The math behind your estimate
The inputs, allowance, and rounding stay visible.
- Volume
- Volume = Length × Width × Thickness (or your known volume)
- Bags
- Bags = Volume × (1 + waste %) ÷ yield per bag, rounded up
- Excess
- Excess = bags × yield − volume incl. waste
Assumptions
- Bag yields use manufacturer published values: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 50 lb ≈ 0.375 ft³, 40 lb ≈ 0.30 ft³, 25 kg ≈ 12.3 L mixed concrete.Source: Quikrete Concrete Mix No. 1101 product data
- Yields vary slightly between manufacturers and mixes (fast-setting, high-strength, crack-resistant). Check your bag's label if you are close to a bag boundary.
Scope note
This tool estimates purchase quantities only. Slab thickness and reinforcement are governed by local building codes and soil conditions.
Frequently asked questions
- How many 80 lb bags of concrete do I need for a 10×10 slab?
- A 10 ft × 10 ft slab at 4 inches thick needs 33.3 cubic feet of concrete. With 10% waste that's 36.7 ft³, and at 0.60 ft³ per 80 lb bag you should buy 62 bags.
- Why do different brands give different bag counts?
- Yield depends on the mix design, not just bag weight. Most standard 80 lb mixes yield about 0.60 ft³, but fast-setting and specialty mixes can differ — the yield printed on the bag wins over any calculator default.
- Why does the calculator always round bags up?
- You can't buy a fraction of a bag, and running short mid-pour creates a weak cold joint between batches. Rounding up plus a small waste factor is cheaper than a second trip to the store with wet concrete waiting.
- At what point should I order ready-mix instead of bags?
- Above roughly one cubic yard (about 45 × 80 lb bags), ready-mix delivery is usually cheaper per yard and far less labor. Get a delivery quote before committing to mixing 50+ bags by hand.
Sources
- Quikrete Concrete Mix product data (yields per bag size)
- Sakrete published mix yields (cross-check against Quikrete figures)