Estimix

Concrete Calculator for Slabs, Bags, and Cost

Use this concrete calculator to estimate cubic yards, bag counts, and ready-mix cost for slabs, patios, and small pours.

Calculator inputs

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Results (live)

Cubic feet (ft³)
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Cubic yards (yd³)
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Estimated ready-mix cost (from $/yd³)
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60 lb premixed bags (est.)
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80 lb premixed bags (est.)
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Enter length, width, and thickness (inches) all greater than zero to see volume, bag counts, and optional ready-mix cost.

Formula and units

Thickness in feet is (thickness in inches) ÷ 12. Volume in cubic feet is length (ft) × width (ft) × thickness (ft). Cubic yards = cubic feet ÷ 27. Estimated 60 lb bags = ceiling of (ft³ ÷ 0.45) and 80 lb bags = ceiling of (ft³ ÷ 0.6) using typical hand-mix box yields. If you add a $/yd³ rate, estimated ready-mix cost = cubic yards × that rate. Your supplier’s test panels and the mix design on the ticket control true yield and strength.

Example calculation

A 20 ft by 10 ft pad at 5 in is 20 × 10 × (5/12) ≈ 83.33 ft³, about 3.09 yd³. That rounds up to roughly 186 sixty-pound bags and 139 eighty-pound bags before waste. If trucked concrete is quoted at $150/yd material-only in your market, the material line is about $464 before minimum load and pump.

Cost explanation

The $/yd³ line is a planning number only: it should match what your batch plant quotes (sometimes material + short haul, sometimes more). It does not include admixtures, hot/cold weather charges, or finishing labor. Premixed bag counts are ordered as whole units from theoretical yield—open one extra pallet on a learning pour if the crew is hand-mixing.

Common mistakes

  • Ordering truck concrete off bag math

    Batched tickets are sold in yards with moisture, air, and slump on a certified report. If you need a truck, always reconcile against yd³ from the takeoff, not the bag run from this box alone.

  • Forgetting capillary break and isolation

    Volume here is a solid block. Vapor, insulation, and edge thickening are not modeled—add a field allowance or detail sheet for frost walls and service penetrations.

  • Confusing specified thickness with as-placed

    Subgrade ruts and stepped sections change average depth. For bid-grade work, use a field rod or as-built of the sub before locking truck counts.

Frequently asked questions

  • Why don’t my bag numbers match the Quikrete table exactly?

    We use a single rounded yield per bag class for quick math. Your SKU’s data sheet, moisture in the sand pile, and how tight you screed the pour all move the true count by a few units.

  • Can I use this for walls or stem walls?

    If you can reduce the pour to a clear rectangle in plan and one thickness, it is a fair first pass. For varying thickness or tall lifts, break the work into more than one run or use a full takeoff tool.

  • Should I add rebar, chairs, and evaporation in this box?

    Rebar and mesh displace a small amount of volume; for budget estimates many crews ignore that steel. Add waste and evaporation in a line item, not by guessing a thicker number here.