How Many Bags of Concrete Do I Need?
Calculate cubic yards and pre-mixed bags needed for slabs, footings, columns, or steps. Supports 40 lb, 60 lb, and 80 lb bags.
Project Type
How to Calculate Concrete Needed
To calculate concrete volume, multiply length × width × depth (all in feet) to get cubic feet. Divide by 27 to convert to cubic yards. For bags, divide cubic feet by the bag volume (0.45 cu ft for 60 lb bags).
For projects under 1 cubic yard, pre-mixed bags are the most practical option. For larger projects, ordering ready-mix concrete from a supplier is significantly cheaper per yard.
Standard Thickness by Project Type
| Project | Standard Thickness | Notes |
|---|---|---|
| Sidewalk / walkway | 4 inches | Foot traffic only |
| Patio | 4 inches | Add rebar or mesh for stability |
| Driveway (cars) | 4–6 inches | 6 inches for regular vehicle traffic |
| Driveway (RV/trucks) | 6–8 inches | Heavier loads need more thickness and reinforcement |
| Shed foundation | 4 inches | Check local code for load requirements |
Calculating Footings for Posts
Post footings use a different formula since they're cylindrical, not rectangular: Volume (cubic feet) = π × radius² × depth. A typical fence post footing is 10 inches (0.83 ft) in diameter — so radius is 0.42 ft — and 24 inches (2 ft) deep. Volume = 3.14 × 0.42² × 2 ≈ 1.1 cubic feet per hole. For 10 posts, that's about 11 cubic feet, or roughly 0.4 cubic yards — about six 60-lb bags.
Concrete Bag Sizes and Coverage
| Bag Size | Weight | Coverage at 4" depth |
|---|---|---|
| Small bag | 40 lb | ~0.30 cu ft |
| Standard bag | 60 lb | ~0.45 cu ft |
| Large bag | 80 lb | ~0.60 cu ft |
For large pours (over 1 cubic yard), ordering ready-mix concrete by the yard is more cost-effective than bags. One cubic yard = 27 cubic feet = about 45 bags of 80-lb concrete. Ready-mix concrete costs $125–$175 per cubic yard delivered, vs. $4–$6 per bag ($180–$270 per yard equivalent). Minimum delivery quantities vary by supplier — most have a 1 yard minimum, some charge extra for partial loads.
When to Use Bagged vs. Ready-Mix Concrete
Bagged concrete is ideal for small projects: fence posts, mailbox bases, stepping stones, small slabs under 50 sq ft, and repair work. Ready-mix is better for larger slabs, driveways, patios over 100 sq ft, and any pour requiring consistent batch-to-batch color. Ready-mix also lets you pour larger areas continuously — multiple batches of bagged concrete mixed at different times can show slight color and consistency variations in the finished slab.
Common Concrete Ordering Mistakes
- Mixing up cubic feet and cubic yards — 1 cubic yard = 27 cubic feet. Ordering "10" when you meant cubic yards but the supplier heard cubic feet (or vice versa) is a very common and very expensive mistake on delivered/ready-mix orders.
- Treating post holes as cylinders when they taper — Auger holes are rarely a perfect cylinder top to bottom. Measure the actual average diameter, not just the top, or you'll consistently overestimate.
- Assuming all bags yield the same — An 80 lb bag yields about 0.6 cubic feet, a 60 lb bag about 0.45 cubic feet — mixing bag sizes in your count without adjusting the math leads to a short order.
- No waste buffer on a delivered pour — Ready-mix trucks charge a short-load fee if you come up short and need a second small delivery. Order 5–10% extra rather than cutting it exact.
- Pouring in bad weather — Concrete poured below 40°F or above 90°F cures unevenly and can crack. Check the forecast for the full 48–72 hour initial cure window, not just pour day.
Frequently Asked Questions
How do I calculate cubic yards of concrete for a slab?
Multiply length (ft) x width (ft) x thickness (ft — divide inches by 12), then divide by 27 to convert cubic feet to cubic yards.
How many 80 lb bags of concrete equal one cubic yard?
About 45 bags of 80 lb pre-mixed concrete make one cubic yard. For 60 lb bags it's about 60 bags; for 40 lb bags, about 90 bags.
How thick should a concrete slab be?
A standard patio or walkway slab is 4 inches thick. Driveways carrying vehicle weight should be at least 6 inches, often with reinforcement.
Should I order ready-mix or use bags for a small project?
For projects under about 1 cubic yard, bagged concrete is usually more practical since most ready-mix trucks have a 1-yard delivery minimum.