Concrete Calculator

The product this calculator is modeled on lists many uses. Among them are foundation walls and footings, sidewalks, curbs, steps, ramps, and walkways. Others are appliance platforms, pipe and post footings, floor slabs, and patios. The list also covers pools, fish ponds, stepping stones, splashblocks, bird baths, riprap, slope protection, and driveway repairs. Three of those shapes, a flat slab, a footing, and a post hole, match the pours this calculator computes. ³

For a slab or a footing, the calculator treats the pour as a rectangular box: length times width times thickness. Length and width are entered in feet, but thickness is entered in inches. The calculator changes inches to feet first, using the rule that 12 inches equal 1 foot. ¹

For a round column or post hole, the calculator uses the cylinder formula instead. Volume equals pi times the radius squared times the height. That formula holds whether or not the cylinder stands perfectly straight. The calculator finds the radius by halving the diameter you enter. It then multiplies by the number of columns. ⁶

The waste allowance you enter is a percent, and a percent is simply the number 0.01. Whatever percent you type adds that many hundredths of the volume as a margin. It's not a separate unit of its own. That convention comes from the guide that governs how measurements are written and expressed. ²

Before you pour, stake out the planned area and remove sod or soil to the desired depth. Nail and stake forms securely in place, then tamp and compact the sub-base until firm. ³

Dampen the sub-grade before concrete is placed, and avoid standing puddles of water. Fill the form to its full depth in one continuous pass, since partial-depth fills can leave a cold joint. After the pour fills the form without air pockets, strike it off and float it right away. Use a straight board, moving it back and forth in a saw-like motion. Then cut control joints about 1 in deep every 6 ft to 8 ft. ³

Curing is one of the most important steps in concrete construction. Proper curing increases strength and durability, while a poor curing job can undo an otherwise well-placed pour. ³

Protect fresh concrete from freezing during its first 48 hours. Use plastic sheeting or insulation blankets if temperatures are expected to fall below 32°F. ³

A round slab and a round column are really the same shape, just poured in different directions. Volume works the same for either one: multiply pi by the radius squared, then by the height. That holds whether the cylinder stands up straight or lies on its side. ⁶

Some pours aren't a solid cylinder — a tube form, or a circular slab poured around a pipe or a knockout, leave a hole in the middle. For those, work out the volume of the whole outer circle first, then subtract the volume of the inner circle you aren't filling. That difference, times the height, is what you're actually pouring. ⁵

Concrete mix is cement and aggregate, pre-blended, built for general structural work. All you add is water. ³

Water is the only thing you add to the mix. Plan on roughly 6 to 9 pints of water for every 80 lb bag; for other bag sizes, use Table 2. ³

If you're mixing by machine, give it about 1 cubic foot of drum capacity for every 80 lb bag you load in at once. ³

An estimate like the one above assumes a flat, even sub-base. It doesn't add anything extra for waste or for a job site that isn't perfectly uniform. ⁴

Quantity and/or coverage calculations for a particular product can be found on that product's own data sheet. ⁴

This page doesn't quote a price. Cost per square foot, the total cost of a slab, and cost in general aren't in the sources behind this calculator — a NIST measurement standard and a QUIKRETE product data sheet — so no such figures appear here. ³

It also doesn't cover coloring your concrete, a "pros' magic number," when concrete was first invented, or how to work out a square column. The data sheet behind this page describes one general-purpose mix in inches, feet, and pounds — nothing about pigments, history, or square shapes. ³

As for concrete versus cement, or the types of concrete: the data sheet only says concrete is cement blended with aggregate for general structural work. It doesn't go further into how the two terms differ or list other types, so this page stops there too. ³

You may be looking for the difference between an online tool like this one and a physical construction calculator, how to add color to your concrete, how to figure a square column, the different types of concrete, or when concrete was invented. This page's sources are silent on all five. The closest they come is saying that the mix this calculator is built around is one pre-blended product of cement and aggregate for general structural use, not a lineup of types to choose between ³. Beyond that, none of it is covered here.