How we verify the math
Last updated: May 2, 2026
Every number on this site comes from somewhere. Manufacturer spec sheets, building code data, or our own field measurements. When two sources disagree, we use the more conservative number, the one that has you buying slightly more material, not slightly less. This page lists the references behind every calculator.
Concrete
Bag yields come from Quikrete's published technical data: 0.6 ft³ per 80 lb bag, 0.45 ft³ per 60 lb, and 0.30 ft³ per 40 lb. Sakrete's spec sheets show the same numbers within a few percent. PSI ratings (3,000 / 4,000 / 5,000) follow ACI 318 conventions.
Pavers and patio base
Paver dimensions and pattern coverage come straight from Belgard and Pavestone product sheets, both public, both regularly updated. Base depth recommendations follow ICPI (Interlocking Concrete Pavement Institute) field guidelines: 4 inches of compacted base for pedestrian patios, 6 inches for vehicle traffic, 1 inch of bedding sand on top.
Mulch and topsoil
Bagged-mulch math uses the standard 2 cubic foot bag size. The bulk conversion is the one every yard uses: 1 cubic yard = 27 cubic feet = 13.5 two-cubic-foot bags. Verified against bagged-product yields from the major retailers and delivery norms in Florida and the Northeast.
Fence posts
Hole-sizing rules come from the standard "three times the post width" convention used by every fence contractor we've asked. Depth follows the local frost line (see the next section). When in doubt, dig deeper. Frost heave will find a shallow post and lift it the first February it can.
Frost line and footings
Frost depth values are pulled from adopted state and county building codes, not National Weather Service historical averages. Local code numbers are typically 6 to 12 inches deeper than actual frost depth because they account for worst-case winters and worst-case soil exposure.
When in doubt, call your local building department. They have the only number that matters when you pull a permit.
Decking
Joist span tables come from the American Wood Council's published span charts (these are the same tables most building inspectors use). Composite span limits come straight from Trex and TimberTech installation manuals, composite is more flexible than PT lumber, so the spans are shorter.
Gravel and aggregate
Stone weights use the standard 1.5 tons per cubic yard for ¾-inch crushed stone, 1.35 t/yd³ for pea gravel, 1.3 t/yd³ for river rock. These match aggregate-supplier conversion charts across the Northeast and Florida. Driveway depth recommendations follow a layered approach: 4 inches of compacted #3 base, 2 inches of #57 surface stone for most cars, deeper for trucks and RVs.
When we disagree with a source
Sometimes a manufacturer's spec sheet rounds up; sometimes a code book is decades behind real-world practice; sometimes two credible sources contradict. Our rule: pick the answer that costs you a few extra dollars, not a wasted Saturday. If the conservative choice is a bag too many or an extra inch of base, that's the one we use.
Caught a mistake?
If you ever run our math against your project and we're off, email us at requests@materialmath.io. We fix the calculator the same day and credit the catch in the changelog. You can also use the contact form.