To buy the right amount of paint you need three numbers: the area to paint, the number of coats and the paint’s spreading rate, which is printed on its technical data sheet. This guide walks through the sum our calculator does, using a bedroom and a bathroom as examples, and shows why the same bedroom can need 6 or 8.5 litres depending on the paint and the wall.
Updated on 3 November 2026. General information, not a substitute for an assessment by a professional.
The essentials
- Litres = area (m²) × number of coats ÷ spreading rate (m² per litre).
- The spreading rate is on each paint’s data sheet and is given per coat. On the sheets we read it ranges from 8 to 16 m² per litre.
- Wall area is the room’s perimeter times the ceiling height, minus doors and windows. The ceiling is length × width.
- The sheets we read ask for 2 to 3 coats.
- Rough walls, new plaster or a strong colour change use more paint. If unsure, do the sum with the lowest spreading rate.
- Primer is a separate tin with its own spreading rate. Count it separately.
1. Measure the room
Measure the length (L), width (W) and ceiling height (H) in metres. The wall sum is:
Walls = 2 × (L + W) × H − doors and windows
The calculator subtracts 1.2 × 1.2 m (1.44 m²) for each window and 0.8 × 2.0 m (1.6 m²) for each door. These are typical sizes, not standard ones. If you have a balcony door or a very large window, measure it and subtract the real figure.
The ceiling adds L × W. If the bathroom or kitchen is tiled, count only the part of the wall you will paint.
2. Read the spreading rate on the data sheet
Each maker gives a range, because consumption changes with the wall. CIN and Barbot both say the rate depends on the surface and on application conditions. These are the figures on the sheets we read:
| Paint (maker) | Spreading rate per coat | Coats stated |
|---|---|---|
| Vinylmatt (CIN), matt | 11 to 15 m²/L | 2 to 3 |
| Barbomat (Barbot), matt | 10 to 14 m²/L | 2 to 3 |
| Barbosilk (Barbot), satin | 12 to 16 m²/L | 2 to 3 |
| Cozinhas e Casas de Banho (Barbot), matt kitchen and bathroom paint | 8 to 12 m²/L | 3 |
| Acrilvip Mate (Robbialac) | 10 to 12 m²/L | 1 thinned + 1 |
| Robbiotel (Robbialac) | 12 to 14 m²/L | not stated in the leaflet |
| Dyrustar (Dyrup), matt | 12 to 16 m²/L on smooth surfaces, 8 to 12 on rough | 2 to 3 |
The Dyrustar sheet shows the effect of texture clearly: the same paint covers nearly 30% less on a rough wall. The Vinylmatt sheet gives another clue: its opacity class was measured at a practical rate of 6.5 m²/L for two coats, which is about 13 m² per litre per coat.
3. The sum, step by step: a bedroom
A bedroom of 3.4 × 3.0 m, 2.6 m ceiling height, one window, one door, ceiling included. Two coats of Barbomat.
- Perimeter: 2 × (3.4 + 3.0) = 12.8 m.
- Walls before deductions: 12.8 × 2.6 = 33.28 m².
- Openings: 1.44 + 1.60 = 3.04 m². Walls to paint: 30.24 m².
- Ceiling: 3.4 × 3.0 = 10.2 m². Total: 40.44 m².
- Two coats: 40.44 × 2 = 80.88 m² of painting.
- At the sheet’s lowest rate, 10 m²/L: 80.88 ÷ 10 = 8.09 L. The calculator always rounds up to the next half litre: 8.5 L.
- At the highest rate, 14 m²/L: 80.88 ÷ 14 = 5.78 L, rounded to 6 L.
The same room, with the same paint, can need anything from 6 to 8.5 litres. Barbomat comes in 1, 5 and 15 L tins, so 8.5 L means combining tins. Compare the price per litre of each size in the shop.
4. Another example: a bathroom
A bathroom of 2.2 × 1.8 m, 2.6 m ceiling height, one door, no window, ceiling included. Three coats of Barbot’s kitchen and bathroom paint, rated at 8 to 12 m²/L.
- Walls: 2 × (2.2 + 1.8) × 2.6 = 20.8 m², minus the door (1.6 m²) = 19.2 m².
- Ceiling: 2.2 × 1.8 = 3.96 m². Total: 23.16 m².
- Three coats: 69.48 m².
- At 8 m²/L: 8.69 L, so 9 L. At 12 m²/L: 5.79 L, so 6 L.
