Concrete Calculator
Figure the cubic yards and 80-lb bags a slab needs from its dimensions, with a waste allowance — and when to stop counting bags and call the truck.
4 in for patios and walkways; 5–6 in for driveways.
Covers uneven subgrade and spillage. 10% is standard.
At 74 bags this is ready-mix truck territory. Mixing a yard or more by hand is a genuine endurance event, and the truck is often cheaper than the bags once you price your afternoon.
Volume first, then a reality check
The math is straightforward — length times width times thickness gives cubic feet, twenty-seven of those make the cubic yard concrete is sold by. The judgment call is what comes after the number, and it splits at roughly one yard.
Below it, bags are sensible. Above it, the calculator says so plainly, because forty-plus bags is not a shopping quantity — it is two tons of material on a schedule set by chemistry.
What bag math actually commits you to
Each 80-lb bag needs hauling twice — cart to car, car to site — then mixing, placing and screeding inside concrete’s working window. A yard of bag-mix concrete is a full, hard day for two people with a rented mixer. Price the ready-mix truck before defaulting to bags; once you count the mixer rental and your Saturday, the truck’s short-load fee often wins.
The pour is unforgiving in one specific way
Order the waste-included figure, always. Every other estimating mistake on this site costs a second trip to the store — this one does not, because concrete placed an hour apart does not bond. The seam where a fresh batch meets a half-set pour is called a cold joint, it is structural, and it is forever. One extra bag returned beats one bag short every time.
Same tape measure, gentler stakes — the mulch and gravel calculator runs identical volume math for the landscape half of the weekend, and the paint calculator covers the walls once the slab cures.
How this is calculated
Volume (cu ft) = length × width × (thickness in inches ÷ 12) Cubic yards = volume ÷ 27, plus the waste allowance 80-lb bags = volume ÷ 0.6 cu ft per bag
Frequently asked questions
- How many bags of concrete do I need?
- An 80-lb bag yields about 0.6 cubic feet, a 60-lb bag about 0.45. A modest 10×12 patio at 4 inches thick needs roughly 45 bags of 80-lb mix once waste is counted — which is around 3,600 pounds of material to haul, mix and place before it sets.
- How thick should my slab be?
- Four inches handles patios, walkways and shed pads. Driveways want five to six, and anything carrying vehicles also wants reinforcement and a proper gravel base. Thickness is the input that moves the total fastest — going 4 to 6 inches is a 50% jump in material.
- When should I order a ready-mix truck instead of bags?
- The usual crossover is around one cubic yard — roughly 45 bags. Past that, mixing by hand becomes an endurance event with a clock running, since concrete does not pause while you rest. Trucks carry short-load fees below about 3 yards; the calculator flags when you are in that conversation.
- Why add a waste factor?
- Subgrades are never perfectly level, forms bow slightly, some mix stays in the mixer and wheelbarrow. Ten percent is standard. Running short mid-pour is the one failure with no good fix — a cold joint where the fresh batch meets the half-set one is permanent.