Paint Calculator: How Much Paint Do You Need?

Buying paint should be simple—until you're halfway through the second coat and the last can runs dry. Or you overbuy and end up storing half-used buckets for years.

The good news? Paint is predictable when you calculate it properly. In this guide you’ll learn the exact formula for walls and ceilings, how coverage rates really work, how many coats you should plan for, and how to add a smart safety margin.

The Basics: Coverage, Coats and What Counts as "Area"

Paint calculations are based on three numbers:

• Paintable area: total wall area (and optionally ceiling) minus large openings
• Coverage rate: how much area one liter/gallon covers (printed on the tin)
• Number of coats: usually 2 for walls, sometimes more depending on color and surface

Typical coverage rates

• Walls (interior): often around 8–12 m² per liter (or ~325–450 ft² per gallon)
• Ceilings: often similar, but porous ceilings may absorb more
• Primer: coverage varies widely depending on surface

Step 1 – Measure Walls (and Ceiling) Correctly

For most rooms, you can calculate wall area using room perimeter and wall height.

Wall area (metric)

Wall area (m²) = (perimeter in m × wall height in m) − openings

Example: A rectangular room 5.0 m × 4.0 m with 2.4 m wall height

• Perimeter = 2 × (5.0 + 4.0) = 18.0 m
• Wall area = 18.0 × 2.4 = 43.2 m²

Wall area (imperial)

Wall area (ft²) = (perimeter in ft × wall height in ft) − openings

Example: A room 16 ft × 13 ft with 8 ft wall height

• Perimeter = 2 × (16 + 13) = 58 ft
• Wall area = 58 × 8 = 464 ft²

Ceiling area (optional)

If you are painting the ceiling, add:

Ceiling area = room length × room width

Step 2 – Use the Paint Formula

Once you know the total paintable area, calculation is straightforward.

Core formula

Paint needed = (area × coats) ÷ coverage

Where coverage is m² per liter or ft² per gallon, depending on your units.

Example (metric)

• Wall area = 43.2 m²
• Coats = 2
• Coverage = 10 m² per liter

Paint = (43.2 × 2) ÷ 10 = 8.64 liters

Round up to practical can sizes (for example 10 L total), and consider a small safety margin.

Example (imperial)

• Wall area = 464 ft²
• Coats = 2
• Coverage = 400 ft² per gallon

Paint = (464 × 2) ÷ 400 = 2.32 gallons

Step 3 – Choose the Right Number of Coats

The biggest variable in paint planning is coats. Here are practical defaults that work in most projects:

• 2 coats: the standard for interior walls on a similar color
• 3 coats: strong color change (dark → light), patchy walls, or cheaper paints
• 1 coat: only for touch-ups, or when the system is designed for it (and the surface is already uniform)

When primer changes the math

Primer often makes your topcoat coverage more consistent and can reduce the need for a third coat. However, primer itself is still paint you must budget for.

Common Mistakes That Make You Run Short

1. Using floor area instead of wall area

Paint is about vertical surface area, not floor space. A 20 m² room can easily have 40–50 m² of wall area depending on height.

2. Ignoring texture and porosity

Rough walls, textured ceilings, brick and previously unsealed surfaces all reduce coverage. If you’re unsure, assume lower coverage and add extra.

3. Forgetting the second (or third) coat

Many paint calculators look “too high” because DIYers think in one coat. In reality, two coats is the baseline for durable, even results.

4. Not rounding up to real can sizes

Shops sell standard sizes (1 L, 2.5 L, 5 L, 10 L, 1 gal, 5 gal, etc.). Always round up to the next practical combination—and keep a little extra for touch-ups.

Conclusion

Calculating paint accurately comes down to a repeatable process:

• Measure your wall area using perimeter × height (and add ceiling if needed).
• Multiply by the number of coats.
• Divide by the coverage rate on your paint.
• Round up to real can sizes and add a small safety margin.

Ready to get a precise number for your project? Use our paint calculator to get instant results and reduce waste.