Paint Needed Calculator: How Much Paint Do I Need?

This calculator works out how much paint you need for a room or wall by taking your total paintable area, the number of coats, and the paint’s coverage per litre, then returning the litres required. You enter the wall dimensions (or total area), select how many coats you plan to apply, and input the coverage rate found on your paint tin; the calculator gives you the exact litres to buy, plus a helpful reminder to round up. Below, we explain the simple formula behind it, walk through a worked example, and cover the practical details that make the difference between a smooth project and a frustrating trip back to the store.

Formula

The core formula is straightforward:

Paint needed (L) = (Wall length × Wall height × Number of coats) ÷ Coverage per litre

In symbols: P = (L × H × C) / R

• L = length of the wall in metres (m)
• H = height of the wall in metres (m)
• C = number of coats (unitless, typically 1 or 2)
• R = coverage rate in square metres per litre (m²/L), printed on the paint can

If you are painting multiple walls, simply add their lengths together, or enter the total area directly by using a length of 1 and setting the height to the total area (though it’s easier to sum the lengths first). For ceilings, treat them as a wall with length = room length and height = room width.

Worked Example

Using the default values from the calculator above:

• Wall length = 6 m
• Wall height = 2.5 m
• Coats = 2
• Coverage = 10 m²/L

First, calculate the area for one coat: 6 m × 2.5 m = 15 m².

Then multiply by the number of coats: 15 m² × 2 = 30 m² (this is the total area to paint).

Finally, divide by the coverage: 30 m² ÷ 10 m²/L = 3.0 L.

So you need 3.0 litres of paint. The calculator shows the same result. In practice, you would buy a 3.7 L tin (a common size) or two 1.5 L tins, because paint is sold in fixed sizes and you should always have a little extra for touch-ups.

How to Measure Your Walls Correctly

Accurate measurements are the foundation of a correct estimate. Here’s what to do:

• Use a metal tape measure, not a fabric one, for straight, reliable readings.
• Measure the length of each wall at floor level, where it’s easiest to get the tape straight.
• Measure the height from the floor to the ceiling, not to the top of a chair rail or picture rail.
• For each wall, multiply length by height to get its area, then add all wall areas together.
• Subtract large areas you won’t paint: doors (about 1.6 m² each) and windows (about 1.2 m² each) – but only if you’re not painting the frames.

A common mistake is to measure the room’s perimeter and multiply by height, but forget to subtract door and window openings. That’s fine if you’re painting the trim too, but for walls alone it overestimates by 10–15%.

What Does “Coverage� Mean on the Tin?

Coverage is the area that one litre of paint will cover when applied at the recommended film thickness. It is always printed on the can, usually as “10 m²/L� or “100 sq ft per gallon� in the US. This number assumes:

• The surface is smooth and not overly porous.
• The paint is applied with a roller or brush at the recommended thickness.
• The temperature and humidity are within normal ranges.

In reality, coverage varies with surface type. Fresh plaster or bare drywall can absorb 20–30% more paint than a previously painted wall. Rough surfaces like brick or textured plaster have a larger effective area, so they need more paint. If your walls are new, unpainted, or very porous, reduce the coverage rate by 20% (e.g., use 8 m²/L instead of 10 m²/L) to be safe.

How Many Coats Do You Need?

Most paint jobs need two coats for even colour and durability. One coat is only enough when:

• You are painting over the same colour with the same type of paint.
• The old paint is in good condition and the new colour is very similar.
• You are using a high-hide paint specifically designed for one-coat coverage.

If you are making a dramatic colour change (e.g., dark blue to light cream), you may need three coats, or a primer first. Primers are not included in this calculator—they have their own coverage rate and should be calculated separately. A good rule: when in doubt, add 10% to your final litre figure to cover touch-ups and mistakes.

Common Mistakes and How to Avoid Them

• Mixing units: If you measure in feet, convert to metres (1 ft = 0.3048 m) or use a calculator that accepts feet. Never mix feet and metres in the same formula.
• Forgetting the ceiling: Ceilings are often painted separately and need their own calculation. For a rectangular room, ceiling area = length × width.
• Ignoring the number of coats: The formula multiplies by coats, but many people forget to double the area for a two-coat job.
• Rounding down: Paint is sold in fixed sizes (1 L, 2.5 L, 3.7 L, 5 L, 10 L). Always round up to the nearest available tin size. Buying 3 L when the tin is 2.5 L means you’ll run out halfway through the second coat.

Limits of the Formula

This formula gives a solid estimate, but it cannot account for every variable. It assumes a flat, rectangular wall with no major obstacles. Real walls have windows, doors, sockets, and sometimes angled ceilings. The formula also doesn’t factor in paint loss from brush or roller absorption, drips, or spills—which typically add 5–10% to your needs. For a more precise estimate, measure each wall individually, subtract openings, and then add a 10% buffer. If you are painting a textured surface or using a sprayer, add 20% because sprayers waste more paint due to overspray.

Frequently Asked Questions

How much paint do I need for a 12×12 room?

A 12×12 foot room with 8-foot ceilings has about 384 square feet of wall area (perimeter 48 ft × height 8 ft). With a typical coverage of 350 sq ft per gallon for one coat, you’d need about 1.1 gallons per coat, so for two coats you’d need about 2.2 gallons—so buy 2.5 gallons. In metric, that’s roughly 3.65 m² of wall, and at 10 m²/L you’d need about 3.65 L per coat, so 7.3 L for two coats, so buy 7.5–8 L.

Should I buy more paint than the calculator says?

Yes, always round up to the next tin size and add 10% for touch-ups. Paint that sits in a cupboard is cheap insurance; running out mid-project means a second trip and a possible colour mismatch if the batch differs.

Does primer count as a coat?

No, primer is a separate product with its own coverage. Use the same formula but with the primer’s coverage rate and the number of primer coats you plan (usually one). Do not mix primer litres with paint litres.

How do I calculate paint for a ceiling?

Measure the length and width of the ceiling, multiply them to get the area, then divide by the coverage and multiply by the number of coats. For a 4 m × 3 m ceiling with 10 m²/L coverage and two coats, you need (4×3×2)/10 = 2.4 L.

What if my walls are textured or unpainted?

Textured surfaces and bare drywall absorb more paint. Reduce the coverage rate by 20% (e.g., from 10 m²/L to 8 m²/L) or add 20% to the final litres. For heavily textured plaster, consider 30% extra.

Is it better to use a paint calculator or just guess?

A calculator removes the guesswork and helps you avoid buying too much or too little. It’s especially useful for large rooms or when you’re budgeting. The formula is simple enough to do by hand, but a calculator reduces arithmetic errors and lets you quickly test different coverage rates or coat numbers.

Related

• Asphalt Cold Patch Calculator: How Much Do I Need?
• Asphalt Kingdom Paint Calculator Guide
• Asian Paint Calculator: How to Estimate Paint

in w | Wall length | m | 6 | 0
in h | Wall height | m | 2.5 | 0
in c | Number of coats | - | 2 | 1
in cov | Paint coverage | m²/L | 10 | 1
out paint | Paint needed | L | (w * h * c) / cov | 1
out area | Total area to paint | m² | w * h * c | 1