How Many Bags of Concrete Do I Need?
Concrete is sold by the bag, but you measure the job by volume. Here is how to turn cubic feet into a bag count - and how to know when you have crossed the line into ready-mix territory.
By the Calculate My Reno Team / Published
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Open the Concrete Calculator
Concrete bags are priced by weight but used by volume, so the job is converting your pour into a bag count - and adding a little for waste.
The formula
bags = ceil( volume × wastage ÷ bag yield )
Work out the volume of your slab, footing or pad (length × width × thickness), add a wastage allowance for spillage and an uneven base, then divide by the yield of one bag. The concrete calculator does all of it, including odd shapes like circles and columns.
Bag yields
| Bag | Yield (set concrete) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | ~90 |
| 60 lb | 0.45 cu ft | ~60 |
| 80 lb | 0.60 cu ft | ~45 |
(Metric: a 20 kg bag yields roughly 0.009 m³, a 25 kg bag about 0.0115 m³.)
Worked example
A 10 ft × 10 ft slab, 4 inches thick, 80 lb bags:
- Volume: 10 × 10 × (4 ÷ 12) = 33.3 cu ft (about 1.23 cubic yards).
- Add 10% for an uneven sub-base: about 36.7 cu ft.
- Bags: 36.7 ÷ 0.60 = 62 bags.
Sixty-two bags is a lot of mixing - which brings up the next point.
Know when to stop bagging
Mixing concrete by hand is hard work, and bags cost more per unit than ready-mix. The practical switchover is about a cubic yard (~45 eighty-pound bags). Below that, bags win on flexibility; above it, ready-mix delivered by the truck is cheaper and far less effort. See Quikrete vs Sakrete vs ready-mix for that decision.
Tips
- Buy one or two spare bags - running out mid-pour leaves a cold joint in the slab.
- For footings and posts, the hole is what you fill - use the calculator’s circle/column shape and the hole diameter, not the post size. See how much concrete for a fence post.
- A standard concrete mix is fine for slabs; use a fast-setting mix for posts.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10 ft × 10 ft slab at 4 inches thick is about 33 cubic feet (1.23 cubic yards). At 0.60 cu ft per 80 lb bag that is around 56 bags before wastage - past the point where ready-mix is usually easier and cheaper. Add ~10% for an uneven sub-base. Enter your exact slab in the concrete calculator for the precise count.
How much does a bag of concrete cover?
By volume yield: a 40 lb bag makes about 0.30 cubic feet of set concrete, a 60 lb bag about 0.45 cu ft, and an 80 lb bag about 0.60 cu ft. How much area that covers depends on your thickness - 0.60 cu ft is a 2 ft × 2 ft pad at roughly 1.75 inches thick.
How many bags of concrete make a cubic yard?
A cubic yard is 27 cubic feet, so about 45 bags at 80 lb, 60 bags at 60 lb, or 90 bags at 40 lb. Mixing that many by hand is a serious job, so a cubic yard is roughly where ready-mix delivery becomes the better option.