How Many Bags of Concrete Do I Need?
The conversion that decides your whole shopping trip: how much volume is in a bag.
An 80 lb bag of concrete mix yields about 0.60 cubic feet, so one cubic yard — 27 cubic feet — takes 45 bags. A 60 lb bag yields 0.45 cubic feet and takes 60 bags to the yard; a 40 lb bag yields 0.30 and takes 90.
Estimates only. Textured walls, high ceilings, sprayers and major colour changes can all increase the amount you need. Always buy slightly more than the calculation suggests.
Bags needed by slab size and bag weight
| Pour | Volume | Cubic yards | Bags | Cheaper |
|---|---|---|---|---|
| 4 x 8 slab at 4", 80 lb bags | 11.7 cu ft | 0.4 cu yd | 20 × 80 lb | Bags |
| 4 x 8 slab at 4", 60 lb bags | 11.7 cu ft | 0.4 cu yd | 27 × 60 lb | Bags |
| 4 x 8 slab at 4", 40 lb bags | 11.7 cu ft | 0.4 cu yd | 40 × 40 lb | Ready-mix |
| 10 x 10 slab at 4", 80 lb bags | 36.7 cu ft | 1.4 cu yd | 62 × 80 lb | Ready-mix |
| 10 x 10 slab at 4", 60 lb bags | 36.7 cu ft | 1.4 cu yd | 82 × 60 lb | Ready-mix |
| 20 x 20 slab at 4", 80 lb bags | 146.7 cu ft | 5.4 cu yd | 245 × 80 lb | Ready-mix |
10 % waste at 4 inches thick. The bag count scales exactly with bag yield: halving the bag weight doubles the count.
The yields, which are not negotiable
Every manufacturer publishes the same figures within a rounding error, because the mix is essentially the same. Learn these four numbers and you can estimate any bagged pour in your head:
- 80 lb bag — 0.60 cubic feet
- 60 lb bag — 0.45 cubic feet
- 40 lb bag — 0.30 cubic feet
- One cubic yard — 27 cubic feet
So a yard is 45, 60 or 90 bags depending on the size you buy. The bag size does not change the price per unit of concrete much — it changes how many trips you make and how much your back hurts.
The weight is the real problem
Most people underestimate this. A cubic yard of concrete weighs about 4,000 lb — two tons.
In 80 lb bags that is 45 bags, which is 3,600 lb of material you have to load, unload, carry, open and mix. And a yard is not a large pour: it is a 10x10 slab four inches thick.
Practical consequences:
- Do not plan to mix more than about 30 bags by hand in one session. That is 2,400 lb of lifting plus the mixing, and it takes most of a day. Beyond that, order ready-mix or hire help.
- A wheelbarrow holds about 6 cubic feet, which is ten 80 lb bags. That is your mixing batch, not the whole job.
- Check the vehicle. 45 bags is well over a tonne. It will not go in a normal car boot, and it may exceed the payload of a small van.
When bags stop making sense
Bags are the right answer for fence posts, small footings, patching and anything too small or too awkward for a truck to reach.
They stop making sense when:
- The volume passes about one cubic yard — that is where ready-mix becomes cheaper, because of the supplier's minimum billing.
- You cannot place it fast enough. This is the limitation people forget. Concrete must be placed, levelled and finished before it starts to set, which is roughly an hour after mixing. You cannot mix 60 bags one at a time and keep a single continuous pour — you will end up with cold joints between batches.
- The finish matters. A large slab poured in bag-by-bag batches never levels as well as one placed continuously from a truck.
Frequently asked questions
How many bags of concrete make a yard?
Forty-five 80 lb bags, 60 of 60 lb, or 90 of 40 lb. An 80 lb bag yields 0.60 cubic feet and a yard is 27 cubic feet.
How many 80 lb bags of concrete for a 10x10 slab?
About 62 bags at 4 inches thick with a 10 % waste allowance. The slab is 33.3 cubic feet and each bag yields 0.6 cubic feet.
How much does an 80 lb bag of concrete cover?
About 0.6 cubic feet. At 4 inches thick that is roughly 1.8 square feet per bag, so a 10x10 slab needs about 62 bags.
How many bags of concrete can I mix at once?
A wheelbarrow holds about 6 cubic feet, which is ten 80 lb bags. In practice, plan on no more than about 30 bags in a single session by hand before the work becomes unsafe and the pour quality suffers.