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Concrete Bag Calculator — How Many Bags Do I Need?

Calculate how many 40, 60, or 80 lb bags of concrete you need for a slab, footing, or post — plus total weight and cost estimate.

Bags Needed
23 bags
(exact: 22.22 bags — rounded up, since you can't buy a partial bag)
Volume: 10 cu ft = 0.37 cu yd
Total weight: 1,380 lbs (0.69 tons)

Bags needed = Total volume (cu ft) ÷ bag yield (cu ft), rounded up to the next whole bag. Standard yields: 40 lb bag ≈0.30 cu ft, 60 lb bag ≈0.45 cu ft, 80 lb bag ≈0.60 cu ft (Quikrete product specifications). A full cubic yard (27 cu ft) needs 90 bags of 40 lb, 60 bags of 60 lb, or 45 bags of 80 lb. Consider buying a little extra beyond the calculated minimum to cover spillage, over-excavation, and uneven subgrade.
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Reference Values

Last verified:
Category Range What It Means Status
40 lb bag 0.30 cu ft yield Lightest standard bag size — easier to lift and hand-mix, but needs the most bags for a given volume. Okay
60 lb bag 0.45 cu ft yield Middle-ground size — the most common choice for DIY slabs, footings, and posts. Good
80 lb bag ★ 0.60 cu ft yield Most volume-efficient per bag — fewest bags to carry, but heaviest to lift and mix (near or above the 50 lb single-person lifting guideline many safety agencies recommend). ★ Best
1 cubic yard cross-check 27 cu ft = 90 bags (40 lb) / 60 bags (60 lb) / 45 bags (80 lb) Useful sanity check: divide 27 cu ft by a bag's yield to confirm the bag count needed for one full cubic yard of concrete. Good

Source: Quikrete.com official product yield specifications for 40 lb, 60 lb, and 80 lb Concrete Mix bags (yield per bag: 0.30, 0.45, and 0.60 cu ft respectively) — the industry-standard figures printed on bag labels and used across concrete-quantity calculators.

Worked Examples

Small Fence Post Footings

Total Volume
1.5 cu ft (direct entry)
Bag Size
40 lb
5 bags (200 lbs)

1.5 ÷ 0.30 = 5.00 → already a whole number, so no rounding waste. 5 × 40 lb = 200 lbs total.

Sidewalk Slab (10 ft × 3 ft × 4 in)

Length
10 ft
Width
3 ft
Thickness
4 in
Bag Size
80 lb
17 bags (1,360 lbs)

Volume = 10 × 3 × (4 ÷ 12) = 10.00 cu ft. 10.00 ÷ 0.60 = 16.67 → rounds up to 17 bags. 17 × 80 lb = 1,360 lbs.

Small Patio Slab (8 ft × 8 ft × 3.5 in)

Length
8 ft
Width
8 ft
Thickness
3.5 in
Bag Size
60 lb
Price per Bag
$4.50
42 bags (2,520 lbs, ≈1.26 tons, ≈$189.00)

Volume = 8 × 8 × (3.5 ÷ 12) = 18.67 cu ft. 18.67 ÷ 0.45 = 41.48 → rounds up to 42 bags. 42 × 60 lb = 2,520 lbs. 42 × $4.50 = $189.00.

One Full Cubic Yard (Cross-Check)

Total Volume
1 cu yd (27 cu ft)
Bag Size
80 lb
45 bags (3,600 lbs)

27 ÷ 0.60 = 45.00 → exactly 45 bags, matching the standard 1 cu yd = 45 (80 lb bags) cross-check with zero rounding waste. 45 × 80 lb = 3,600 lbs (1.8 tons).

Deck Footing (12 in × 12 in × 12 ft deep post hole)

Length
12 ft
Width
1 ft
Thickness
12 in
Bag Size
60 lb
27 bags (1,620 lbs)

Volume = 12 × 1 × (12 ÷ 12) = 12.00 cu ft. 12.00 ÷ 0.45 = 26.67 → rounds up to 27 bags. 27 × 60 lb = 1,620 lbs.

