How BricoDato checks the estimate
The calculation starts with wall area, subtracts openings when appropriate, applies coats and coverage, and keeps waste explicit instead of hiding it in the measurements. Always prefer the coverage stated on the actual paint data sheet.
What you need to know
- Calculate wall area as perimeter × height and subtract openings you will not paint.
- Add the ceiling separately if you are painting it too.
- Use the coverage in the product data sheet and account for the number of coats and touch-ups.
Run the calculation with your measurements
Enter room dimensions, openings, coats and your paint’s actual coverage.
Calculate wall area
For a rectangular room, calculate the perimeter: 2 × (width + length). Multiply it by the height and subtract the area of doors and windows you will not paint.
Add the ceiling if you are painting it too
Ceiling area is width × length. Add it once to the area for each coat.
Apply the number of coats and coverage
Litres are estimated with: area × coats ÷ coverage in m²/L. Use the coverage from the technical data sheet, not a generic value if you already know the product.
Full example
Room size 4 × 5 m, height 2,5 m, with 4 m² of doors and windows. The perimeter is 18 m; gross wall area is 18 × 2.5 = 45 m². After subtracting openings: 41 m². For 2 coats and coverage of 10 m²/L: 41 × 2 ÷ 10 = 8.2 L. With a 10% margin: 9.02 L. In practice, buy a pack size that covers at least that amount.
Common mistakes
- Using floor area as if it were wall area.
- Forgetting the second coat.
- Subtracting too many small openings and ending up short.
- Using a generic coverage figure when the paint specifies another.
- Ignoring very porous walls, major colour changes or primer.
- Rounding down because it “almost reaches”.
Before you buy
Check batch numbers if you need several containers, keep a small reserve for touch-ups, and calculate primer separately because it does not always have the same coverage.
Final check
Before accepting the result, review these points to catch measurement, interpretation or safety errors before buying materials or doing the work.
- Use technical-data-sheet coverage
- Calculate primer and finish separately
- Count real coats and colour changes
- Round to a commercial pack size without running short
Make a small job sheet before buying
Record calculated area, substrate condition, chosen product, stated coverage, number of coats, pack size and colour reference. This avoids rebuilding the calculation if you return to the shop or need a touch-up months later.
Also do a small test before committing the whole room. It lets you check adhesion, coverage, sheen and colour under the real lighting. If the wall absorbs unevenly or defects appear, correct the preparation before using the finish coat.
Turn the guide into a decision
Use your measurements, the product data sheet and the real conditions in your home. If a decision affects safety, fixed installations or a fault you cannot identify, consult a professional.
Related guides
A room estimate you can recalculate
A paint estimate starts with the wall surface, not the floor area. Measure every wall and subtract large openings only if they will not be painted. Keep ceiling paint separate because its coverage and colour may differ from the walls.
Example to adapt
For a 4 m × 3 m room with walls 2.5 m high, perimeter is 14 m and gross wall area is 35 m². Subtract a 2 m² door and 3 m² of windows: 30 m² remain. Two coats at a hypothetical 10 m² per litre per coat require 6 litres before any allowance. If you add 10% for losses, plan for about 6.6 litres, then select available tin sizes.
| Input | Example | Why it matters |
|---|---|---|
| Coats | 2 | A second coat doubles the coverage demand |
| Coverage | 10 m²/L per coat | Use the technical sheet, not the headline tin size |
| Ceiling | 12 m² separately | Calculate this with its own paint type and coat count |
Checks before acting
- Check whether primer or stain-blocking treatment is needed before the finishing coats.
- Allow for rough, porous or previously dark surfaces; a test patch is more informative than a universal coverage claim.
- Compare total project cost per coated square metre, including primer and extra coats.
- Keep leftover paint in a sealed labelled container for future touch-ups.
Keep a useful record: Record measured wall and ceiling areas, opening deductions, number of coats, stated product coverage and tin sizes.