Concrete Calculator
Volume is arithmetic. The decision that actually costs people a Saturday is whether to buy bags or order a truck, and no calculator tells you. One cubic yard is forty-five 80 pound bags — about a ton and a half of mixing by hand. This calculator gives the volume, the bag count, and a straight answer on which side of that line you are on.
- Concrete needed
- 1.36 cubic yards 36.67 ft³ including 10% waste — 10 ft × 10 ft × 4"
- Order ready-mix
- 1.5 yd³ That is 62 bags of 80 lb mix. Nobody mixes that by hand — most suppliers have a 1 yd³ minimum plus a short-load fee, and it is still cheaper and more consistent.
- Volume before waste
- 33.33 ft³ 1.23 cubic yards
- 80 lb bags
- 62 0.6 ft³ each
- 60 lb bags
- 82 0.45 ft³ each
- Approximate weight
- 5,317 lb Cured concrete weighs about 145 lb per cubic foot — check what your forms and subgrade are carrying
The bag-to-truck decision
Somewhere around half a cubic yard, bagged concrete stops being sensible, and the reason is labour rather than cost per yard.
A cubic yard is 27 cubic feet, which is 45 bags of 80 pound mix — 3,600 pounds to carry, open, and mix in batches. At a realistic pace of two or three bags per batch and several minutes per batch, that is a full day for one person and a hard one for two.
The quality problem is worse than the labour. Batches mixed by hand vary in water content, and water content is the single biggest determinant of concrete strength. A slab poured over four hours in twenty separate batches has twenty different strengths and cold joints wherever one batch set before the next arrived.
Ready-mix arrives as one consistent batch, placed in minutes rather than hours, and is usually cheaper per yard even after the short-load fee that most suppliers charge below their minimum — commonly one yard.
Bags remain the right answer for post holes, small pads and repairs, where the total is under half a yard and the pour is naturally divided anyway.
Why running short is worse than running over
Concrete has a working window, and it closes on its own schedule regardless of how the job is going.
If fresh concrete is placed against concrete that has already begun to set, the two do not bond. The result is a cold joint: a plane of weakness through the pour that will crack there eventually, and that transmits water in a slab or a foundation.
In warm weather that window is short — under an hour in hot dry conditions. A trip to the supplier for four more bags is enough to create one.
The asymmetry is stark. Excess concrete costs you the price of the excess and somewhere to put it, and a spare form or a couple of post holes solves that. A cold joint through a footing or a slab is permanent and cannot be repaired without demolition.
This is why every experienced pour orders over, and why the waste allowance here defaults to ten percent rather than zero.
What the volume calculation does not tell you
Volume is arithmetic. Everything that determines whether the concrete performs is separate from it.
Strength is specified as compressive strength at 28 days, commonly 2,500 to 4,000 psi for residential work. Bagged mixes state theirs on the bag; ready-mix is ordered to it. Higher is not automatically better — high-strength mixes are stiffer to place and can crack more from shrinkage.
Water is the variable that ruins pours. Adding water makes concrete easier to place and directly weakens it: the water-to-cement ratio governs strength, and a mix loosened on site to make finishing easier can lose a third of its rated strength.
Air entrainment matters wherever concrete freezes. Entrained air gives water somewhere to expand into during freeze-thaw cycles, and exterior flatwork in cold climates without it spalls within a few winters.
Curing is the step most often skipped. Concrete gains strength by hydration, not by drying, and it needs to stay moist for days. Concrete that dries out in the first week never reaches its rated strength no matter what was in the mix.
What this is based on
- Standard bag yields: 40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³
- Cured normal-weight concrete at approximately 145 lb per cubic foot
Volume only. This calculator does not address mix strength, reinforcement, curing or frost depth. Structural elements should be designed to the IRC or by a qualified engineer.
Frequently asked questions
How many 80 lb bags are in a cubic yard?
Forty-five. An 80 pound bag yields about 0.6 cubic feet and a cubic yard is 27 cubic feet. That is roughly 3,600 pounds of material to carry and mix, which is why most people switch to ready-mix well before a full yard.
When should I order ready-mix instead of bags?
Around half a cubic yard is the practical crossover. Below that, bags are convenient and the mixing is manageable. Above it, ready-mix is usually cheaper per yard, far more consistent, and finishes in one continuous pour rather than in batches that set at different rates.
Why add a waste allowance?
Because the hole is never exactly the size you dug it. Subgrade dips, forms bow slightly, some concrete spills and post holes are always over-excavated. Ten percent is normal for a formed slab; twenty is reasonable for holes in rough ground.
Does this include reinforcement?
No. Rebar and mesh displace a negligible amount of concrete and are not deducted. Sizing and spacing of reinforcement is a separate structural question.
Is running short or over worse?
Running short, by a wide margin. A cold joint forms where fresh concrete meets concrete that has begun to set, and it is a permanent weak plane. Order slightly over and have somewhere to put the excess.