This calculator turns a small concrete pour into counts of 60 lb, 80 lb, or 25 kg premixed bags, including an adjustable waste allowance. Use it for post holes, steps, repair pads, and other pours where a ready-mix truck minimum would cost more than hand mixing. It does not design the footing, choose a mix strength, include reinforcement, gravel, forms, mixer hire, water, or the labour of placing repeated batches. US mode uses feet, inches, and common pound bags; UK mode uses metric dimensions and 25 kg bags, with yield based on a typical product. Bag yields vary by manufacturer, so check the printed yield or ask the retailer to confirm the count before loading the order.
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.
For several post holes, calculate one hole from its real diameter and depth, then multiply before adding waste. Do not use the outside post dimensions as the concrete volume without subtracting the post itself when accuracy matters.
Every bag count rounds up because partial bags are not dependable planning units. Compare the final bag weight and number of batches with access, mixer capacity, and the time available to place concrete before it begins setting.
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.
Calculator
Enter your dimensions below. Results update as you type. Costs are planning ranges — confirm with a local supplier before you order.
Pour area
ft
ft
in
Add cost details
%
Results
Volume with waste
0.87 yd³
60 lb bags
53 bags (60 lb)
80 lb bags
40 bags (80 lb)
Bag mix suits small pads and post holes
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. See how we calculate and the disclaimer.