If the walls are tiled up to the ceiling, only the ceiling is left: 3.96 × 3 ÷ 8 = 1.49 L, rounded to 1.5 L. To choose the paint for this room, see which paint for each room.
5. What makes you use more
- Rough or porous walls. Use the low end of the range, as the Dyrustar sheet shows.
- A colour change. Robbialac recommends an opacity product (VipOpac) when the base colour is uneven or when repainting over medium and strong colours. Barbot names its own primer (Barboplac) to hide a dark colour. Allow for that product or for an extra coat.
- New or powdery surfaces. They need primer before the topcoat (section 6).
6. Primer: when and how much
Primer prepares the wall and evens out absorption. The data sheet for Barbot’s Barbolite Hydro primer says it evens out the surface’s absorption, which lets the topcoat go further. It covers 10 to 15 m²/L in a single coat. For the example bedroom (40.44 m²) that is between 3 L (at 15 m²/L) and 4.5 L (at 10 m²/L).
When the sheets call for it:
- New surfaces of render, plaster, cement or concrete (Barbot); powdery or alkaline surfaces and plasterboard, each with its own primer (CIN).
- Old paint in good condition: Barbot only asks for primer if the film is very glossy or glazed.
- Dry stains from damp, nicotine or smoke: CIN names a specific primer, once the cause is fixed. See damp walls.
- Mould: no paint solves it alone. Clean it and deal with the cause first, as the mould on walls guide explains.
7. How long to wait between coats
| Paint | Touch dry | Recoat |
|---|---|---|
| Vinylmatt (CIN), at 20 °C and 60% humidity | about 30 min | 3 to 4 h |
| Barbomat (Barbot) | not stated | 4 h between coats |
| Barbosilk (Barbot), at 23 °C and 55% | 1 h | 2 h |
| Kitchen and bathroom paint (Barbot), at 23 °C and 55% | 1 h | 4 h |
| Acrilvip Mate (Robbialac), at about 20 °C | 1 h | 4 to 6 h |
| Robbiotel (Robbialac) | not stated | 3 to 4 h |
| Dyrustar (Dyrup) | 30 min | 4 h (fully dry in about 12 h) |
CIN and Barbot ask for temperatures above 5 °C and relative humidity below 80%. Dyrup advises against painting below 10 °C. The times in the table apply to the stated conditions; in cold, damp weather, expect longer.
If you are asking for quotes, bring the area you worked out. See how to compare painting quotes and what interior painting costs.
What we could not confirm
- The spreading rate on your wall. Only a test shows it. Dyrup itself recommends trying the product first.
- Whether the sheets’ rates count the thinned paint. CIN thins the first coat by 10% with water and Dyrup by 15 to 20%, but none of the sheets we read says whether the rate includes that water.
- The current Dyrustar sheet. We read the December 2017 edition, from a reseller’s copy. A newer version may exist on dyrup.pt.
- The full Robbiotel data sheet. We only read the maker’s leaflet, which has no printed date.
- The revision of the kitchen and bathroom sheet. The file name says “Rev. 5”, the document says “Rev. 08”, dated 29/11/2024.
- Door and window sizes. They are the calculator’s typical sizes, not from a standard.
- Tin prices. We do not publish them.
Sources (read on 3 November 2026)
- CIN, Boletim Técnico Vinylmatt 10-250 (technical bulletin, in Portuguese), July 2026 revision (EN 13300 classification, surface preparation, application, spreading rate, drying).
- Barbot, Ficha Técnica 1302 Barbomat, Rev. 15, 12/11/2024 (spreading rate, coats, interval, preparation).
- Barbot, Ficha Técnica 1200 Barbosilk, Rev. 13, 15/11/2024 (spreading rate, coats, drying).
- Barbot, Ficha Técnica 1601 Cozinhas e Casas de Banho, Rev. 08, 29/11/2024 (spreading rate, 3 coats, drying, dark colour preparation).
- Barbot, Ficha Técnica Primário Barbolite Hydro, Rev. 03, 16/12/2024 (spreading rate, single coat, when to apply).
- Robbialac, Informação Técnico-Comercial Acrilvip Mate, CC-VIP 120 21.1, 24/06/2021 (spreading rate, coats, drying, VipOpac).
- Robbialac, Robbiotel leaflet, no printed date (spreading rate, time between coats).
- PPG Dyrup, Informação Técnico/Comercial Dyrustar, ref. 5555, ITC no. 839, December 2017, copy on the Live Place reseller site (smooth and rough spreading rates, drying, thinning, application conditions).