Paint Estimate Calculator: How to Measure

A paint estimate calculator works out how much paint you need for a room or surface. You enter the room’s dimensions, the number of doors and windows, and the number of coats, and it returns the total area to cover and the litres of paint required. This page gives you a working calculator, the formula behind it, and practical tips to avoid buying too much or too little.

Formula

The calculator uses the standard wall-painting formula:

Total wall area = (2 × length × height) + (2 × width × height) − (door area × number of doors) − (window area × number of windows)

Paint needed = (total wall area × number of coats) ÷ coverage per litre

Where:

• length (m) – the longest wall dimension of the room
• width (m) – the shorter wall dimension
• height (m) – floor to ceiling height
• door area – assumed to be 1.9 m² (a standard interior door is about 0.9 m × 2.1 m)
• window area – assumed to be 1.5 m² (a typical window is about 1.2 m × 1.2 m)
• coverage – 10 m² per litre is the common figure for most emulsion paints on smooth plaster

If your paint label says a different coverage, replace the 10 in the formula with that number. The calculator’s default of 10 m²/L is a safe middle ground for interior wall paint.

Worked Example

Using the default values: room 5 m long, 4 m wide, 2.5 m high, 1 door, 2 windows, 2 coats.

1. Calculate the gross wall area: (2 × 5 × 2.5) + (2 × 4 × 2.5) = 25 + 20 = 45 m²
2. Subtract door and window areas: 45 − (1 × 1.9) − (2 × 1.5) = 45 − 1.9 − 3 = 40.1 m²
3. Multiply by number of coats: 40.1 × 2 = 80.2 m²
4. Divide by coverage per litre: 80.2 ÷ 10 = 8.02 L

The calculator rounds to one decimal place, so it shows 8.0 L. That is the amount of paint you need for two coats on the walls, not including the ceiling or trim.

Why Use a Paint Calculator?

Estimating paint by eye almost always leads to overbuying or underbuying. Overbuying wastes money and leaves you with cans you may never use; underbuying forces a trip back to the store mid-project, and the second batch may not match perfectly if the store mixed it from a different tint batch.

A calculator removes the guesswork. It gives you a single number you can take to the shop, and you can adjust it for the actual coverage of the paint you choose. It also helps you compare costs between different paints: if one brand covers 12 m²/L and another covers 8 m²/L, the cheaper per litre may actually cost more per room.

How to Measure Your Room Correctly

Accurate measurements matter more than the formula. Here is what to do:

• Measure wall length and width at the baseboard, not at eye level, because walls can be slightly out of plumb.
• Measure height from floor to ceiling. If you have crown moulding or a tray ceiling, measure the tallest point and subtract the moulding height if you won’t paint it.
• Count doors and windows that are fully within the wall area. If a window is unusually large, measure it and subtract its actual area instead of using the default.
• For a room with an open plan or an angled ceiling, break the wall into rectangles and triangles, calculate each separately, and add them together.

If you are painting a ceiling, measure the floor area (length × width) and use that as the ceiling area. Ceilings typically need the same coverage per litre as walls.

Common Mistakes and How to Avoid Them

• Forgetting the number of coats. Most paints need two coats for even colour, especially over a darker shade. Always multiply by the number of coats you plan to apply.
• Using the wrong coverage. Coverage varies with paint quality, surface texture, and colour. Rough plaster or unpainted drywall absorbs more paint; a high-quality paint may cover 12–14 m²/L, while a budget paint may only cover 7–8 m²/L.
• Not subtracting doors and windows. A room with large windows has significantly less wall area. Skipping this step can overestimate by 10–20%.
• Mixing units. If you measure in feet, you must convert to metres or use a formula designed for feet. One metre equals 3.28 feet; one square metre equals 10.76 square feet. The calculator here expects metres.
• Rounding too early. Round only at the final step. Rounding each intermediate number can shift the result by half a litre.

How to Read the Result

The result is the theoretical amount of paint for smooth, sealed surfaces. In practice, you should add about 10% for waste, touch-ups, and uneven application. For the worked example, 8.0 L plus 10% is 8.8 L, so you would buy a 10 L can or two 5 L cans.

Paint is sold in standard can sizes: 1 L, 2.5 L, 5 L, and 10 L (in many countries). Choose the can size that gets you closest to your estimate without going under. If the estimate is 8.8 L, a 10 L can is the practical choice.

If you are painting over a very dark colour with a light one, you may need an extra coat, and a primer is often recommended. The calculator does not include primer; add it separately using the same area but a single coat.

Limits of the Formula

This formula assumes a rectangular room with flat walls and standard doors and windows. It does not account for:

• Textured walls (e.g., stucco, brick, or wood panelling) that absorb more paint
• Openings like arches, built-in shelves, or fireplaces
• Ceilings, trim, and baseboards – those are separate areas
• Different paint types (e.g., gloss, satin, or masonry paint) that have different coverage rates

For a non-rectangular room, measure each wall individually and add the areas, then subtract doors and windows. For a very porous surface, reduce the coverage per litre by 20–30%.

Frequently Asked Questions

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

A 12 ft × 12 ft room with 8 ft ceilings (about 3.7 m × 3.7 m, 2.4 m high) has roughly 4 walls of 3.7 m × 2.4 m = 35.5 m² total, minus one door (1.9 m²) and one window (1.5 m²) gives about 32 m². For two coats, that’s 64 m², which at 10 m²/L means 6.4 L. You would likely buy a 5 L can and a 2.5 L can, or a single 10 L can if you plan to do the ceiling.

Should I include the ceiling in the calculation?

No, unless you are painting the ceiling. The formula here covers walls only. For a ceiling, measure the floor area (length × width) and use that as the square footage. Ceilings usually need the same number of coats as walls.

What if my paint label says a different coverage per litre?

Use that number instead of 10. For example, if the label says 12 m²/L, divide the total area (area × coats) by 12. The calculator’s default is a conservative estimate; many quality paints cover more.

How do I convert from square feet to square metres?

Multiply square feet by 0.0929 to get square metres. For a quick mental estimate, divide square feet by 10. So a 400 sq ft room is about 40 m². If you measure in feet, you can also use a formula with feet and then convert the final litres (1 gallon = 3.785 L, 1 quart = 0.946 L).

Why does the calculator show 8.0 L but I think I need more?

The calculator gives the theoretical amount for smooth walls. Real-world factors – roller texture, paint quality, and touch-ups – add waste. Add 10% to the result, and if you are painting a dark colour over light, plan for a third coat. It is always safer to buy a slightly larger can than to run out mid-project.

Can I use this calculator for exterior paint?

Yes, but exterior surfaces are rougher and more porous, so coverage is often lower (around 6–8 m²/L). Also, you must subtract the areas of windows and doors on the exterior walls. Measure each wall’s height and width, including gables, and use the same formula with a lower coverage value.

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in w | Wall length | m | 5 | 0
in d | Room width | m | 4 | 0
in h | Wall height | m | 2.5 | 0
in dr | Doors | count | 1 | 0
in win | Windows | count | 2 | 0
in coats | Coats | count | 2 | 1
out area | Total wall area | m² | (w * h * 2 + d * h * 2) - (dr * 1.9 + win * 1.5) | 1
out paint | Paint needed | L | (area * coats) / 10 | 1