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Concrete Calculator: How Many Bags Do I Need?

Enter your slab, footing or column's dimensions and we'll tell you the cubic yards and bag count.

Concrete needed1.36 yd³36.7 cu ft
80 lb bags62
60 lb bags82
40 lb bags123

Over 1 cu yd — consider ordering ready-mix delivery instead of bags.

How it works

For a slab or footing, volume is length × width × thickness (thickness converted from inches to feet). For a round column, volume is π × radius² × height, where the radius comes from your diameter in inches divided by 24 to land in feet. We multiply by your quantity, add your waste margin on top, then convert cubic feet to cubic yards by dividing by 27. Bag counts are the total cubic feet divided by each bag size's yield, rounded up — you can't buy a fraction of a bag.

Example

A 10 ft × 10 ft slab, 4 inches thick, with a 10% waste margin: volume is 10 × 10 × (4 ÷ 12) = 33.3 cu ft, plus 10% waste = 36.7 cu ft, which is 1.36 cubic yards. That's 62 bags of 80 lb mix (or 82 of 60 lb, 123 of 40 lb) — right at the point where ready-mix delivery starts to make sense.

Tips

  • Order 5–10% extra even beyond the calculator's waste margin for your first pour — it's much cheaper than stopping mid-pour to buy more bags.
  • Mix bags in small batches (1–2 at a time) so you're not fighting concrete that's already starting to set before you place it.
  • Dig and form your slab or footing to the exact planned dimensions before ordering — rough excavation often ends up slightly larger than planned.
  • For columns (fence posts, deck footings), measure the sonotube's actual inside diameter, not the size printed on the label.
  • Get a ready-mix quote even if you're close to the 1-cubic-yard line — pricing varies a lot by region and delivery minimums.

Common mistakes

  • Using diameter where the formula needs radius (or vice versa) — this calculator takes diameter so you don't have to convert.
  • Forgetting the waste margin entirely and coming up short on bag count by 10%+.
  • Mixing bag brands or strengths within the same pour, which can cause uneven curing and color.
  • Not accounting for quantity — if you're pouring 6 identical footings, multiply, don't just compute one and eyeball the rest.
  • Assuming every 80 lb bag yields the same 0.6 cu ft — always check the bag's actual printed yield.

FAQ

How much waste margin should I add?
10% is a safe default for most DIY pours — it covers spillage, uneven sub-grade, and forms that are slightly off from your measurements. Add more (15–20%) for irregular ground or a first-time pour.
When should I order ready-mix instead of bags?
Once a job needs more than about 1 cubic yard, ready-mix delivery is usually cheaper and far less physical work than mixing dozens of bags by hand — most ready-mix trucks have a minimum order anyway, so it's worth a quote once you're in that range.
Do all 80 lb bags yield the same volume?
No — the ~0.60 cu ft figure is a typical average, but it varies by brand and mix type (regular vs. high-strength vs. fiber-reinforced). Check the yield printed on your specific bag and re-run the calculator if it's noticeably different.
How do I calculate a round column or footing?
Measure the diameter (not the radius) in inches and the height in feet. The calculator converts diameter to a radius in feet automatically — you don’t need to do that math yourself.

Related guides

Cost to Build a Deck

What footings and framing actually cost.

Drywall Repair Cost Calculator

Patch vs. full-wall pricing.

Best Levels

Buying guide for getting it straight.

Estimates only. Bag yields vary by brand and mix type — always check the yield printed on your specific bag. Local building codes may set minimum footing depth/width requirements — check before you pour.