How the calculation works
A worked example
Walls with a 16 m perimeter and 3 m height cover 48 m². Two coats at 8 m² per litre per coat need 12 litres before waste or opening deductions: four 3.6 L cans.
How much wall area does a paint can or 18 L drum cover?
Pack coverage = litres in the pack × coverage per litre per coat ÷ coats. At an assumed 8 m²/L per coat, a 3.6 L can covers 14.4 m² for two coats and an 18 L pack covers 72 m² for two coats, before waste. This is wall or ceiling surface area, not house floor area.
The table is a calculation example with different capacities, not guaranteed coverage or a list of sizes sold by one brand. With a 10% waste allowance in the order, divide each area by 1.10 to find the area one pack can cover under that allowance. Use the litre capacity on the label; “gallon” and “drum” alone do not establish the quantity.
| Pack capacity | One coat | Two coats |
|---|---|---|
| 3.6 L | 28.8 m² | 14.4 m² |
| 4 L | 32 m² | 16 m² |
| 16.2 L | 129.6 m² | 64.8 m² |
| 18 L | 144 m² | 72 m² |
Using Jotun or another manufacturer’s coverage data
Jotun’s Fenomastic My Home Rich Matt sheet, code 29600 issued 6 June 2023, gives a theoretical 7.3–13.3 m²/L per coat, affected by film thickness, surface and application. This is an example for a particular product and edition; it does not validate 8 m²/L for every paint.
Check the current local sheet for your chosen product and set coverage per coat and coat count separately. To compare estimates, keep area constant and try the coverage values the painter considers suitable for the surface. Primer, filler and topcoat should each retain their own consumption calculation.
Wall paint starts with perimeter and height
To estimate room walls, add length and width, multiply by two, then multiply by wall height. A 4 × 4 m room with 3 m walls has a perimeter of 16 m and a gross wall area of 48 m². The 16 m² floor area describes the rectangular ceiling, not the walls.
Deduct doors and windows that will not receive this paint. If you include the ceiling, add its area only when it uses the same product and colour. Keep doors, woodwork, feature walls and other coatings separate when their material or finish differs. This makes the final can list usable at the shop.
Work from surface area to litres
Using 48 m² of walls, two coats and coverage of 8 m² per litre for each coat, the calculation is 48 × 2 ÷ 8 = 12 litres. At 10% waste it becomes 13.2 litres. Four cans of 3.6 L supply 14.4 litres, so the purchase quantity is larger than the calculated consumption.
The example excludes openings and the ceiling so each step is easy to inspect. It is not a fixed amount for every bedroom. Enter the room's actual dimensions, coats and product coverage rather than adjusting a generic ‘cans per room’ figure that hides those assumptions.
See the difference an opening or ceiling makes
A 0.9 × 2.1 m door removes 1.89 m² and a 1.5 × 1.2 m window removes 1.8 m². The net walls in the example become 44.31 m². With two coats, coverage of 8 and 10% waste, the requirement is 12.18525 litres, shown as about 12.19 litres. It still rounds to four 3.6 L cans.
Adding the 16 m² ceiling raises the surface to 60.31 m² and the requirement to about 16.59 litres under the same assumptions. One 18 L pack covers that calculated quantity. If the ceiling has a different colour or paint system, calculate it as a separate order instead of pooling the litres.
Coverage and coats deserve a closer look
Coverage is entered as square metres covered by one litre in one coat. Check the chosen product's information and how it applies to your surface. Texture, absorption, preparation and application conditions can affect practical consumption. The 8 m²/L starting value is editable and is not a manufacturer guarantee.
For 48 m², two coats and 10% waste, changing coverage from 8 to 6 increases consumption from 13.2 to 17.6 litres. Keeping coverage at 8 but using three coats gives 19.8 litres. These comparisons show the quantity effect; the product system and the painter determine which preparation and coats are appropriate.
Choose packs by litres, even when they are called gallons
The default small pack is 3.6 L and the large pack is 18 L. These are editable supplier pack sizes. A 3.6 L can described as a ‘gallon’ is not a conversion to a US or imperial gallon. Compare products by the capacity printed in litres and update both pack fields as needed.
The mixed-pack result starts with large packs and fills the remainder with small packs. If the small packs would provide at least another large pack's volume, it uses that large pack instead. At 35 litres with these defaults, the result is two 18 L packs. It estimates volume coverage, not the cheapest price combination.
Keep colours separate and preserve the assumptions
When several rooms use exactly the same paint, combine their litres before rounding to cans. Rounding every room independently may leave more unused material. Do not combine quantities across colours, finishes or products just because the packs have the same capacity. Save the product and colour references with the result.
This calculator estimates the selected paint layer only. It does not automatically include primer, filler, sealers or surface repairs. Print or share the measurements and assumptions for the painter to review before buying. Record whether waste and ceilings were included so they are not added again during that review.
Before you place the order
- The 8 m²/L coverage setting is an assumption. Check the product information, surface preparation and a representative trial area.
- Calculate primer, filler and finishing paint separately; their quantities are not interchangeable.
- A supplier's ‘gallon’ pack label is not a universal volume conversion. Read its litre capacity.
- Combine rooms before rounding cans only when they share the same product, colour and finish.
Sources and product references
These references explain the method or a specific product. Match the product and specification to your project before ordering.
- Jotun: Fenomastic My Home Rich Matt technical data, code 29600
Edition dated 6 June 2023, page 2: an example of theoretical spreading rate per coat. Use the current local sheet for your actual pack.
Prepared by the KSA Home Calc team. Content and defaults await professional review before launch. Updated: .