Concrete Mix Calculator
Calculate concrete volume for slabs, footings, or post holes. Results include cubic yards and the number of 60lb or 80lb bags.
Recommended Concrete Thickness by Application
The thickness of your pour is as important as the mix. Too thin and it cracks; too thick wastes money. Use this guide alongside the calculator.
| Application | Thickness | ~80lb bags/sq yd | Notes |
|---|---|---|---|
| Patio / foot traffic | 4 in (10 cm) | ~7.5 bags/sq yd | Minimum for outdoor slabs |
| Driveway (passenger cars) | 4–5 in | ~7.5–9 bags/sq yd | Use 5 in for longevity |
| Driveway (trucks / RVs) | 6 in | ~11 bags/sq yd | Add rebar for heavy loads |
| Garage floor | 4–5 in | ~7.5–9 bags/sq yd | Slope 1/8 in/ft for drainage |
| Fence post hole | Post-dependent | 1–2 bags/post | 1/3 of post length underground |
| Structural footing | 8–12 in+ | Varies by load | Check local building codes |
Tips for a Successful Concrete Pour
- →Always add 10% extra. Running short mid-pour forces you to stop and mix more, creating a cold joint — a visible weak seam in the finished slab. Order more than you think you need.
- →Use the right water ratio. Adding too much water weakens the concrete significantly. Follow the bag’s instructions. A slump test (how far concrete settles in a cone) is the professional way to check consistency.
- →Cure properly. Keep concrete moist for at least 7 days after pouring. Cover with plastic sheeting or use a curing compound. Concrete reaches ~70% strength at 7 days and ~95% at 28 days.
- →Add rebar or mesh for structural pours. For driveways, footings, and any load-bearing slab, use steel reinforcing bar or welded wire mesh to prevent cracking under load.
Frequently Asked Questions
How do I calculate how much concrete I need?
For a rectangular slab: multiply length × width × depth (all in the same unit). For a circular form: use π × radius² × depth. This calculator handles all shapes and converts the result to cubic yards and bag count automatically.
How many bags of concrete per cubic yard?
An 80lb bag yields approximately 0.60 cubic feet of concrete, so you need about 45 bags per cubic yard. A 60lb bag yields ~0.45 cubic feet (60 bags/yd³). A 40lb bag yields ~0.30 cubic feet (90 bags/yd³). For large projects, ordering ready-mix concrete by the truck is far more economical.
Should I order extra concrete?
Yes — always add 10% to cover spillage, uneven sub-base depth, and mixing variation. For small or complex pours, consider 15%. Running short and having to do a second pour creates a visible cold joint (weak seam) in the finished concrete.
What is the difference between 60lb and 80lb bags?
80lb bags yield more concrete per bag and are more economical per unit volume. 60lb bags are lighter and easier to handle, which matters when mixing by hand. 40lb bags are for small repairs and patches. All three are typically Quikrete or similar pre-mixed concrete (cement + aggregate + sand).
When should I use ready-mix instead of bags?
Ready-mix concrete delivered by truck becomes economical at around 1 cubic yard (27 cubic feet) or more. Below that, bagged concrete is usually cheaper when you factor in delivery fees. For a driveway (typically 3–4 cubic yards) or larger, always get ready-mix.
What thickness should a concrete slab be?
Foot traffic / patio: 4 inches (10cm). Driveway (passenger cars): 4–5 inches. Driveway (trucks / heavy vehicles): 6 inches. Garage floor: 4–5 inches. Structural footings: check local building codes — typically 8–12 inches deep.
Is this concrete calculator free?
Completely free. Enter your dimensions and get your concrete volume and bag count instantly.
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