Paint calculator

What this calculator does

It turns a room into a shopping list in three steps. First the area: the perimeter of the room times the wall height gives the gross wall, and every door and window you enter is subtracted at the size you give it. Then the paint: the paintable area times the number of coats, divided by the coverage per coat from your tin, plus an allowance you set. Finally the purchase: that quantity divided by the tin size you chose, rounded up, because tins cannot be split.

The ceiling is optional and, when you include it, is treated as a second product. It gets its own coats, its own coverage and its own tin size, and its tins are rounded up on their own. That matters: half a litre left in a wall tin is no use for a ceiling in a different colour or finish, so pooling the two would understate what you have to buy.

How to measure and use it

1. Pick your units. Metric takes metres, square metres per litre and litre tins; US customary takes feet, square feet per US gallon and US-gallon tins. Switching converts the numbers, so the room stays the same size.
2. Measure the room's length and width along the floor, and the wall height from floor to ceiling.
3. Count the doors and windows and measure one of each. If they differ, use the biggest or run the odd wall separately — the default sizes are examples, not standards.
4. Set the number of coats. Two is usual over a similar colour; going lighter over dark, or painting bare plaster, often takes three.
5. Copy the coverage from the tin or its data sheet, per coat.
6. Choose the tin size you actually intend to buy, and add a price if you want the cost.
7. Decide whether the ceiling is in, and give it its own paint if it is.
8. Read the paintable area, the paint the job uses, and the whole tins to buy.

A worked example

A 14 ft × 12 ft room with 8 ft walls has a 52 ft perimeter, so the gross wall is 416 sq ft. One 3 ft × 6.67 ft door is 20.01 sq ft and two 3 ft × 4 ft windows are 24 sq ft, which leaves 371.99 sq ft to paint. Two coats at 350 sq ft per US gallon consume 2.13 US gal, and a 5% allowance takes that to 2.23 US gal. In 1 US gallon tins that is three tins — 3 US gal bought and 0.77 US gal left over.

The 168 sq ft ceiling is where the rounding bites hardest. Two coats at the same coverage consume 0.96 US gal, or 1.01 US gal after the allowance. That fraction over a gallon forces a second 1 US gallon tin: you buy 2 US gal to use just over one. Buying a larger tin, or accepting one very light coat, are both real answers — the calculator shows you the choice rather than hiding it.

The same room in metric: 4 m × 3 m with 2.5 m walls is 35 m² gross, a 0.9 m × 2.0 m door and two 1.2 m × 1.2 m windows take off 4.68 m², and 30.32 m² is left. Two coats at 9 m² per litre consume 6.74 L, or 7.07 L with the allowance, which is three 2.5 L tins and 7.5 L bought.

What the doors, the coats and the ceiling change

Openings matter more than people expect in small rooms. In the example above, one door and two windows take 44 sq ft off 416 — about a tenth of the wall, and roughly a quarter of a tin. In a room with patio doors the deduction can be a third of a wall. That is why the sizes are fields rather than fixed values: an assumed door is fine until your door is a pair of glazed doors.

Coats multiply everything. Going from two coats to three is a 50% increase in paint, which in the example turns three tins into four. Coverage works the other way: a paint rated at 400 sq ft per gallon rather than 350 cuts the same job by about an eighth. Both numbers come from you, so it is worth being honest about the surface — a strong colour change or bare, thirsty plaster will not achieve the figure on the label.

Adding the ceiling rarely adds proportionally to the cost, because the ceiling is a small area that still has to clear a whole tin. That is not a flaw in the arithmetic; it is what buying in tins does to small jobs.

Used versus bought: two different numbers

The calculator reports both, deliberately. The paint the job uses is the honest estimate of consumption, allowance included. The paint you buy is that figure rounded up to whole tins of the size you picked, and it is almost always more. Confusing the two is how people end up one tin short on a Saturday afternoon, or with four part-used tins in a garage.

The gap is worth looking at before you buy. If the job needs 2.23 US gal, three 1-gallon tins leave 0.77 gal spare, while a 5-gallon pail would leave 2.77 gal. If it needs 1.01 gal, the second tin is nearly all surplus. Trying one size larger and one size smaller in the tin field takes a few seconds and often changes the answer.

Common mistakes

• Mixing units — entering a room in feet and a window in inches. If the openings come out larger than the wall, the calculator refuses the input rather than guessing, and that is usually the reason.
• Reading a coverage figure as if it were per job rather than per coat. Coverage on a tin is per coat, and the calculator multiplies by the number of coats itself.
• Assuming a gallon is a gallon. A US gallon is 3.785 L; the imperial gallon still used in the UK and Canada is about a fifth larger, so the same number means different amounts of paint.
• Using the wall paint's coverage for the ceiling. Ceiling paint is often a different product with a different spreading rate.
• Forgetting that a light colour over a dark one is a coats problem, not a coverage problem. Add the coat rather than shaving the coverage.

What it does not do

This first version covers walls and, optionally, the ceiling. Trim, skirting boards, door and window frames and primer are not included — they are a different product on a different unit of measure, and guessing at them would make the total look more precise than it is. It also takes no view on preparation, and it does not know your paint: coverage and coats are your assumptions, and a surface that drinks paint will beat any figure on a label.