◆ Calculators ◆ Concrete Bag Calculator: How Many Bags for a Slab
Concrete Bag Calculator: How Many Bags for a Slab
Work out how many bags of concrete mix a slab needs, plus the mixing water — with the yield per bag and its source spelled out.
QUIKRETE Concrete Mix is designed for pouring concrete 2 in (50 mm) thick or more, per data sheet No. 1101. Thinner values still calculate.
Yields are the manufacturer’s approximate figures. The number printed on the bag you buy takes precedence.
The data sheet names no waste figure, so this one is yours to set. 0 % is a valid answer, not a placeholder.
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How the number is worked out
The steps from input to result
A slab of concrete is priced and carried in bags, not in cubic feet, so the number that matters at the counter is the bag count. Length times width times thickness gives the volume, and the volume divided by what one bag yields gives the bags — everything else on this page exists to make those two steps checkable.
The yield is where estimates diverge, so here is the source in full: QUIKRETE Concrete Mix, data sheet No. 1101, states that an 80 lb bag "yields approximately 0.60 ft³", with 0.30 for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.675 for 90 lb. The mix is made to ASTM C387, the specification for packaged dry combined materials, and its unit weight of roughly 140 lb/ft³ follows ASTM C138. The word "approximately" is the manufacturer’s own: a yield is not a constant, and the figure printed on the bag in your hand beats the one on this page.
The water figure comes from Table 2 of the same data sheet and is a starting point rather than a target — 6 pt for an 80 lb bag, up to 9 pt once the mix is workable. The data sheet also sets a floor for the pour itself: this mix is designed for concrete 2 in (50 mm) thick or more. Thinner slabs are still calculated here, with that limit shown beside the result, because a manufacturer’s recommendation is not an arithmetic boundary.
The formula
Written out, so you can check it by hand
Bags = round up( length (ft) × width (ft) × thickness (in) ÷ 12 × (1 + waste ÷ 100) ÷ yield per bag (ft³) )
Worked example
Worked with the preset values
A 10 ft by 10 ft slab poured 4 in deep holds 10 × 10 × (4 ÷ 12) = 33.33 ft³ of concrete, which is 1.23 cubic yards. At approximately 0.60 ft³ per 80 lb bag that comes to 33.33 ÷ 0.60 = 55.56 bags, so you buy 56 — a bag is not sold by the half. Mixed at the starting water content of 6 pt per bag, those 56 bags take 336 pt of water in total, and up to 504 pt at the maximum given in Table 2 of data sheet No. 1101.
What this calculator does not do
Named, not left out
- 01Yields are the manufacturer’s approximate figures for QUIKRETE Concrete Mix, data sheet No. 1101. The number printed on the bag you actually buy takes precedence over the one used here.
- 02Only QUIKRETE Concrete Mix is covered. Whether Sakrete, Sika or a store brand yield the same volume per bag has not been checked and is not claimed here.
- 03This calculates flat slabs of even thickness. Footings, stairs, columns, post holes and slabs poured to a fall follow different rules and are not covered.
- 04No waste allowance is preset, because the data sheet names none. Uneven sub-grade, spillage, over-excavation and forms that bow are yours to add as a percentage.
- 05The water figure is a starting point, not a target. Final consistency decides how much goes in, up to the maximum per bag given in Table 2.
- 06Reinforcement, load capacity, control joints and frost depth are structural planning rather than arithmetic. This calculation does not replace it.
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Updated: 8/29/2026