Understanding coverage rates
Coverage rate is the single most important variable in this calculation, and it is always printed on the product's data sheet or tin label. The values below are typical ranges reported across manufacturer data sheets for common paint types on a primed, smooth surface.
| Paint type / surface | Typical coverage (one coat) |
|---|---|
| Interior emulsion/latex, smooth primed wall | 10 – 12 m²/L |
| Exterior masonry paint, primed | 6 – 10 m²/L |
| Textured or unprimed masonry | 4 – 8 m²/L |
| Gloss/satin woodwork paint | 12 – 16 m²/L |
- Coverage figures are industry-typical and vary by brand, sheen, tint base and surface porosity — always confirm the value on the specific product's data sheet before ordering paint for a large job.
- Rough, textured or unprimed surfaces absorb more paint and reduce effective coverage, sometimes by 30% or more compared with a smooth primed wall.
- This calculator estimates material quantity only; it does not address surface preparation, primer needs, or drying/recoat times, which also affect the final result.
What does a paint calculator do?
A paint calculator converts a room or wall's surface area into a volume of paint, accounting for the number of coats applied and the manufacturer's stated coverage rate for the specific product. Coverage is normally expressed as square meters per liter (m²/L) or square feet per gallon, and it varies by paint type, surface porosity and application method — flat wall paint on primed drywall covers more area per liter than a textured or unprimed masonry surface.
This calculator first subtracts the area of doors, windows and other openings from the total wall area to get the paintable area, since those surfaces are not painted. It then multiplies the paintable area by the number of coats (most interior walls need two coats for even color and hide) and divides by the coverage rate to get the total liters required.
How to use this paint calculator
- Enter the total wall area in square meters (height × width, summed for all walls being painted).
- Enter the combined area of doors, windows and other unpainted openings — this is subtracted from the total.
- Enter the number of coats. Two coats is standard for most interior walls; a single coat may suffice with a tinted primer or when repainting a similar color.
- Enter the coverage rate from the paint tin or manufacturer's data sheet (default 11 m²/L is a typical mid-range value for interior emulsion — always check your specific product).
- Read the liters needed and the number of 2.5 L or 5 L cans to buy, rounded up to the next whole can.
The formula behind paint quantity
Paintable area is the total wall area minus openings. Liters needed equals paintable area multiplied by the number of coats, divided by the coverage rate per liter. Can quantities are rounded up (ceiling), never down, so the estimate always covers the job.
Worked example: a wall area of 40 m² with 4 m² of doors and windows gives a paintable area of 36 m². At 2 coats and a coverage rate of 11 m²/L, the paint needed is (36 × 2) ÷ 11 ≈ 6.5 L. That rounds up to three 2.5 L cans (7.5 L) or two 5 L cans (10 L) — buying the larger cans wastes less on this job size.
Common mistakes
- Forgetting to subtract doors and windows, which overstates the area that actually needs paint.
- Using the coverage rate for one coat as if it applied to the total across all coats.
- Assuming the same coverage rate for a textured or unprimed wall as for a smooth primed one.
- Rounding down to save money on cans, which risks running short mid-coat with a visible color-match seam.
- Ignoring that a second coat in a different sheen (e.g., matte over gloss) may need more paint to achieve full hide.
Domande frequenti
How much paint do I need for a 40 m² wall?
For a 40 m² wall with 4 m² of doors and windows, the paintable area is 36 m². At 2 coats and a typical coverage rate of 11 m²/L, you need (36 × 2) ÷ 11 ≈ 6.5 liters — round up to three 2.5 L cans or two 5 L cans.
How many coats of paint do I need?
Two coats is standard for most interior walls to achieve even color and full hide, especially when changing color or covering a stain. A single coat can work when repainting a similar shade over a sound existing coat, or when using a tinted primer first.
What is a typical paint coverage rate?
Interior wall emulsion on a smooth, primed surface typically covers 10–12 m² per liter, according to manufacturer data sheets. Textured, porous or unprimed surfaces reduce this significantly, sometimes to 4–8 m² per liter, so always check the specific product's label.
Should I round up or down when buying paint cans?
Always round up. Running short mid-job risks a visible seam where a new batch or can doesn't perfectly color-match the previous one, and most retailers will not accept opened cans for return.
Does this calculator account for primer?
No — this calculator estimates finish-coat paint quantity only. If the surface needs priming (new drywall, a major color change, or a stained area), calculate primer separately using the primer product's own coverage rate.
Fonti
- Typical coverage rates are drawn from manufacturer paint data sheets published by major interior/exterior paint brands (e.g., coverage stated in m²/L or ft²/gal on product labels).
- Painting and Decorating Contractors of America (PDCA) — general surface preparation and application guidance for coating estimation.