Worked example: a 14 × 12 ft concrete patio
Hypothetical educational example, not a client project. The pour is a 14 × 12 ft rectangle at 4 inches with the calculator’s 10% allowance. Bag yield is the QUIKRETE 1101 figure of about 0.60 cubic feet per 80 lb bag. The gravel base is a different order.
Inputs
| Length | 14 ft |
|---|---|
| Width | 12 ft |
| Thickness | 4 in |
| Waste | 10% |
| Bag | 80 lb, 0.60 cu ft (QUIKRETE 1101) |
Arithmetic
- Area = 14 × 12 = 168 sq ft.
- Volume = 168 × 4 ÷ 12 = 56 cu ft = 56 ÷ 27 = 2.074 cu yd before waste.
- With 10% = 61.6 cu ft = 2.281 cu yd. The calculator displays 2.28 cu yd.
- Bags = 61.6 ÷ 0.60 = 102.67, which rounds up to 103 bags.
Required, waste and purchase
- Required
- 2.28 cu yd (61.6 cu ft), equal to 102.67 bags of 80 lb mix at 0.60 cu ft.
- Purchased
- 103 × 80 lb bags. The extra fraction of a bag is purchase rounding, not a second waste percent. At the same volume the calculator also shows 137 × 60 lb and 206 × 40 lb.
- Units
- US feet, inches, cubic yards and 80 lb bags. One cubic yard is 27 cubic feet.
Assumptions
- The 10% figure is the top of the 5% or 10% over-run described by UNL Extension G82-630, which this site already cites on the slab calculator. It is not a code minimum.
- The sub-base is not inside the 103 bags.
What this example does not decide
- Not a structural slab design and not a price comparison with ready-mix.
- Above about 1 cubic yard the slab calculator suggests comparing a delivered load. This example does not choose one.
Where to go next
- Concrete slab calculator — Run these inputs
- Concrete volume topic — Formula and waste sources
- How much concrete do I need? — Longer volume walkthrough
- Gravel calculator — Base stone is a separate volume
- Concrete calculators — Tool list
Questions
Does 103 bags include the gravel base?
No. The 103 bags are concrete at 4 inches plus 10%. A crushed-stone base uses the gravel calculator with its own depth and density.
Why 103 bags and not 102?
61.6 ÷ 0.60 is 102.67. The calculator rounds up to a whole bag. That extra part of a bag is not another 10%.
Sources
- Quikrete – Concrete Mix (No. 1101) product data sheet — accessed 2026-10-19: “An 80 lb (36.2 kg) bag yields approximately 0.60 ft³ (17 L).”