Every constant we assume

Material calculators disagree with each other, sometimes by a third or more. The reason is almost always a number neither of them shows you.

Why two calculators give different answers

Ask any material calculator how much gravel a driveway needs and it will do the same arithmetic: length × width × depth, then multiply by a density to convert volume into the tons a supplier actually sells. The geometry is not where they differ. The density is.

Published bulk densities for crushed aggregate run from roughly 95 to 135 lb per cubic foot depending on gradation, moisture and how compacted the material is when it is measured. All of those figures are defensible. Picking a different one is not an error — but not saying which one you picked is.

Here is what that choice does to one ordinary job: a 40 × 12 ft driveway at 4 inches, which is 160 cubic feet of material either way.

Same driveway, same arithmetic: 7.6 tons or 10.8 tons, a 42% difference, decided entirely by a constant that most calculators never print. At forty dollars a ton that is the gap between one delivery and two.

The spread inside each material

This is not only a difference between websites. It is a difference between real materials sold under the same name. Every figure below is carried on the calculator it belongs to, so you can pick the one that matches what your supplier is actually loading.

Every standard this site leans on

79 of our 87 calculators assert at least one value a visitor cannot derive — an ampacity, a span limit, a ventilation ratio, a density. Each of those names the standard it came from, with the edition, on the calculator's own page. Below is the same information the other way round: given a standard, which pages depend on it.

The edition is not decoration. NEC moved its ampacity table between 310.15(B)(16) and 310.16 across recent cycles, and the IRC has reorganised its span tables; a citation without an edition cannot be checked, which is the only thing a citation is for.

The 8 calculators that cite nothing

Some formulas assert nothing external. Area is length times width; there is no standards body for multiplication, and attaching an authoritative-looking reference to arithmetic that does not need one is worse than silence, because it teaches you to stop checking. These say so on their own pages:

• Baseboard Calculator — Perimeter arithmetic plus a waste allowance.
• Driveway Cost Calculator — Price per square foot is a market rate that moves with region, season, fuel and site access.
• Fence Calculator — Post spacing and picket width are choices you make, not values a standard sets.
• Flooring Calculator — Coverage per box is printed on the carton you are holding and changes between production runs of the same product.
• Roof Area Calculator — Geometry only — the sloped area is the footprint times the pitch multiplier.
• Roof Pitch Calculator — Pure trigonometry: the pitch multiplier is the hypotenuse of a right triangle with a 12 in base.
• Square Footage Calculator — This one asserts nothing.
• Wallpaper Calculator — Roll coverage and pattern repeat are properties of the specific paper, printed on its label.

Found one that is wrong?

Every figure here is checkable, which means some of them are checkable as wrong. If you have a supplier spec sheet, a code section or a test report that contradicts something on this site, send it — cited corrections get made and the page says what changed.

Reusing this page: the tables above are generated from the calculators themselves, so they stay current. Link to www.calcpit.com/constants/ rather than copying the figures, and you will not end up quoting a number we have since corrected.

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