Concrete & Masonry
Concrete Bag Calculator
Find out how many bags of concrete you need. Enter the size of your post holes, pad or steps, or a volume you already know, and this concrete bag calculator shows how many 40, 50, 60 or 80 lb bags to buy, how much they weigh, and what they cost.
Result
Enter your measurements and press Calculate. What to buy, the working and the cost will appear here.
How to use the concrete bag calculator
- Choose what you know: the dimensions of the pour, or a volume you have already worked out.
- Enter the size. Use a circle for post holes and tube forms, a rectangle for pads and steps, and set How many for repeated holes.
- Pick the bag size you plan to buy. If the bag lists a different yield, type it into Yield per bag.
- Add the price per bag and press Calculate. Every bag size is compared side by side.
How many bags of concrete do I need?
bags = volume (ft³) × (1 + allowance) ÷ yield per bag (ft³)
Round the answer up to whole bags. A pad that needs 5 ft³ of concrete with 10% extra (5.5 ft³) takes 5.5 ÷ 0.6 = 9.2, so 10 bags of 80 lb, or 5.5 ÷ 0.45 = 12.2, so 13 bags of 60 lb.
Worked example: six deck post holes
Six post holes are 10 inches across and 30 inches deep. The posts will be set in 60 lb bags at $5.25 each, with 10% extra:
- Volume: π × (5 in)² = 0.55 ft²; × 2.5 ft = 1.36 ft³ × 6 = 8.18 ft³
- With allowance: 8.18 ft³ × 1.1 = 9 ft³
- Bags: 9 ft³ ÷ 0.45 ft³ = 20 → buy 20 × 60 lb bags
Buy 20 × 60 lb bags, about 1,200 lb to carry, for $105.00. The post itself takes up some of each hole, so in practice you may have a bag or two to spare. Open this example in the calculator.
Concrete bag yield chart
Typical yields printed on standard concrete mix bags, and how many make a cubic yard (27 ft³):
| Bag | Yield | Bags per cubic yard | Weight per cubic yard |
|---|---|---|---|
| 40 lb bag | 0.3 ft³ | 90 | 3,600 lb |
| 50 lb bag | 0.375 ft³ | 72 | 3,600 lb |
| 60 lb bag | 0.45 ft³ | 60 | 3,600 lb |
| 80 lb bag | 0.6 ft³ | 45 | 3,600 lb |
Fast-setting, high-strength and fiber-reinforced mixes often yield a little less. The bag label always wins.
Which bag size should I buy?
- 80 lb bags are usually the cheapest per cubic foot and need the fewest trips through the checkout, but they are heavy to lift and mix.
- 60 lb bags are a good balance for one person mixing in a wheelbarrow.
- 40 and 50 lb bags are easier to carry and handy for small repairs, but cost more per cubic foot.
- Fast-setting post mix is poured dry into the hole and watered; check its own yield, which is often lower.
Mixing bagged concrete
- Add water gradually as the bag directs; too much water makes weaker concrete.
- A wheelbarrow and hoe handle one or two bags at a time. For more than a dozen bags, a rented drum mixer saves a lot of effort.
- For a cubic yard or more, compare the cost and labor with ready-mix using the concrete calculator.
Assumptions and limitations
- Yields are typical label values; enter your own for other mixes.
- The volume of posts, pipes or rebar inside the concrete is not subtracted.
- Bag weights are dry weights to carry; placed concrete weighs more once water is added.
Concrete bag questions
How many bags of concrete do I need for a fence post?
A 10 in hole 30 in deep holds about 1.4 ft³: roughly 3 bags of 60 lb or 2–3 bags of 80 lb per post, a little less once the post is in.
How many 80 lb bags of concrete are in a yard?
45 bags, at 0.6 cubic feet per bag.
How many 60 lb bags of concrete are in a yard?
60 bags, at 0.45 cubic feet per bag.
How much area does an 80 lb bag of concrete cover?
About 1.8 ft² at 4 inches thick, 2.4 ft² at 3 inches, or 3.6 ft² at 2 inches (0.6 ft³ ÷ thickness in feet).
Formulas, unit conversions and default values are explained on our methodology page. Results are estimates; check manufacturer data and local codes before you buy or build.