Paint Calculator

How it works

Worked examples

1. 1

   Bedroom with four walls, two coats, 1-gallon tins

   1. Walls of 12, 8, 12 and 8 ft add up to 40 ft; 40 × 8 ft = 320 sq ft.
   2. Take off one door and one window, 40 sq ft together: 320 − 40 = 280 sq ft.
   3. Two coats: 280 × 2 = 560 sq ft. At 350 sq ft per gallon, 560 ÷ 350 = 1.6 gallons.
   4. 1.6 gallons in 1-gallon tins rounds up to 2 tins.

   Buy 2 one-gallon tins; about 0.4 gallon is left for touch-ups.

   Try these numbers
2. 2

   Living room in metres, 2.5-litre tins

   1. 12 m of wall at 2.5 m high is 30 m².
   2. Less 3.5 m² of door and window: 26.5 m².
   3. Two coats cover 53 m²; at 10 m² per litre that is 5.3 litres.
   4. 5.3 ÷ 2.5 = 2.12, which rounds up to 3 tins.

   Buy 3 tins of 2.5 litres; 2.2 litres would be left over.

   Try these numbers
3. 3

   Single coat that uses the tins exactly

   1. A 10 by 10 wall is 100 area units.
   2. One coat at 50 per gallon needs 100 ÷ 50 = 2 gallons.
   3. 2 gallons is exactly 2 tins.

   Buy 2 tins with nothing left over — in practice, consider one spare for touch-ups.

   Try these numbers

Inputs and results

Total wall length

The lengths of every wall you are painting, added together, in feet or metres.

Wall height

Floor to ceiling, or to the top of the painted area, in the same unit as the length.

Doors and windows

The area of openings you will not paint, in square feet or square metres. Enter 0 if there are none.

Coats

How many coats you plan to apply over the whole area, as a whole number.

Coverage

Area one gallon or one litre covers, as printed on the tin or product sheet.

Tin size

The volume of one tin you can buy, in gallons or litres.

Paintable area

Gross wall area less doors and windows.

Paint needed

Paintable area times coats, divided by coverage, in gallons or litres.

Whole tins

Paint needed divided by tin size, rounded up because a part tin cannot be bought.

Left over

Paint remaining in the last tin after the job, if coverage matches the tin.

When to use it

• Buying paint for the walls of one room before a trip to the shop, with the tin size you can actually buy.
• Comparing two products whose tins list different coverage figures, to see which needs fewer tins for the same walls.
• Checking whether a second coat changes the number of tins, so you can decide whether to buy one more now.

Common mistakes

• Measuring only one wall when the room has four: the length field expects the total length of every wall you are painting, added together.
• Forgetting to multiply by coats. Two coats need twice the paint of one, and most colour changes need two.
• Entering coverage per tin instead of per gallon or litre. A 2.5-litre tin that covers 25 m² has a coverage of 10 m² per litre, not 25.

What it does not do

• It does not estimate ceilings, trim, skirting boards or doors; measure those separately and run the calculator again with their area.
• It does not know the coverage of a particular brand or colour; it uses the figure you enter from the tin or the product sheet.
• It treats every coat as covering the full area at the same rate, so it does not model a thinned mist coat or a primer with a different spread rate.

Method and assumptions

Assumptions

• All measurements use one unit system: feet with square feet per gallon, or metres with square metres per litre.
• Every coat covers the whole paintable area at the coverage you enter.
• Tins are bought whole, in the size you enter.

Edge cases

• If doors and windows add up to the whole wall area or more, there is nothing to paint and no answer is shown.
• Coats must be a whole number; half coats are not modelled.
• A very small job can still round up to one full tin; the left-over figure shows how much of it you would use.

Sources and corrections

This calculator uses an elementary formula, so it needs no outside data source. The method, the rounding rules and how a reported error is corrected are set out in the site standards.

• Calculation Methodology
• Sources Policy
• Corrections Policy