Paint Calculator

Painting Guide

What do I need to know about Paint Coverage Rates?

Coverage varies significantly by paint type and surface. Standard emulsion (vinyl matt, silk): 10–14 m² per litre. Premium emulsion (Farrow & Ball, Little Greene): 10–12 m² per litre. Masonry paint: 5–8 m² per litre. Gloss or satin (woodwork): 14–18 m² per litre. Always check the tin — the stated coverage assumes smooth, sealed surfaces. New plaster, rough or porous surfaces, and highly absorbent substrates will reduce coverage by 30–50%. Use a good primer coat on new plaster and first-time painted surfaces, since it seals the surface and reduces how much of the finish coat gets absorbed — skipping the primer on new plaster can mean needing an extra full coat of the more expensive finish paint to achieve even coverage.

How Many Coats?

One coat: only suitable for touch-ups, re-painting the exact same colour, or if using a paint-and-primer combination on a well-prepared surface. Two coats: standard for most painting projects — gives even coverage and better depth of colour. Three coats: needed for dark colours over light (especially with deep colours like red, dark blue, and black which have poor opacity), new plaster (one coat to seal plus two finish coats), and highly absorbent surfaces. Using cheap paint and applying three coats rarely matches the result of two coats of a quality paint with good pigment density — spend on the paint before spending on extra coats. Always allow the recommended drying time between coats (check the tin; typically 2-4 hours for emulsion, longer for oil-based products) so each layer cures properly rather than lifting the one beneath it.

What do I need to know about Preparation is the Work?

Professional decorators spend 60–70% of their time on preparation — filling, sanding, cleaning, taping. The most common DIY painting failure is inadequate prep. Fill all holes and cracks with Polyfilla, sand smooth when dry, and prime the filled areas. Wash walls with sugar soap to remove grease, nicotine, and general grime — paint will not adhere properly to dirty surfaces. Sand gloss woodwork with 120-grit before repainting — gloss needs a key to accept new paint. Tape edges with low-tack painter's tape rather than standard masking tape, which can pull off fresh plaster or lift previous paint layers when removed. Press the tape down firmly along its edge with a scraper or old card to stop paint bleeding underneath, and remove it while the final coat is still slightly tacky for the cleanest line.

What do I need to know about Paint Finishes for Different Surfaces?

Matt: ceilings and walls in low-traffic areas — hides imperfections well but not washable. Eggshell or washable matt: the modern compromise — looks close to matt but can be wiped clean. Suitable for most walls. Silk: more sheen, highly washable, shows wall imperfections more. Good for kitchens, bathrooms, hallways. Soft sheen: between eggshell and silk. Satinwood: for woodwork (doors, skirting, window frames). Gloss: traditional high-sheen woodwork finish. Water-based (acrylic) paints have largely replaced oil-based (solvent) paints for woodwork in recent years — they dry faster, have far less odour, don't yellow over time, and are easier to clean up, though traditionalists still favour oil-based eggshell for a slightly harder, more durable finish on high-traffic woodwork like skirting and door frames.

What's the key thing to understand about Why Paint Coverage Estimates Vary and How to Avoid Running Short?

The coverage figure printed on a paint tin (typically expressed as square metres per litre) represents the manufacturer's estimate under reasonably ideal, controlled conditions, and real-world coverage is often somewhat lower for several common reasons. Surface texture and porosity significantly affect how much paint a surface absorbs: a textured, previously unpainted, or particularly porous surface (bare plaster, for example) soaks up noticeably more paint than a smooth, previously painted surface in good condition, sometimes requiring a dedicated primer or mist coat specifically to seal the surface before the finish coats are applied at their expected coverage rate. Colour change also matters considerably: painting a dramatically different colour over an existing one — particularly a dark colour over a light one, or vice versa — typically requires more coats to achieve full, even coverage without the old colour showing through than painting a similar shade over itself, which is a common and easily underestimated reason a paint job uses noticeably more paint than a simple room-area calculation based on tin coverage figures alone would suggest. Application method affects coverage too, since rollers, brushes, and sprayers all apply paint at somewhat different thicknesses and with different amounts of wastage, meaning the same nominal coverage figure can produce different real-world results depending on how the paint is actually applied. Buying a small margin of extra paint beyond a straight room-area calculation, and specifically checking whether a colour change or surface condition is likely to need an extra coat, avoids the common frustration of running short partway through a job and needing to source more paint that may not perfectly colour-match the original batch.

What should I know about Primer, Undercoat, and Why Skipping Prep Coats Costs More Overall?

Skipping primer or undercoat to save on initial cost is one of the most common paint-job mistakes, and it frequently ends up costing more overall rather than less, since bare or previously unpainted surfaces, surfaces with stains or marks, and surfaces undergoing a significant colour change generally need a properly formulated prep coat to achieve a good, durable, even finish with the expected number of topcoat applications — without it, achieving full, even coverage often requires considerably more coats of the more expensive finish paint than would otherwise have been needed, since finish paints generally aren't formulated to seal and even out an unprepared surface the way a primer or undercoat specifically is. Primer specifically seals porous surfaces and provides a consistent surface for the topcoat to adhere to evenly, while undercoat (used mainly with oil-based or some specialist paint systems) provides an opaque base coat, often in a colour closer to the final finish, that reduces the number of topcoats needed to achieve full, even coverage and helps block staining or show-through from the surface beneath. For surfaces in good condition, already painted a similar colour, and not significantly porous or stained, a prep coat may genuinely not be necessary, and it's worth assessing the specific surface honestly rather than defaulting to always adding an unnecessary prep coat — but for any surface with notable colour change, staining, bare patches, or porosity, including the correct prep coat in the overall paint budget from the outset, rather than discovering partway through that coverage is falling short without one, tends to produce a better finish for a lower total cost than skipping it and buying extra topcoat paint to compensate.

Paint Coverage by Room Size Reference Table

As a quick sanity check against the calculator's precise result, typical UK rooms with standard 2.4m ceilings need roughly the following amount of emulsion for two coats, assuming average coverage of around 12m² per litre per coat and allowing for doors and windows.

Worked Example: Paint Needed for a 4m x 4.5m Room With 2.4m Ceilings The perimeter of this room is 2 x (4 + 4.5) = 17m, so the gross wall area at a 2.4m ceiling height is 17 x 2.4 = 40.8m². Subtracting a standard door (1.8m²) and two windows (1.2m² each, 2.4m² total) gives a net paintable area of 40.8 - 1.8 - 2.4 = 36.6m². At a coverage rate of 12m² per litre per coat, one coat requires 36.6 ÷ 12 ≈ 3.05 litres, so two coats need roughly 6.1 litres. In practice, this means buying either three 2.5-litre tins (7.5 litres total) or two 2.5-litre tins plus a 1-litre tin, which also leaves a small amount spare for cutting in around sockets and skirting boards after the main coats are done.

Dark and Saturated Colours Often Need an Extra Coat

The coverage rates used in standard paint calculations generally assume a mid-tone colour being applied over a similar existing colour. Deep, saturated colours — dark navy, black, strong reds — frequently need a third coat to achieve full, even coverage without the original wall colour showing through in patches, particularly when painting over a much lighter existing colour. Many paint manufacturers now sell a tinted undercoat or a "one-coat" formulation specifically designed to reduce this problem, which can be worth the modest extra cost if you're making a dramatic colour change, since buying two extra tins of a dark emulsion to get a third full coat later is usually more expensive than one tin of the correct undercoat first.