Building and garden · Areas
How much paint for walls or a ceiling?
Quantity is a division: net area times number of coats, divided by spreading rate in square meters per liter. Spreading rate is read on the product sheet and depends on the substrate.
Calculate
How the calculator works
You enter an area in m² (already net of openings if you deducted them), a whole number of coats, and a spreading rate. The default 10 m²/L is an order of magnitude for smooth wall paint, not a constant. New plaster, a primer, a stain, or a facade grab very differently. The result is in liters: round to the pack (quart, gallon, 2.5 L, 10 L) and keep a little leftover for touch-ups. A US gallon is 3.785 L; “350 sq ft/gal” is not 10 m²/L — convert both area and rate into the same system.
Formula and method
litres = (S × n) / r
S in m², n the number of coats, r the spreading rate in m² per liter (one coat).
Rate r is defined for one coat on a substrate described by the maker (often “smooth, non-absorbent”). The sheet may give a range (8–12 m²/L). Take the low end on new plaster, brick, or raw wood; the high end on sound old satin paint. Jobsite losses (tray, roller) are handled by rounding the can, not by inflating n. Windows are subtracted from S before the math: a 1.4 × 1.2 m opening is 1.68 m² less.
Worked example
50 m² of walls, two coats at 10 m²/L
S = 50 m², n = 2, r = 10 m²/L.
- Equivalent one-coat area: 50 × 2 = 100 m².
- Liters: 100 / 10 = 10 L.
- Pack: one 10 L can, or four 2.5 L cans, plus leftover for touch-up if the color is machine-tinted.
10.00 L for two coats at 10 m²/L.
Input notes
- Area to paint
- Sum of vertical surfaces or of the ceiling. Ceiling height × perimeter, minus openings. A ceiling is calculated separately if the product differs.
- Number of coats
- Primer counts as a coat if you quantify it here. Otherwise run a second calculation with the primer’s spreading rate.
- Spreading rate
- Copy m²/L from the sheet, for the thinner and tool stated (roller, sprayer). A sprayer often uses more. Accepted range is 4 to 20 m²/L.
Assumptions and limits
Assumptions
- The typed spreading rate holds for each coat the same way.
- Area is already net, or you accept painting the openings too.
- 10 m²/L is only a default; the product sheet wins.
Limits
- No automatic correction for roughness, porosity, or a bright color over a dark ground.
- Tray, sprayer, and rag losses are not a separate percent.
- A textured ceiling or stucco often leaves the 4–20 m²/L range: the math then refuses the rate.
How to read the result
10 L for 50 m² in two coats is exactly the 10 m²/L default. If the sheet says 8 m²/L on plaster, you go to 12.5 L: the difference is a can. Keep the batch number for touch-ups. For the floor of the same room, the flooring tool computes m² and offcuts, not liters.
Common mistakes
Taking the room’s floor area as the area to paint.
Walls are perimeter × height, minus openings. Floor is for flooring, not wall paint.
Using 10 m²/L when the sheet says 7 or 14.
The default 10 is an order of magnitude. Real spreading rate is that of the can and the substrate.
Forgetting the second coat in n, then running out mid-wall.
n = 2 for a common finish. One coat generally will not cover a contrasting color.
Counting doors and windows in the area.
Deduct them, unless you also paint the sashes with the same product.
Comparison
| Rate (m²/L) | Liters | When to lean this way |
|---|---|---|
| 8 | 12.5 | Absorbent substrate, new plaster, dark color |
| 10 (default) | 10.0 | Smooth wall, sheet “about 10 m²/L” |
| 12 | 8.3 | Old low-porosity paint, high end of the sheet |
Related calculations
Frequently asked questions
Where do I read spreading rate?
On the technical sheet or the label, in m² per liter per coat, for a described substrate.
Makers often give a range. The high end assumes a smooth substrate and careful application. The low end is more honest on a porous ground. If the label is in sq ft per gallon, convert to m²/L before typing r. 350 ft²/gal ≈ 8.6 m²/L.
Is primer calculated the same way?
Yes, with its own spreading rate, often lower.
A plaster primer may list 8 m²/L or less. Do one calculation n = 1 with that r, then a second for the finish. Adding two products into one rate has no meaning.
How do I get wall area?
Interior perimeter × ceiling height, minus the sum of openings.
For a 5 × 4 m room, height 2.50 m: perimeter 18 m, gross 45 m². Two 1.5 m² windows and a 1.8 m² door: 45 − 4.8 = 40.2 m². That is what you type. A ceiling is separate (L × W).
Should I add a waste percent?
Not in the formula. Round up to the next can; that is the usual jobsite leftover.
Unlike tile, paint has no “offcuts.” Losses are the tray, the roller, and touch-ups. An opened can of the same machine tint beats a calculation to the centiliter.
Author and update
Written by Rédaction HexaCalc (editorial team). Content last updated: August 25, 2026. No third-party medical or financial review is claimed.
- Pratiques courantes de calepinage et pertes en travaux (peinture, sols, granulats) — Documents techniques unifiés / fiches fabricants
Same room, the floor
Floor area is ordered in m² with a cutting margin, not in liters.
Calculate floor area to orderCategory: Building and garden