How it's calculated
The calculator works out the volume of the concrete you will place, adds an order allowance, then divides by the yield of one bag of mix. It handles four common pours: a flat slab (patio, shed base, walkway), a continuous footing, round columns or post holes, and a run of solid steps.
Volume comes first, in cubic feet. Ready-mix concrete is sold by the cubic yard (27 cubic feet) or the cubic meter, so the result is shown in those units too. Bagged mix is sold by weight, and each bag makes a set volume of concrete. QUIKRETE's data sheet for its Concrete Mix (No. 1101) lists approximate yields of 0.30 cu ft for a 40 lb bag, 0.45 cu ft for 60 lb, 0.60 cu ft for 80 lb and 0.675 cu ft for 90 lb QUIKRETE. The bag count is always rounded up, because a pour that runs short leaves a cold joint.
The order allowance covers spillage, uneven ground, forms that bow and a slab that ends up a little thicker than planned. The National Ready Mixed Concrete Association recommends ordering 4 to 10 percent more than the volume measured from plan dimensions NRMCA. The calculator starts at 10% because a bagged pour can't call for another truck; lower it if your forms and subgrade are accurate.
The formula
- Slab or footing: volume = length × width × thickness (all in feet; inches ÷ 12).
- Round column: volume = π × (diameter ÷ 2)² × height × number of columns.
- Steps: volume = width × run × rise × n(n + 1) ÷ 2, where n is the number of steps. Each step is treated as a solid block from the ground up, so the bottom step is one rise tall and the top step is n rises tall.
- Order volume = volume × (1 + allowance %).
- Cubic yards = cubic feet ÷ 27. Cubic meters = cubic feet × 0.028316846592, from the exact definition 1 ft = 0.3048 m in NIST's SI guide NIST.
- Bags = order volume ÷ bag yield, rounded up to a whole bag.
Worked example
QUIKRETE's own training module works a 10 ft × 6 ft slab, 4 in thick: 10 × 6 × 0.333 ft = 20 cubic feet, which it divides by 0.60 for 80 lb bags and by 0.45 for 60 lb bags QUIKRETE. With no allowance the calculator gives the same 20 cu ft, and rounds up to 34 bags of 80 lb mix or 45 bags of 60 lb mix. The training module rounds its quotients to the nearest bag; buying the whole number above is the safer choice.
Now the default project: a 10 × 10 ft patio slab, 4 in thick, with a 10% allowance.
Concrete to order: 1.36 cubic yards
That is 1.36 cu yd to order, or 62 bags of 80 lb mix. At the example price of $6 a bag the mix costs $372.
A single 12 in round column 4 ft deep needs 0.12 cu yd (π × 0.5² × 4 cubic feet), and three steps 4 ft wide with a 7 in rise and 11 in run need 0.48 cu yd.
Assumptions and limitations
- Bag yields are approximate. They are the manufacturer's figures for QUIKRETE Concrete Mix. Other mixes (high-strength, fast-setting, other brands) print their own yield on the bag; if yours differs, divide the cubic-foot volume by your bag's yield.
- Solid shapes only. Steps are modeled as solid blocks, not a sloped slab with treads. Hollow forms, rebar and embedded objects aren't subtracted (they take up very little volume).
- The allowance is a judgment call. NRMCA's 4–10% range is for contingencies such as waste and over-excavation; a sloping or soft subgrade can need more. NRMCA notes that just 1/8 in of extra thickness on a 4 in slab adds about 3 percent.
- Gravel base not included. A slab usually sits on a compacted base; use the gravel calculator for that.
- Bags vs ready-mix. Past a cubic yard or so, get a ready-mix quote; it is sold by the cubic yard and arrives mixed.
- Prices are examples. Enter what your store charges.
For a wider walk-through of measuring, waste allowances and ordering, see our guide to estimating materials for home projects.