Paint Calculators

This is the one hub on the site with no code behind it. Nobody inspects a paint job, so the only checkable authority is the manufacturer's own data sheet.

The calculators

• Exterior Paint CalculatorGables counted as half base times rise. Coverage from Sherwin-Williams data sheets
• Paint CalculatorCoverage from Sherwin-Williams data sheets, checked against 1604 / wet mils

Everywhere else here there is a table to argue with. Joist spans, header sizes, guard heights and ledger bolts all come out of the IRC, and a page either quotes the cell correctly or it does not. Paint has none of that. No inspector signs off a second coat, no table sets a film thickness for a bedroom, and the trade runs on rules of thumb that nobody wrote down.

So the data sheet is the whole authority, and it turns out to be a stronger one than it looks. Every sheet prints a coverage figure and, beside it, the wet film thickness that figure assumes. Those two numbers are related by fixed arithmetic, which means a published coverage rate can be checked rather than taken on trust. The calculators here do that check on the page and print the result next to the claim.

Where the money actually goes wrong

Not on coverage. On wall area. Measuring the longest wall and multiplying by four turns a 20 by 10 room's true 60 linear feet into 80, a third too much, and that error survives every correct decision made after it. Perimeter is 2 × (length + width), and a room with an alcove or a chimney breast is not a rectangle at all.

Outside it is gables, for the same reason: they are the part of the wall you cannot reach with a tape from the ground, so they get guessed or skipped. Four gable ends on a modest house is real area, and it is all at the top of a ladder.

The allowance nobody publishes

Search every sheet in this section and you will not find a waste figure. What happens between the can and the wall is left entirely to you: the roller nap, the tray, the brush, the overlap where a cut-in meets a roller, the texture of the surface, the inch in the bottom. These calculators put that allowance in a field you can see and change, defaulted rather than hidden, because a number we chose should not be dressed up as a number we read.

What sheen does and does not change

It does not change coverage. Flat and semi-gloss from the same line publish the same rate at the same film thickness. What it changes is how much the wall shows, how it cleans, and how forgiving a second coat is over the first, none of which is a quantity question.

Why the figures here are all one manufacturer

Not a preference. Sherwin-Williams publishes its data sheets at a stable public address with a document number and a revision date on every one, which is what makes a figure citable. Where a brand's coverage number could only be found on a retailer listing or a review site, it is not here. Repeating a number does not source it, and on a page whose only authority is the document, an uncited figure has no authority at all.