Paint can sizes, 5-gallon savings and the leftover problem

The sizes on the shelf

US paint is sold in quarts, gallons and 5-gallon pails; stains and enamels also come in pints and half pints, and sample pots hold 8 fl oz. In metric countries the sizes are 1 L, 2.5 L, 5 L and 10 L. The full table, with exact conversions between the two systems, is in the can sizes and conversions reference.

The price per gallon falls as the can gets bigger, and the step from quarts to gallons is the steep one. At typical prices of $16 a quart, $40 a gallon and $185 for a 5-gallon pail, the paint costs:

Those are example prices; yours will differ, and the ratios between sizes matter more than the prices. A quart usually costs well over a quarter of a gallon, and a pail usually costs a bit less than five gallons.

Rounding up: the simple rule

Every quantity calculator on this site rounds up to cans with a simple rule: whole pails, then whole gallons, then quarts for what is left, with four quarts becoming a gallon and anything over four gallons becoming a pail. It never leaves you short, and it is usually close to the cheapest. Usually, not always.

When the simple rule is not the cheapest

Because quarts are expensive per gallon, three quarts often cost more than a gallon, and a gallon and three quarts can cost more than two gallons. The can size optimizer tries every combination of cans that covers your quantity and picks the cheapest at your prices.

When a 5-gallon pail pays off

A pail makes sense for one colour over a large area: a whole-house interior in one wall colour, a ceiling white for every room, or an exterior. Besides the price, it gives you one batch, so the colour is identical from the first wall to the last. Its drawbacks are weight and leftover. A full pail of latex weighs about 58 lb, which the paint weight calculator works out for a load, and if you only need 4.2 gallons, the pail leaves most of a gallon over.

Professionals often buy pails even for smaller jobs and pour off into a bucket with a roller grid, which is faster than working from gallon cans.

Metric cans

In metric countries the same logic applies with 1 L, 2.5 L, 5 L and 10 L cans. Our calculators round metric quantities up to 10 L tubs first, then a 5 L can, then a 2.5 L and a 1 L can for the remainder, and they turn a remainder over 7.5 L into another 10 L tub and one over 3.5 L into another 5 L can. Small cans cost more per litre, so a job that needs 3.6 L is cheaper as one 5 L can than as a 2.5 L can and two 1 L cans: at our default prices of 42 for 5 L, 24 for 2.5 L and 12 for 1 L, that is 42 against 48. Enter your store’s prices in the can size optimizer in metric mode and it checks every combination.

Boxing: why one batch matters

Paint tinted in different batches can differ slightly in colour, and the difference shows on a wall in raking light. If you buy several gallons, pour them all into one large bucket and stir before you start, then pour back. Painters call this boxing. It costs ten minutes and guarantees one colour across the whole job. If a second batch has to be bought mid-job, finish a whole wall from one batch and change at a corner, never in the middle of a wall.

Converting quantities

Quantity calculators give gallons or litres to two decimals; the can list is what you take to the store. To convert a figure from a data sheet, a recipe or a foreign label, the paint quantity converter handles gallons, quarts, pints, fluid ounces, litres and millilitres and rounds to cans. Two conversions catch people out:

• A UK gallon is about 20% larger than a US gallon. Old British coverage figures “per gallon” are per the larger gallon.
• A 5 L can is 1.32 US gallons, not “about a gallon”. Five litre cans for a job calculated in gallons will leave a lot over.

The leftover problem

Rounding up always leaves some paint. The question is how much to keep, and what to do with the rest.

How much to keep. Keep enough for touch-ups over the next few years: about 10% of what the job used, and never less than a quart for a wall colour. Write the colour name, code, sheen, brand and the rooms it was used in on the lid. The leftover and touch-up planner works it out.

How to store it. Pour the keeper into a smaller airtight container so there is little air above it; a quart of paint in a gallon can skins over. Clean the rim before closing, tap the lid down with a mallet over a cloth, and store it somewhere that does not freeze. Sealed latex paint is typically good for about two years once opened, oil-based paint for longer.

What to do with the surplus. Unopened, untinted cans can often be returned; tinted paint usually cannot. Give surplus to neighbours, community groups or a reuse store. In US states with a paint stewardship law, PaintCare runs drop-off sites at many paint stores for leftover paint. Elsewhere, local rules differ: many places let you dry out small amounts of latex and put it in household trash, while oil-based paint is household hazardous waste. Never pour paint down a drain.

Common mistakes

• Buying quarts to avoid leftovers. Three quarts often cost more than a gallon.
• Buying exactly the calculated amount. Round up; running short costs a trip and a colour mismatch.
• Mixing batches on one wall. Box the paint, or change batches at a corner.
• Keeping the leftover in the original part-empty can. It skins over; decant it.
• Not labelling the leftover. In three years nobody will remember which grey it was.

For the full method of getting the quantity in the first place, see how much paint do I need; for the cost of the whole job, what painters charge.