Concrete Calculators

Concrete is measured in one place and specified in another, and the gap between those two is where most of the mistakes live.

The calculators

• Concrete Block CalculatorNominal module from CMHA CMU-TEC-001-23, mortar types from ASTM C270
• Concrete CalculatorBag yields from the data sheets, strength from IRC Table R402.2
• Concrete Slab CalculatorIRC Section R506 in full: base course, vapor retarder and where the steel sits
• Rebar CalculatorBar table from TxDOT Item 440, checked against its own geometry

Measuring is easy. Length times width times thickness, divided by 27, and you have cubic yards. Every calculator on the internet does that much, and it is not wrong.

Specifying is the other half. When you ring a ready-mix supplier they do not ask how many yards and stop. They ask what strength, and whether you want air. Those come out of the code, they depend on what the concrete is doing and where you live, and a volume figure answers neither.

Three numbers that cannot be published nationally

This is the honest limit of a concrete calculator, and the pages here say so rather than guessing:

• Frost depth. Footings go below it. It is set by local jurisdiction and runs from nothing to over 100 in across the United States. There is no national figure and anybody who gives you one is inventing it.
• Soil bearing capacity. Footing width comes from it. It is a property of the dirt under your specific site.
• Weathering potential. IRC Table R301.2(1) is a map with jurisdiction-level values, and it is the input that decides your required strength and whether air entrainment is mandatory.

All three take one phone call to the building department, and none of them can be read off a web page. Where a page here needs one, it asks you for it instead of assuming.

What is genuinely fixed

A cubic yard is 27 cubic feet. That is a definition. Bag yields are printed on the manufacturers' own data sheets, and they turn out to be remarkably consistent: every Quikrete bag size is exactly 0.0075 cubic feet per pound of dry mix, so a cubic yard is 3,600 lb whatever size bag you carry it in. Those figures can be quoted precisely and checked against the document, which is what the calculators here do.

Strength is not the same thing as durability

The requirement people miss is not a psi. It is air entrainment: deliberate microscopic voids, 5 to 7 percent by volume, that give freezing water room to expand. IRC Table R402.2 footnote d makes it mandatory for exterior walls, porches, steps and garage slabs in moderate and severe weathering areas, and no amount of extra compressive strength substitutes for it. A 4,000 psi mix with no entrained air will still spall the face off a porch in a few winters.

It is also the requirement bagged mix cannot evidence, because the data sheets do not publish an air content at all.