Formula Reference
Reading the three totals
Total Surface Area is the painted surface counted once, whatever the number of coats. Total Coat SF is that surface multiplied by the coats, and it is the quantity the gallons are derived from: a 700 SF room in two coats is 700 SF of surface and 1,400 SF of coat area. Total Gallons is rounded up once from the unrounded sum across every line, which is why it can differ by a tenth or two from adding up the individual rows.
Where the default coverage rates come from
The 350 SF/gal default on walls and 400 SF/gal on ceilings sit inside the ranges manufacturers publish for professional interior latex. Benjamin Moore's data sheet for Ultra Spec 500 Interior Eggshell (N538) gives 350–400 sq ft per gallon at a recommended 4.3 mils wet, 1.8 mils dry. The flat sheen of the same line (T535) gives 400–450 sq ft per gallon at 3.6–4.0 mils wet. Flat goes further because it is specified at a thinner film, which is the reason a ceiling default sits above a wall default rather than any property of ceilings.
Both sheets carry the same sentence: depending on surface texture and porosity. Treat the defaults as placeholders. The moment you know the product and sheen, put its published rate in the Coverage field — the calculator will stop overwriting the value once you have typed in it.
The floor default is a starting number only. Floor coatings run from thin traffic paints to high-build resinous systems whose rated coverage is a fraction of a wall paint's, and no single default is meaningful across that range. Take it from the data sheet.
What the substrate selector actually does
Substrate multiplies the coverage rate before the division: choosing Concrete Block/CMU with a 350 SF/gal product gives an effective 227.5 SF/gal. Smooth drywall, metal and previously painted surfaces apply no reduction; textured drywall, smooth concrete, wood, block and brick reduce it progressively. These multipliers are this calculator's own assumptions. No manufacturer publishes coverage as a multiplier, so we cannot cite one, and we are not going to invent a source for a number that does not have one.
What is published is how large the effect gets. Benjamin Moore rates its Ultra Spec Hi-Build Masonry Block Filler (571) at 75–100 sq ft per gallon, applied at 16–21 mils wet, against 350–400 for the same brand's interior wall paint. Open-face block is not a coverage-multiplier problem; it is a separate product on a separate line at its own rate. Use the multiplier for texture, not for bare masonry.
Openings, waste and packaging
The calculator does not deduct doors, windows or louvres. Wall area is the full perimeter times the height. On a typical partitioned interior that is close enough and it is how most painters count; on a glazed elevation or a corridor of full-height storefront it is not. Where openings are large, add the wall as an Accent Wall line instead — that type takes a single length and height, so you can enter net segments and keep the arithmetic honest.
Waste sits at 10% by default with a 5–20% override per line. It is applied to gallons, so it covers what stays in the tray, the hose and the roller sleeve, the mis-tint, and the touch-up kept back for the punch list. It does not cover a second product, a primer coat, or the extra material a rougher-than-expected substrate will drink.
Specialty items convert to square feet before anything else happens. A door defaults to 42 SF, which is a 3'0" × 7'0" leaf painted on both faces (21 SF × 2). Trim converts linear feet and a face width in inches to square feet. The other defaults — door frames at 10 SF, window frames at 6 SF, the trim face widths — are the calculator's starting values with no published basis behind them. Measure a representative one on the drawings and overwrite them.
What this output must not be used for
These are ordering and pricing quantities, not a specification. Do not bid from them without verifying dimensions against the drawings and the coverage rate against the data sheet for the exact product and sheen. The calculator does not read a specification, does not decide whether a primer, sealer or block filler is required, does not check a required dry film thickness, and does not verify VOC or emissions compliance for the project. Every one of those can change the gallon count, and two of them can fail an inspection.
Frequently asked questions
It depends on the product, the sheen and the surface. Benjamin Moore's data sheet for Ultra Spec 500 Interior Eggshell gives 350 to 400 square feet per gallon at a recommended 4.3 mils wet film thickness; the flat sheen of the same line gives 400 to 450 square feet per gallon at 3.6 to 4.0 mils wet. Both sheets qualify the figure with "depending on surface texture and porosity". Those are one coat on a sound, smooth, previously finished surface. Do not price a job off a generic number — open the data sheet for the exact product and sheen you are specifying and enter that rate.
Five times the one-gallon rate on the product's data sheet, for one coat. At 350 square feet per gallon that is about 1,750 square feet per coat, and at 400 square feet per gallon about 2,000. Two coats halve it. A pail is also the point at which rounding starts to cost real money: this calculator sums the unrounded gallons for a product line, rounds up once, then splits the result into 5-gallon pails and 1-gallon cans, so a line that lands at 10.2 gallons buys two pails and one can rather than three pails.
Take the surface area, multiply it by the number of coats, divide by the coverage rate on the product data sheet, and add a waste allowance. For a room painted wall to wall the area is (2 x length + 2 x width) x height. This calculator does that per area and per product line, but it does not deduct door and window openings, so on a heavily glazed elevation you should enter net wall segments rather than a full perimeter. Verify quantities against the drawings before you order or bid.
Material and labour are estimated off two different square-foot numbers, which is the mistake worth avoiding. Surface area is the area painted once. Coat area is that surface multiplied by the number of coats, and it is the number gallons are derived from. A 700 square foot room in two coats is 700 square feet of surface and 1,400 square feet of coat area. This calculator reports both. It gives you quantities only — it does not price labour, and it is not a substitute for checking the specification for primer, block filler or a required dry film thickness.
Far less than wall paint, and on bare block the first product is usually not paint at all. Benjamin Moore rates its Ultra Spec Hi-Build Masonry Block Filler at 75 to 100 square feet per gallon at 16 to 21 mils wet, against 350 to 400 for the same brand's interior wall paint. Open-face concrete block has to be filled before a topcoat rate means anything. Price the filler as its own line at its own published rate rather than relying on a coverage multiplier.
Coverage rates and film thicknesses on this page are taken from manufacturer technical data sheets: Benjamin Moore Ultra Spec 500 Interior Eggshell (N538), Ultra Spec 500 Interior Flat (T535), and Ultra Spec Hi-Build Masonry Block Filler (571). They are cited as examples of what a professional interior latex and a block filler are rated at, not as the product you should be pricing. Substrate multipliers, the specialty-item default areas and the waste percentages are this calculator's own assumptions and are labelled as such above.