Batch a mix from scratch — portland, sand, stone and water in real bags and tons. If you want to know how many yards to order, that is a different question.
Proportions are by volume of dry loose material, which is how the trade quotes them. One 94 lb bag of portland is almost exactly one cubic foot, so a "part" is easy to batch by the bag.
Cement is the grey powder that binds. Concrete is what you get after adding sand, stone and water. In a normal mix the cement is about a seventh of the volume, so the two words are not interchangeable and the numbers are nowhere near each other.
Most people searching for a cement calculator want concrete. If you are ordering for a slab, that is the concrete calculator. This page is for when you are batching the mix yourself out of separate materials — a small pour, a remote site, or a job where you want to control the mix.
Pile up a cubic foot each of cement, sand and stone and you do not get three cubic feet of concrete. Sand drops into the gaps between the stones, cement paste fills the gaps between the sand grains, and the trapped air disappears.
The standard allowance is about 1.54 cubic feet of dry loose material per cubic foot of finished concrete. That bulking factor is built into every figure on this page, which is why the dry ingredients always add up to more than what you asked for.
Ready-mix suppliers talk about a five sack or six sack mix, meaning bags of cement per cubic yard. It is the same information as a volume ratio, coming from the other direction.
| Ratio | Bags / yd | Strength |
|---|---|---|
| 1 : 1½ : 3 | 7.6 | 4500 psi |
| 1 : 2 : 3 | 6.9 | 4000 psi |
| 1 : 2 : 4 | 5.9 | 3000 psi |
| 1 : 3 : 5 | 4.6 | 2500 psi |
| 1 : 3 : 6 | 4.2 | 2000 psi |
A 1:2:4 mix works out to 5.9 bags a yard, which is why the trade calls it a six sack mix. Strengths are nominal at 28 days with good practice and clean materials.
Strength is governed more by the water-cement ratio than by anything else in the mix. Around 0.50 by weight is a sensible general figure — for a 94 lb bag that is about 47 lb of water, close to 5⅔ gallons.
Extra water makes concrete easier to place and dramatically weaker. Going from 0.50 to 0.60 can cost a quarter of the compressive strength. If it is too stiff to work, the answer is a plasticiser or a different mix, not the hose.
About 6 for a 1:2:4 mix and about 7 for 1:2:3, using 94 lb bags.
Usually yes, and for anything under half a yard you should — 60 lb premix bags yield 0.45 cubic feet each, so a yard is about 60 bags. Batching from raw materials pays off on volume, on remote sites, or when you need a mix the shelf does not carry.
Damp sand bulks up substantially — moist sand can occupy a fifth more volume than the same weight dry, which throws off volume batching. Either allow for it or batch by weight if the mix matters.