Can I add the wall paint and the ceiling paint together?

Only if they are the same product in the same colour. Then add the two litres figures before rounding to tins. BuildBatch keeps them separate because they are usually different paints.

Does the 10% extra mean I will have paint left over?

Usually the rounding to whole tins leaves more over than the allowance does. In the worked example the allowance is 0.69 L and the lowest-cost set of tins leaves about 0.91 L.

How much paint do I need?

Multiply the area you are painting by the number of coats, divide by the paint’s coverage per litre, and add a little for waste. Then round up to the tins you can actually buy, which is where an estimate turns into a shopping list.

The sum

Paint needed in litres = area × coats ÷ coverage × (1 + allowance).

Area is the net wall area or the ceiling area, in m².
Coats is a whole number, usually 2.
Coverage is the m² one litre covers in one coat. It comes from the tin.
Allowance is the extra for waste and touch-ups, as a fraction: 10% is 0.1.

Walls and ceiling are worked out separately because they are usually different paints. If yours are the same product in the same colour, add the two litres figures together before rounding to tins.

The starting figures, and why they are examples

BuildBatch starts every paint at 2 coats, 10 m² per litre per coat, 10% extra and a 2.5 L tin. These are examples, so that you can get an answer without a tin in your hand. They are labelled as examples on screen and in the copied list, and the figure on your tin beats them every time. The 10 m² per litre is on the cautious side of what most emulsions state, so an unedited estimate errs towards slightly too much paint rather than too little.

Worked example: the walls

A room 4.2 m × 3.6 m × 2.4 m high, with one door 0.826 m × 1.981 m and one window 1.2 m × 1.1 m.

Perimeter: 2 × (4.2 + 3.6) = 15.6 m
Gross wall area: 15.6 × 2.4 = 37.44 m²
Openings: 1.636 + 1.32 = 2.956 m²
Net wall area: 37.44 − 2.956 = 34.484 m²
Two coats: 34.484 × 2 = 68.968 m² of coating
At 10 m² per litre: 68.968 ÷ 10 = 6.897 L
Plus 10%: 6.8968 × 1.1 = 7.586 L

From litres to tins

Say the paint you have chosen comes in 1 L, 2.5 L and 5 L tins, and you have typed in the prices you were quoted: £20, £32 and £55. Those are example prices typed in by the user, not prices BuildBatch knows.

Lowest total cost: 1 × 5 L + 1 × 2.5 L + 1 × 1 L = 8.5 L for £107.00, leaving about 0.91 L over.
Fewest tins: 2 × 5 L = 10 L for £110.00, leaving about 2.41 L over.

The rule is simple. With a price for every size, the combination with the lowest total cost wins; ties go to the least left over, then the fewest tins. With any price missing, the least left over wins. The comparison is only between the sizes and prices you entered. It says nothing about other shops or other paints.

With a single tin size, as in the plain paint calculator, 7.586 L in 2.5 L tins is 3.03 tins, so 4 tins and 10 L.

Worked example: the ceiling

Ceiling area: 4.2 × 3.6 = 15.12 m²
Two coats at 10 m² per litre: 15.12 × 2 ÷ 10 = 3.024 L
Plus 10%: 3.326 L
Lowest cost with the same tins: 1 × 2.5 L + 1 × 1 L = 3.5 L for £52.00, about 0.17 L over
Fewer tins: 1 × 5 L for £55.00

What changes the answer most

Coverage. At 12 m² per litre instead of 10, the walls need 6.322 L rather than 7.586 L. Read the tin.
Coats. A third coat adds half as much again.
Openings. The door and window together are worth about 0.65 L here. Skipping them costs little; measuring them costs nothing.
The allowance. 10% of 6.897 L is 0.69 L, less than the smallest tin. Rounding up to whole tins usually leaves more over than the allowance adds.

Do it with BuildBatch

Open the Room Refresh Batch with paint chosen. Type the room, add the doors and windows, enter the tin sizes and prices you have been given, and it works out the combination for you, with the fewer-tins alternative beside it.

What this guide does not cover

Primer, undercoat or a mist coat on bare plaster, which need their own figures from their own tins. Woodwork. Textured or very absorbent surfaces, where the tin’s coverage figure may not hold. Whether a particular paint suits the room. Check first with the tin, and with your merchant if you are unsure.