How this is calculated
Each bag yields a fixed volume once mixed — about 0.45 ft³ for a 60 lb bag and 0.60 ft³ for an 80 lb bag in US sizing. Divide your total volume (with waste) by that yield and round up.
Once you pass roughly one cubic yard (0.75 m³), ready-mix delivery usually beats the cost and labour of hand-mixing dozens of bags. The result note flags that threshold.
Formula & methodology
Volume = L × W × D × (1 + waste %). Bags = ceil(Volume ÷ bag yield).
Worked example
An 8 ft × 8 ft slab, 4 in thick, 10% waste
- 1.Volume = 8 × 8 × (4÷12) = 21.3 ft³
- 2.With 10% waste: 21.3 × 1.10 = 23.5 ft³
- 3.80 lb bags: 23.5 ÷ 0.60 = 40 bags (rounded up)
- 4.Ready-mix would be ~0.87 yd³ — a truck may still charge a short-load fee.
Frequently asked questions
How many bags for a 10×10 slab?
At 4 in thick with 10% waste, about 50 eighty-pound bags or 67 sixty-pound bags. Use the calculator with your exact depth — driveways often go thicker than patios.
60 lb vs 80 lb bags?
Eighty-pound bags cover more volume per bag but are heavier to move. The calculator shows both counts so you can buy what your store stocks.
When is bag mix a bad idea?
Large monolithic slabs, driveways, and anything over ~1 yd³ are faster and often cheaper with ready-mix. Bag mix also makes consistent colour harder on big pours.
Does this include the gravel base?
No. Sub-base gravel is a separate order. Use the Gravel Calculator for that layer.
What waste percentage should I use?
10% is a fair default for a simple rectangle. Odd shapes, multiple pours, or sloppy subgrades need 12–15%.
UK bag sizes?
Switch to metric for 25 kg bag yields. Yields vary slightly by brand — check the label on the bag you buy.
Last updated 2026-09-01. Formulas are planning methods — confirm with a supplier before you order.