Where Our Coverage and Waste Numbers Come From

Manufacturer-published figures

These come directly off a spec sheet, a bag label or a box, and this site's job is to apply them correctly, not to re-derive them.

• Paint coverage, roughly 350-400 sq ft per gallon on a smooth, previously-painted wall — printed on the can, and the figure the paint and ceiling paint calculators start from.
• Nail count per pound by size and type — 8d common at roughly 106 per pound, 12d common at roughly 69, 16d common at roughly 49, 16d sinker at roughly 60 — standard figures printed on nail box labels and used directly by the nail calculator.
• Bagged concrete yield per bag size — a nominal cubic-foot figure printed on the bag for a properly hydrated mix, used by the concrete slab, post and steps calculators before any water-ratio adjustment.
• Backer board sheet size — commonly a 3 by 5 foot module rather than the 4 by 8 foot module most other sheet goods use, which is a manufacturer packaging choice specific to that product category.
• Epoxy coating coverage per kit at a stated mil thickness — epoxy floor coverage and cure time explained has the actual sensitivity of that figure to film thickness.

Code-derived figures

These trace to a building code requirement or a common amendment of one, which means they are legal minimums in most jurisdictions rather than engineering optimums, and a local inspector's specific requirement can differ from the common baseline used here.

• Ice barrier membrane extending a set distance up-slope from the eave, commonly required to reach at least 24 inches inside the building's exterior wall line, used by the ice-and-water shield calculator as a common minimum rather than a universal one.
• Continuous electrical loads sized at 125% of their rating, with a breaker only loaded to 80% of its rating for a continuous load — the pairing that drives the generator, EV charger and radiant floor circuit calculators, and a figure that is not adjustable by preference the way a waste percentage is.
• Attic net free ventilating area, commonly required as a ratio against the attic floor area rather than as a flat vent count — used by the attic ventilation calculator, which treats the ratio as a starting requirement to size vent area from, not a suggestion.
• Frost-depth-driven footing depth, which varies by region rather than being one national number — the deck footing and concrete post calculators take frost depth as a local input precisely because this site cannot know your jurisdiction's figure for you.

Anywhere a calculator touches one of these, it carries a code-critical notice above the tool, because a wrong number here is a different kind of wrong than a wrong number on a waste percentage — one fails an inspection or a load, the other just means a second trip to the store.

Trade-convention figures

These are the numbers a trade has settled on through practice rather than a single citable source, and they are the tier this site is most careful to label honestly as convention rather than dressing up as measurement.

• 6.86 bricks per square foot for modular brick at a 3/8 inch mortar joint — actually derived on this site from the brick module's own dimensions, and cross-checked against the figure brick suppliers commonly quote, which is why the brick calculator states both the derivation and the match.
• 10% as a starting waste allowance across many materials, adjusted up or down by the causes explained in waste factors by material — a genuinely common trade default, not a universal law.
• 6 to 8 foot post spacing for a wood privacy fence, tighter in high-wind regions — settled by decades of practice against wind load and rail span rather than by a single spec, and the fence calculator treats it as an adjustable input rather than a fixed constant.
• Screw penetration of roughly two-thirds of its length into the receiving member — a long-standing carpentry rule of thumb rather than a code minimum, used as guidance rather than as a hard cutoff by the fastener-related calculators.
• 3/8 inch as the default mortar joint width across masonry calculators — the width modular brick is dimensioned around, and the most common joint in the trade, though 1/4 to 1/2 inch joints are all genuinely used.

Where this site does its own arithmetic instead of quoting a source

A handful of figures on this site are not looked up anywhere — they are computed from geometry that is checkable by anyone with the same inputs. The 6.86 bricks per square foot figure above is one: it comes from dividing 144 square inches by the brick module's own area, not from a supplier's table, even though it happens to match one. The pitch multiplier used across every roofing calculator is another — it is the Pythagorean relationship between a roof's rise and run, true by definition rather than by convention, and it is the same for every roof at a given pitch regardless of manufacturer or region.

These derived figures are, if anything, the most trustworthy tier on the site, precisely because they don't depend on anyone's claim — they depend on arithmetic anyone can redo. How to read a material spec sheet and coverage rates explained cover the tier above this one: figures that do depend on a manufacturer's claim, and how to read that claim correctly.

How to check any of these yourself

Every calculator on this site states its sourcing on the page itself, in a sources section beneath the formula, naming which of the three tiers above the figure sits in. If a figure looks wrong for your specific product or region, the sourcing note tells you whether it's worth adjusting the calculator's input (a manufacturer figure that your specific product's spec sheet states differently), checking your local code office (a code-derived minimum that your jurisdiction amends), or simply treating it as a starting point rather than a fixed answer (a trade convention that your situation genuinely differs from). Contact us if a figure looks wrong for a product you can point to directly — that is the one category of correction this site actively wants to hear about.