How to Use This Calculator

  1. 1

    Choose how to enter your volume

    Pick "Slab / Footing Dimensions" to enter length, width, and thickness, or "I already know my total volume" if you already have cubic feet or cubic yards from a plan or post-hole chart.

  2. 2

    Enter your dimensions or volume

    For dimensions mode, enter length and width in feet and thickness in inches (the calculator converts inches to feet automatically). For direct volume, enter the number and choose cubic feet or cubic yards.

  3. 3

    Select your bag size

    Choose 40 lb, 60 lb, or 80 lb — each has a different yield in cubic feet, so the bag count changes even though the total concrete volume needed stays the same.

  4. 4

    Read your bag count and optional cost

    The result shows bags needed (rounded up to a whole bag), total weight in pounds and tons, and — if you enter a price per bag — an estimated total material cost.

What Each Value Means

Bag Yield (cu ft per bag)
The volume of mixed concrete one bag produces, in cubic feet. Standard yields: 40 lb bag ≈0.30 cu ft, 60 lb bag ≈0.45 cu ft, 80 lb bag ≈0.60 cu ft, per Quikrete's published product specifications.
Bags Needed (bags)
Total volume needed (cu ft) divided by the selected bag's yield, rounded up to the next whole bag since partial bags can't be purchased or reliably measured mid-pour.
Total Weight (lbs / tons)
Number of bags needed multiplied by the bag's weight (40, 60, or 80 lb) — useful for planning transport, since a pallet of 80 lb bags gets heavy fast and many bags exceed comfortable single-person lifting weight.

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Frequently Asked Questions

How many 80 lb bags of concrete are in a cubic yard?
45 bags. A cubic yard is 27 cubic feet, and an 80 lb bag of concrete mix yields about 0.60 cubic feet, so 27 ÷ 0.60 = 45 bags exactly. For 60 lb bags (0.45 cu ft yield) it's 60 bags per cubic yard, and for 40 lb bags (0.30 cu ft yield) it's 90 bags per cubic yard. These figures come from Quikrete's published yield specifications and are the industry-standard numbers used across concrete calculators.
How many bags of concrete do I need for a fence post?
It depends on the hole size, but a common 10-inch-diameter, 2-foot-deep post hole holds roughly 0.9 cubic feet of concrete — about 3 bags of 40 lb mix, 2 bags of 60 lb mix, or 2 bags of 80 lb mix (all rounded up). Enter your hole's diameter and depth as a cylinder volume, or just plug the total cubic feet into this calculator's direct-volume mode if you already know it from a post-hole size chart.
What's the difference between 40 lb, 60 lb, and 80 lb bags of concrete?
They're the same concrete mix — cement, sand, and gravel aggregate — just packaged in different quantities. A 40 lb bag yields about 0.30 cubic feet, a 60 lb bag about 0.45 cubic feet, and an 80 lb bag about 0.60 cubic feet. Bigger bags are more volume-efficient (fewer bags and less packaging per cubic foot) but heavier to lift, mix, and pour — many DIYers choose 60 lb bags as the best balance between manageable weight and not needing dozens of bags for a mid-size project.
Why does this calculator round up instead of down?
Because bags are sold whole — you can't buy 16.67 bags, only 17. Rounding down would leave you short of concrete mid-pour, which is a real problem since concrete needs to be placed and finished before it starts setting. Rounding up to the next full bag guarantees you have enough mix to complete the pour in one go, even if it means a small amount of leftover dry mix.
Should I buy extra bags beyond the calculated amount?
Yes, a small buffer is smart. This calculator already rounds each pour up to the next whole bag, but real-world projects lose material to spillage while mixing, uneven or over-excavated subgrade that needs more depth than planned, and minor measurement error. Buying 1-2 extra bags beyond the calculated total for small projects (or an extra 5-10% for larger ones) is cheap insurance against a mid-project hardware store run — unopened bags are easy to return, but a half-finished slab is not.