Concrete Bags Calculator
Find how many bags of concrete you need — enter your slab, footing or column dimensions and the bag size to get the bag count for 40, 60 and 80 lb bags, plus an estimated cost.
United States — national average · 2026 prices
Material only unless stated as installed. Excludes tax, permits and local labour. Verify with local suppliers — see how we calculate.
How this calculator works
Every figure above comes from the formula below, applied to the assumptions listed. Nothing is looked up from a hidden table — you can reproduce the result by hand.
Formula
cubic feet = length × width × thickness (in) ÷ 12 × (1 + waste ÷ 100)
bags = cubic feet ÷ bag yield, rounded up
bag yield: 80 lb = 0.60 · 60 lb = 0.45 · 40 lb = 0.30 cu ft
Assumptions & limits
- Bag yields follow the QUIKRETE Concrete Mix data sheet. Other brands and mix types (fast-setting, high-early-strength) yield slightly differently — check your bag.
- Bag prices assume $6.00 per 80 lb, $4.75 per 60 lb and $3.75 per 40 lb bag.
- Default waste allowance is 10%, higher than the ready-mix default because part-bags and spillage cost proportionally more on small jobs.
- Counts are rounded up to whole bags — you cannot buy two thirds of a bag.
- For a post hole, the figure is the hole volume. Subtract the post itself for a closer number.
Worked example
An 8 ft × 8 ft shed pad, 4 inches thick, 10% waste, using 80 lb bags
- Cubic feet = 8 × 8 × 4 ÷ 12 = 21.3, plus 10% waste = 23.5 cu ft
- 80 lb bags = 23.5 ÷ 0.60 = 39.1 → 40 bags
- 60 lb bags = 23.5 ÷ 0.45 = 52.1 → 53 bags
- 40 lb bags = 23.5 ÷ 0.30 = 78.2 → 79 bags
Result:40 × 80 lb bags — roughly $216 to $276 of mix. At this bag count ready-mix is worth pricing too.
Built and maintained by Christoph Ballmer, Honeygoat GmbH
Method and constants last checked against the calculator code on August 17, 2026. Independent trade review is not yet in place for this calculator — see how we handle review.
Read the site-wide calculation methodology · estimate vs. quote and safety limits
How many bags of concrete do I need?
Start with the volume of your slab in cubic feet — length × width × thickness, all in feet — then divide by the yield of the bag you're buying. An 80 lb bag of concrete mix yields about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. The calculator above does this from your dimensions, rounds up, and adds a waste allowance so you don't come up short.
bags = (length × width × thickness ÷ 12, in cu ft) ÷ bag yield
Bag yield and coverage
| Bag size | Yield | Covers at 4″ | Bags per yd³ |
|---|---|---|---|
| 80 lb | 0.60 cu ft | ≈ 1.8 sq ft | 45 |
| 60 lb | 0.45 cu ft | ≈ 1.35 sq ft | 60 |
| 40 lb | 0.30 cu ft | ≈ 0.9 sq ft | 90 |
Buying a specific brand? See the Quikrete and Sakrete calculators, which use the same yields. For bigger pours, the concrete calculator compares bags against ready-mix and also handles footings and round post holes.
Bag sizes, yields and cost
| Bag size | Yield | Bags per yd³ | Typical price (2026) |
|---|---|---|---|
| 80 lb | 0.60 cu ft | 45 | $6–$8 |
| 60 lb | 0.45 cu ft | 60 | $5–$7 |
| 40 lb | 0.30 cu ft | 90 | $4–$6 |
The 80 lb bag is the cheapest concrete per cubic foot — if you can lift it repeatedly. Mixing forty of them is real physical work; the 60 lb size is the common compromise, and 40 lb suits small patches and anyone who prefers more trips over heavier lifts.
Common mistakes
- Mixing too wet. Soupy concrete pours easily and finishes weak — extra water is the most common strength killer. Follow the water line on the bag and err dry.
- Forgetting the post takes up space. For fence posts, the concrete fills the hole MINUS the post — subtract it, or use the round-column mode in the concrete calculator.
- Storing opened bags. Cement pulls moisture from the air and hardens in the bag. Buy what the job needs; a rock-hard bag is a total loss.
- Pouring in the cold. Below about 40°F, curing stalls — cold-weather pours need protection or a warmer week.
- Bagging a job that wants a truck. Past roughly one cubic yard — 45 bags of 80 lb — ready-mix beats bags on price and your back: compare in the concrete cost calculator.
Yields are the standard data-sheet figures for general-purpose mix; fast-setting and high-strength products differ. The printed yield on your bag is the authority.
Frequently asked questions
How many bags of concrete do I need?
Work out the volume in cubic feet (length × width × thickness, all in feet), then divide by the bag yield: about 0.60 cu ft for an 80 lb bag, 0.45 for a 60 lb bag, and 0.30 for a 40 lb bag. The calculator does this from your dimensions and rounds up, including a waste allowance.
How many 80 lb bags of concrete make a yard?
About 45. A cubic yard is 27 cubic feet and an 80 lb bag yields roughly 0.60 cu ft, so 27 ÷ 0.60 ≈ 45 bags. For 60 lb bags it is about 60 per yard, and for 40 lb bags about 90 per yard.
How much does an 80 lb bag of concrete cover?
An 80 lb bag of standard concrete mix yields about 0.60 cubic feet of finished concrete. That covers roughly 1.8 square feet at 4 inches thick, or 1.2 square feet at 6 inches. The calculator converts your area and thickness into the exact bag count.
How many bags of concrete for a fence post?
For a typical 4-inch post in a 10-inch-wide hole 2 feet deep, plan on roughly one to two 50–60 lb bags per hole. Wider or deeper holes need more. Enter the hole as a round column above (diameter and depth) to size it exactly, and remember to subtract the post itself for a close figure.