Everyday Life · Practical Life · Home & DIY
Paint Coverage Calculator
Calculates the amount of paint required to cover a given wall or surface area, accounting for the number of coats and paint coverage rate.
Calculator
Formula
V is the total volume of paint required (in litres). A is the total surface area to be painted (in square metres). C is the number of coats to be applied. R is the coverage rate of the paint (in square metres per litre), typically found on the paint tin label.
Source: Standard trade painting practice; coverage rates per BS EN ISO 2814 (hiding power and spreading rate of paints and varnishes).
How it works
Every tin of paint carries a coverage rate — usually expressed in square metres per litre (m²/L) — which describes how far one litre of that product will spread at the recommended film thickness. This figure varies widely: thin emulsions and primers typically cover 10–14 m² per litre, while thick masonry paints or specialist coatings may cover only 4–8 m² per litre. The coverage rate is measured under laboratory conditions, so real-world results can differ depending on surface texture, application method, and how generously the paint is applied.
The core formula is straightforward: multiply the total surface area (in m²) by the number of coats you intend to apply, then divide by the paint's coverage rate. This gives you the theoretical minimum volume in litres. For example, if you are painting 30 m² with two coats of a paint that covers 12 m²/L, you need (30 × 2) ÷ 12 = 5 litres. The calculator also outputs a second figure with a 10% waste allowance added, which accounts for paint left in rollers, spillage, touch-ups, and the inherent variability of real surfaces.
In practice, most professional decorators add between 10% and 15% to their calculated requirement. Rough or porous surfaces such as bare brick, new plaster, or heavily textured render will absorb considerably more paint than smooth, previously painted walls. For these surfaces, applying a diluted mist coat first and then adjusting the coverage rate downward — to perhaps 8 m²/L — will give a more accurate estimate. The calculator is equally useful for ceilings, doors, skirting boards, fences, and garden furniture; simply enter the correct surface area and the appropriate coverage rate for the specific product you are using.
Worked example
Suppose you want to paint a medium-sized bedroom. The room has four walls, each measuring 4 metres wide by 2.5 metres high, giving a total raw wall area of 4 × 4 × 2.5 = 40 m². You subtract the door (2 m²) and two windows (1.5 m² each), leaving a net paintable area of 40 − 2 − 3 = 35 m².
You have chosen a mid-sheen emulsion with a coverage rate of 12 m² per litre and plan to apply two coats. Entering these values: V = (35 × 2) ÷ 12 = 5.83 litres. Adding the 10% waste allowance gives 6.42 litres. You would therefore purchase two 2.5-litre tins (5 litres) and one 1-litre tester pot, or simply buy a 7.5-litre trade tub to have a comfortable surplus for touch-ups.
If the same room had textured artex ceilings on the walls, a more conservative coverage of 8 m²/L would be appropriate: V = (35 × 2) ÷ 8 = 8.75 litres, rising to 9.63 litres with the waste factor — nearly 40% more paint than the smooth-wall estimate.
Limitations & notes
This calculator assumes a uniform, continuous surface at the coverage rate printed on the tin. Real-world coverage varies significantly with surface porosity (new plaster absorbs far more than previously painted walls), application method (spray application wastes more than rolling), and colour changes (covering dark colours with light ones almost always requires an extra coat). The 10% waste buffer is a common industry rule of thumb; for very small areas, spray application, or heavily patterned surfaces, a 15–20% buffer may be more realistic. The formula also does not account for different products used across separate coats — for instance, a primer coat at 8 m²/L followed by a topcoat at 13 m²/L should be calculated separately for each product. Always cross-reference your result with the specific product datasheet, as some specialty coatings (fire-retardant paints, anti-mould treatments, floor paints) have coverage rates outside the typical 10–14 m²/L range.
Frequently asked questions
What is a typical paint coverage rate per litre?
Most standard interior emulsions cover between 10 and 14 square metres per litre when applied to a smooth, previously painted surface. Masonry and exterior paints typically cover 4–8 m²/L due to their thicker consistency, while gloss and satinwood paints for woodwork often cover 12–16 m²/L. Always check the specific coverage stated on the tin you intend to use, as it varies significantly by product and brand.
How many coats of paint do I normally need?
For most interior repaints in a similar colour, two coats is the industry standard and gives full, even coverage. If you are making a dramatic colour change — particularly going from a dark colour to a light one — three coats may be necessary. New plaster should always receive a diluted mist coat first (1 part water to 4 parts emulsion) before two full coats, so budget for three applications in total.
Should I include windows and doors in my surface area measurement?
No — you should subtract the area of windows, doors, and any other features you are not painting. A standard internal door is approximately 2 m², and a typical window is 1–1.5 m². Deducting these from your total wall area gives a more accurate and economical paint estimate. For complex room shapes with alcoves or chimney breasts, measure each section individually and sum the areas.
Why does the calculator add a 10% waste allowance?
No painting job uses exactly the theoretical minimum volume of paint. Paint is left coating the inside of the tin, absorbed into roller sleeves, and used for touch-ups after the main work is finished. Brush strokes and roller application are less efficient than the flat-surface test conditions used to determine the coverage rate. A 10% buffer is the standard trade allowance for smooth interior surfaces; for textured surfaces or spray application, 15–20% is more appropriate.
Can I use this calculator for exterior painting?
Yes, but use a lower coverage rate. Exterior masonry paint typically covers 4–8 m²/L due to the rougher, more absorbent nature of brick and render. Bare or weathered surfaces will absorb considerably more than surfaces in good condition. For exterior timber such as fencing or decking, use the coverage rate specific to the exterior wood paint or decking oil you have chosen, and factor in an extra coat if the timber is new or untreated.
Last updated: 2025-01-15 · Formula verified against primary sources.