How to Calculate Concrete for Any Project
I built this concrete calculator because getting the volume wrong is an expensive mistake — too little concrete means a second trip to the store mid-pour, and too much means you're stuck with heavy surplus bags. Whether you're filling post holes for a deck, pouring a slab for a garage, or setting fence posts, the principle is the same: calculate the volume in cubic feet or cubic metres, then convert to bags.
For rectangular slabs, multiply length × width × depth (all in the same unit). For cylindrical post holes, use π × radius² × depth. This calculator handles both shapes and converts directly to bag counts for common sizes (40 lb, 60 lb, and 80 lb bags in imperial; 20 kg and 25 kg in metric). Always add 5–10% to your volume estimate to account for uneven subgrades and spillage.
Choosing the Right Concrete Mix
Not all concrete mixes are equal, and using the wrong one is a common mistake. Here's a quick guide:
- General purpose (2500–3000 PSI): sidewalks, patios, and steps not subject to heavy loads.
- High strength (4000–5000 PSI): driveways, garage slabs, structural footings, and cold climates where freeze-thaw cycles stress the concrete.
- Fast-setting mix: post holes and fence posts — you pour it dry, add water, and it sets in 20–40 minutes without mixing.
- Fiber-reinforced mix: reduces surface cracking on slabs and is a good upgrade for patios in hot, dry climates.
Practical Tips for Pouring Concrete
Timing matters a lot. Avoid pouring concrete in temperatures below 4°C (40°F) or above 32°C (90°F) without taking special precautions. Cold concrete sets too slowly and can freeze before curing; hot weather causes rapid evaporation that weakens the surface. If you must pour in extreme temperatures, use insulating blankets in cold weather and wet curing compounds or shade in hot weather.
For slabs larger than about 10 × 10 feet, consider renting a concrete mixer rather than hand-mixing bags. Consistency of mix across a large pour affects strength and appearance significantly. Large slabs (over 100 sq ft) are often better handled with ready-mix concrete ordered by the cubic yard from a supplier, which is more economical above about 1 cubic yard.
Frequently Asked Questions
How many 80 lb bags of concrete do I need per cubic foot?
One 80 lb bag of standard concrete mix yields approximately 0.60 cubic feet of mixed concrete. So for one cubic foot of finished concrete you'll need roughly 1.67 bags — in practice, order 2 bags per cubic foot to be safe. An 80 lb bag covers a 2" deep pour over about 3.6 square feet.
How thick should a concrete slab be?
For foot traffic and light use (patios, walkways) a 4-inch slab is the standard minimum. Garage floors and driveways that bear vehicle loads should be 5–6 inches thick. Slabs subject to heavy equipment should be at least 6 inches and should include rebar or wire mesh reinforcement. Going thicker than necessary wastes material, but going thinner risks cracking under load.
Do I need to add rebar or wire mesh?
For most residential slabs (patios, pathways), wire mesh or fiber reinforcement is sufficient to control cracking. For structural slabs, driveways, or any slab carrying vehicle weight, rebar at 12–18 inch spacing is recommended. Rebar should be positioned in the middle third of the slab depth — use plastic rebar chairs to keep it elevated off the subgrade during the pour.
