About the concrete calculator
Small jobs like a shed pad or post footings are often poured from premix bags. Larger slabs are usually cheaper and easier with ready-mix delivered by truck. The break-even point depends on local prices, which is why this calculator lets you enter your own.
Bag yield is printed on every bag of premix. Enter it (or tap a common value if it matches your bag) to get a bag count.
How to use this calculator
- Choose a slab or round footing.
- Enter the dimensions, thickness or depth, and how many you’re pouring.
- Set a waste allowance.
- Enter the yield per bag from the label, and prices if you want to compare.
Worked examples
- 10 × 10 ft, 4 in thick
- 10 × 10 × 0.333 = 33.3 cu ft = 1.23 cu yd
- 80 lb bags at 0.60 cu ft: 33.3 ÷ 0.6 = 55.6
1.23 cu yd, or 56 bags of 80 lb premix.
Measurement guidance
Check thickness at several points
Subgrade is rarely perfectly flat. Measure form depth in several spots and use the average.
Minimum orders
Ready-mix suppliers may have minimum loads or short-load fees. Include those in your ready-mix price for a fair comparison.
Frequently asked questions
How many bags of concrete make a yard?
One cubic yard is 27 cu ft. At 0.60 cu ft per 80 lb bag, that’s 45 bags; at 0.45 cu ft per 60 lb bag, 60 bags.
How much extra concrete should I order?
5–10% is a common allowance for uneven subgrade, form flex, and spillage.
When is ready-mix cheaper than bags?
It depends on local pricing and delivery fees. Enter both prices and the calculator shows which is cheaper for your volume.