Cement Calculator

First find the volume of the pour: length times width times thickness for a slab or footing, or pi over 4 times the diameter squared times the depth for a round column or post hole. That gives the wet volume of concrete.

Multiply the wet volume by a dry factor of about 1.54 to allow for the voids and shrinkage as the materials pack together, then split that dry volume between cement, sand and gravel according to your mix ratio. For a 1:2:3 mix there are 6 total parts, so cement is one-sixth of the dry volume.

Finally, divide the cement volume by the bag size — a 94 lb bag of portland cement is 1 cubic foot — and round up to whole bags. Add 5 to 10 percent for waste.

The ratio is parts of cement to sand to gravel by volume. A 1:2:3 mix is the classic all-purpose concrete, good for slabs, driveways and patios and reaching roughly 2,500 to 3,000 psi. For higher strength, a 1:1.5:3 mix uses more cement; for less demanding work like a base course, a leaner 1:3:6 is cheaper.

More cement per part of aggregate means stronger, but also more expensive and more prone to shrinkage cracking if not cured well. The water-to-cement ratio matters just as much as the aggregate ratio — around 0.5 by weight gives workable, durable concrete.

Mortar is different: it has cement and sand but no gravel, typically 1:3 for render or screed and 1:4 for laying brick and block.

For a standard 1:2:3 mix, a cubic yard of concrete needs roughly 7 bags of 94 lb portland cement when you batch it yourself from cement, sand and gravel. Richer mixes use more and leaner mixes use fewer.

A cubic yard is 27 cubic feet of finished concrete. Multiplying by the 1.54 dry factor gives about 41.6 cubic feet of dry material; for a 1:2:3 mix one-sixth of that is cement, which is about 6.9 cubic feet — and since a 94 lb bag is 1 cubic foot, that is about 7 bags.

If you buy premixed concrete in bags instead, an 80 lb bag yields about 0.6 cubic foot, so a cubic yard takes about 45 of them — which is why ready-mix or batching your own is usually cheaper for large pours.

Cement is the binder — a fine gray powder, usually portland cement, that reacts with water and hardens. On its own it is rarely used structurally; it is an ingredient in the other two.

Concrete is cement plus sand plus gravel (coarse aggregate) plus water. The gravel gives it compressive strength and bulk, making it the material for slabs, footings, columns and driveways. Mortar is cement plus sand plus water, with no gravel, so it is smoother and used to bond bricks and blocks or to render and screed.

So cement is the glue, concrete is the structural material, and mortar is the jointing material. This calculator handles both concrete mixes and mortar mixes.

A water-to-cement ratio of about 0.5 by weight is a good starting point for workable, durable concrete. A 94 lb bag of cement therefore takes roughly 47 lb of water, which is about 5.6 gallons.

Lower water ratios make stronger, more durable concrete but a stiffer, harder-to-place mix; higher ratios are easier to pour but weaker and more prone to cracking and surface problems. The ratio counts all the water, including moisture already in damp sand.

Add water gradually and stop when the mix is plastic and workable rather than soupy. Too much water is one of the most common causes of weak, dusty concrete, so err on the dry side and adjust.

It depends mostly on the volume. For small jobs — a few post holes, a small pad — buying premixed bags of concrete and adding water is simplest and often cheapest once you account for your time and a mixer rental.

For medium jobs, batching your own from bulk cement, sand and gravel is usually the lowest material cost, but it is labor-intensive and you need to get the proportions and mixing right. For large pours, a ready-mix truck is normally the best value and quality, since a yard or more of hand-mixed concrete is a lot of work and risks cold joints if you can’t place it fast enough.

Most ready-mix suppliers have a one-yard minimum and short-load fees, so compare the all-in cost for your specific volume.

Four inches is the standard thickness for most residential slabs — patios, walkways, sheds and garage floors that carry normal foot traffic and light vehicles. For driveways that carry heavier vehicles, 5 to 6 inches is better, and heavy loads may call for more plus reinforcement.

Thickness has a big effect on how much concrete you need, since the volume scales directly with it: going from 4 to 6 inches adds 50 percent more concrete. It also drives strength far more than a small change in mix ratio does.

Also plan a compacted gravel base beneath the slab and control joints to manage cracking. For anything structural or load-bearing, follow an engineer’s design and your local building code rather than a rule of thumb.

The 1.54 factor converts the finished wet volume of concrete into the dry volume of loose materials you need to buy. When dry cement, sand and gravel are mixed with water, the fine particles fill the gaps between the larger ones and the whole mix compacts, so the materials take up more space dry than the concrete does once placed.

In other words, about 1.54 cubic feet of dry ingredients yields roughly 1 cubic foot of solid concrete. The exact factor varies a little with the aggregates, but 1.54 is the standard figure for concrete; mortar, which has finer material and no gravel, uses about 1.33.

This is why simply adding up the cement, sand and gravel by their wet volume would leave you short — the dry factor accounts for the voids that disappear during mixing.