conditions, wastage and cut-offs all affect the true figure.
How many coats should I allow?
Two coats cover most repaints in a similar colour. Bare plaster needs a mist coat first, and a strong colour change or a porous surface can need a third. Woodwork systems often run primer, undercoat and topcoat. Changing the coats input scales the litres and tins in proportion.
Does this replace professional advice?
No. This tool is a planning estimator. For works that affect structure, Building Regulations, Party Wall, gas, electrics, drainage to a sewer, or similar, consult a suitably qualified professional.
Can I change the unit prices?
Yes — every price field is editable. Plug in your supplier's quote to get a total that matches your project.
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