What this tool calculates
The wall area is the summed wall width multiplied by a shared height. Subtract only the opening area you choose to exclude. Multiply by the coat count, divide by the label’s square-foot coverage per gallon, and apply your chosen allowance.
The prefilled 350 sq ft per gallon and 10% allowance are example inputs, not a recommendation or a product performance claim.
Worked example
40 ft of wall width × 8 ft high − 20 sq ft of openings = 300 sq ft. Two coats at an example 350 sq ft per gallon with 10% extra require about 1.886 gallons. The supported purchase combination rounds to 2 gallon cans.
What is not included
Ceilings, primer, trim, texture-dependent performance and price optimization are not included unless separately accounted for. The smallest supported purchase volume is not necessarily the cheapest order.
Why the label matters: manufacturers publish coverage as a range that depends on surface texture and porosity, for example 400–450 sq ft per gallon for one premium interior paint (Benjamin Moore Regal Select 550 data sheet) and 350–400 sq ft per gallon for another (Ultra Spec 500 N538 data sheet). The number on your can, at the coat count its instructions call for, is the input to use. Our arithmetic example and interface are original project work.
Not sure what to measure? Read how to measure a room for paint, wallpaper and flooring.
What this page decides: how many whole cans cover the wall area you typed, at the coverage figure you typed, in the can sizes you chose. What it does not decide: the coverage figure, the coat count or the allowance, all three of which are yours to set from the product and the surface. It prints the same warning under every result, and the warning is the boundary of the answer: “Ceiling, primer and trim are not included unless measured separately.”
Questions people ask
How much paint per square foot?
Of wall, at a 350 sq ft/gal label in two coats with a 10% allowance: 0.80 fluid ounces per square foot, which is 0.00629 gallons. At a 250 label it is 1.13 fl oz, at 400 it is 0.70, at 450 it is 0.63. One coat instead of two halves all four.
Per square foot of floor there is no answer, because the walls standing on a floor can be any height and any perimeter. Three rooms with the same 400 sq ft of floor and 8 ft ceilings take 4, 4 and 5 gallons at a 350 label, depending on shape; the per-square-foot guide works through all three.
Which square footage does this calculator want?
Wall area, never floor area. "Room dimensions" takes the room’s length, width and wall height and adds the four walls for you; "One wall" takes a single wall; "Total area" takes a wall figure you worked out yourself. None of the three has a field for floor area.
I know my wall square footage. How many cans is that?
Type it into "Total area" with "Area of doors & windows" set to 0 if your figure already has the openings taken off, or read it off the table in wall square feet to paint cans, which runs from 100 to 1,000 sq ft of wall at four label coverages. The same table shows where the purchase steps: about every 40 sq ft of wall at a 350 label, so 397 sq ft and 398 sq ft are a quart apart.
Why does a 350 sq ft/gal can not cover 350 sq ft of my wall?
Because the label figure is one coat on a prepared surface. Two coats and a 10% allowance put the useful figure at 159 sq ft of wall per gallon at a 350 label, 113.6 at 250, 181.8 at 400 and 204.5 at 450. The number on the can is not the number of square feet your wall uses up.
Rooms worked through with this calculator
Eight worked examples, each one a real room run through the fields above. They show the arithmetic by hand first, then where this page agrees.
- The three places paint math slips — one coat instead of two, the top of a printed range when your wall sits lower, and rounding early: on one bedroom those are worth a gallon, up to a gallon, and a quart.
- Your can prints a range, not a number — a 14-by-16 living room at both ends of the ranges four manufacturers actually print: 4 gallons plus 2 quarts at one end, 2 gallons plus 3 quarts at the other.
- Do I subtract the window? — what the “Area of doors & windows” field above is worth: a quart at every label, and the default 20 buys the same cans as a measured 44.25.
- A 5-by-8 bathroom: the can size beats the label — every coverage figure from 250 to 450 lands on one gallon plus a quart or less, so whether the shop sells quarts decides one can or two.
- Two rooms at once: count coats before cans — one coat or two moves the order by 1.75 to 3 gallons; buying for both rooms together instead of apart moves it by one quart.
- A 12-by-15 ceiling, two coats — ceiling paint labels say 400 to 450 square feet a gallon, which is why the ceiling is a separate can from the walls above.
- One maker, four products, two coverage numbers — the same company publishes 350–400 on one wall paint and 400–450 on another, and its own calculator tells you it has already assumed two coats and left the ceiling out.
- Primer is a separate can — one coat at a typical primer figure takes the same 2 gallons and 1 quart as two coats of a 400-square-foot paint, so it needs a run of its own rather than an extra coat on this one.