How Many Bags of Concrete Do I Need? 40, 60 and 80 lb Bag Charts
Find how many 40, 60 or 80 lb bags of concrete you need for slabs, posts and footings, with yield charts, worked examples and when to switch to ready-mix.
Premixed concrete in bags is ideal for small jobs: setting fence posts, pouring a pad for an air-conditioning unit or building a few steps. The question is always how many bags to carry home. The answer comes from the volume of your pour and the yield of each bag.
Bag yields
A bag's yield is the volume of concrete it makes once mixed with water. Typical yields for standard concrete mix are:
| Bag size | Yield | Bags per cubic foot | Bags per cubic yard |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 3.33 | 90 |
| 60 lb | 0.45 ft³ | 2.22 | 60 |
| 80 lb | 0.60 ft³ | 1.67 | 45 |
Yields vary a little between brands and products (fast-setting and high-strength mixes can differ), so check the bag label.
The calculation
- Find the volume in cubic feet: length × width × thickness, with the thickness converted from inches to feet.
- Add 5–10% for spillage, uneven ground and form bulges.
- Divide by the bag yield and round up.
bags = volume (ft³) × 1.1 ÷ yield per bag
The concrete calculator shows all three bag sizes at once for slabs, round columns and stairs.
Bags for common slabs (4 inches thick, 10% extra included)
| Slab size | Volume | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 3 × 3 ft (AC pad) | 3.0 ft³ | 6 | 8 |
| 4 × 4 ft | 5.3 ft³ | 10 | 14 |
| 4 × 8 ft | 10.7 ft³ | 20 | 27 |
| 8 × 8 ft | 21.3 ft³ | 40 | 53 |
| 10 × 10 ft | 33.3 ft³ | 62 | 82 |
Fence and deck posts
For a round hole, volume = 3.1416 × radius² × depth. Subtract the post's volume if you want to be precise. A 10-inch hole 3 ft deep holds about 1.64 ft³; a 4 × 4 post (actual size 3.5 × 3.5 in) takes up about 0.26 ft³ of that, leaving 1.38 ft³. With 10% extra that's about 3½ bags of 60 lb or 2½ bags of 80 lb per post, so buy four and three respectively. Many fast-setting post mixes come in 50 lb bags with their own yield; read the label.
When to stop using bags
Around one cubic yard, bags become hard work: 45 bags of 80 lb is 3,600 lb to lift, carry, mix and place, usually within the working time of the concrete. Above that, a ready-mix truck is usually faster and often cheaper, though small loads carry a short-load fee. Some suppliers also offer trailer loads of mixed concrete for medium-sized jobs. See our guide to ordering ready-mix concrete.
Mixing tips
- Add water gradually; too much water weakens concrete. Follow the bag's water range.
- A rented electric mixer handles two 80 lb bags at a time and saves a lot of effort.
- Wear gloves, eye protection and long sleeves. Wet cement is caustic and can burn skin.
- Wet the forms and the base slightly on hot days so they don't pull water from the mix.
Concrete vs. mortar vs. sand mix
Concrete mix contains gravel and is used for slabs, posts and footings. Sand or topping mix has no coarse aggregate and suits thin layers under about 2 inches. Mortar is for laying brick and block. Buying the wrong product is a common mistake; check the bag's intended uses.
Summary
Work out the cubic feet, add 10%, divide by 0.6 for 80 lb bags, 0.45 for 60 lb bags or 0.3 for 40 lb bags, and round up. For anything approaching a yard, price a truck delivery as well.
Further reading from official sources
- Concrete basics and resources – American Concrete Institute (ACI)
- Cement and concrete technical resources – Portland Cement Association (PCA)
