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TNToolsNexus

Concrete calculator

Slab volume in cubic yards plus exact 80/60/40 lb bag counts from the yields printed on the bags — with an honest bags-vs-ready-mix threshold.

You need

1.36 cubic yards

33.33 ft³ measured + 10% waste = 36.67 ft³

  • 62 × 80 lb bags (yield 0.6 ft³ each)
  • 82 × 60 lb bags (yield 0.45 ft³ each)
  • 123 × 40 lb bags (yield 0.3 ft³ each)

At 1.36 yd³, price a ready-mix delivery too — mixing 62 bags by hand is a long day, and trucks get competitive around 1–2 yards.

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Concrete quantity with the numbers that matter at the store: cubic yards for the ready-mix call, and bag counts at the real printed yields (80 lb → 0.60 ft³, 60 lb → 0.45, 40 lb → 0.30) for the DIY route — waste included, rounding up, honest about when bags stop making sense.

How to estimate concrete

  1. Enter the slab’s length and width in feet and thickness in inches (4 in is the patio standard — see the FAQ).
  2. Keep the 10% waste default unless your subgrade is rough (then 15%).
  3. Read the cubic yards (what ready-mix suppliers quote) and the bag counts per size.
  4. Over about a yard? Price the truck — the calculator flags it.

The formula

yd³ = length(ft) × width(ft) × thickness(in)/12 × (1 + waste%) ÷ 27. Bags: volume ÷ printed yield, rounded up. Footings work the same way — treat each as its own small slab (length × width × depth) and add the volumes.

Pour-day realities

Concrete starts setting when water hits it, so all the material must be ready before the first bag is mixed — a 60-bag pour is a two-person, all-morning job with a rented mixer. Ready-mix arrives workable and uniform but bills a short-load fee under its minimum; the crossover sits right around the 1-yard mark this calculator flags. Get the slab’s footprint from the square footage calculator if it isn’t a simple rectangle.

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Frequently asked questions

How do I calculate concrete for a slab?
Length × width × thickness, all in feet (inches ÷ 12), gives cubic feet; divide by 27 for cubic yards. A 10 × 10 ft patio at 4 inches is 10 × 10 × 0.333 = 33.3 ft³ = 1.23 yd³ before waste.
How many bags of concrete do I need?
Divide the volume by the yield printed on the bag: 80 lb bags yield 0.60 ft³, 60 lb bags 0.45 ft³, 40 lb bags 0.30 ft³. The 10 × 10 × 4 in slab with 10% waste (36.7 ft³) needs 62 × 80 lb bags — which is why the ready-mix question in the next answer matters.
Bags or a ready-mix truck?
Rule of thumb: under about half a yard, bags win on convenience; approaching 1 yard and beyond, price a ready-mix delivery — 60+ bags means hours of continuous mixing, and a slab poured in stages can cold-joint. Many suppliers have a 1-yard minimum plus a short-load fee; it is still often worth it.
How much waste should I allow?
10% is the standard for slabs on grade — subgrades are never perfectly level, forms bow slightly, and running short mid-pour is the worst outcome in concrete work. Uneven excavation or a gravel base that will absorb paste justifies 15%.
How thick should a slab be?
Common practice: 4 inches for patios, walkways, and shed bases; 5–6 inches for driveways and anything carrying vehicles. Local codes and soil conditions override rules of thumb — check before pouring anything structural.

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