How much concrete does an 80-lb bag make?

An 80-lb bag of pre-mixed concrete yields approximately 0.6 cubic feet of cured concrete. That's enough to fill a 10" diameter × 11" deep hole. For larger pours (over 1 cubic yard), consider ordering ready-mix from a truck.

When should I use bags vs ready-mix?

Pre-mixed bags are practical for 1-10 footings. For more than about 1 cubic yard of concrete (roughly 45 bags of 80-lb), ordering a ready-mix truck is more economical and produces better results because all the concrete is mixed consistently.

What's the difference between Sonotubes and post holes?

Sonotubes are cardboard forms that create clean cylindrical footings with smooth sides and a flat bottom. Post holes are simply holes dug in the ground. Sonotubes prevent soil from falling in, keep the concrete clean, and extend the footing above ground if needed.

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Formulas Used

Round Footing

V = π × r² × depth (÷ 1728 for cu ft)

Calculate volume of a cylinder using radius and depth in inches, then convert to cubic feet.

Rectangular Footing

V = L × W × depth (÷ 1728 for cu ft)

Multiply length, width, and depth in inches, then convert to cubic feet.

Bags from Volume

80-lb bags = cu ft ÷ 0.6

One 80-lb bag of pre-mixed concrete yields approximately 0.6 cubic feet of cured concrete.

Concrete Volume Calculations for Footings

Whether you're building a deck, fence, pergola, or mailbox post, concrete footings anchor your structure to the ground. Getting the right amount of concrete means not over-buying heavy bags or running short mid-pour.

Pre-Mixed Concrete Bag Coverage

| Bag Size | Coverage | Weight per Bag | |----------|----------|----------------| | 50-lb | 0.375 cu ft | 50 lbs | | 60-lb | 0.45 cu ft | 60 lbs | | 80-lb | 0.6 cu ft | 80 lbs | | 90-lb | 0.68 cu ft | 90 lbs |

Quick Reference: 80-lb Bags per Round Footing

| Diameter | 24" Deep | 36" Deep | 48" Deep | |----------|----------|----------|----------| | 8" | 1 | 2 | 2 | | 10" | 2 | 2 | 3 | | 12" | 2 | 4 | 5 | | 14" | 3 | 5 | 6 | | 16" | 4 | 6 | 8 | | 18" | 5 | 8 | 10 |

Tips for Mixing and Pouring

Mix in a wheelbarrow or mixer — not in the hole
Add water gradually — mix should be like thick peanut butter
Fill holes in 6-8" lifts and rod each layer to remove air bubbles
Crown the top slightly above grade so water sheds away
Insert post anchors while concrete is wet (for post-base hardware)
Let cure 24-48 hours before applying load

Frequently Asked Questions

Related Calculators

Fence Material Calculator

Calculate posts, rails, pickets, and concrete bags for any fence project — with gate support and waste factor.

Deck Footing Calculator

Calculate concrete volume, bag count, and bearing capacity for deck footings — with frost line check and soil type.

Concrete Footing Calculator