Concrete calculator
Slab, footing, or post holes. Get cubic yards, bag count for your bag size, and a clear call on bags versus a ready-mix truck.
You need
0 cubic yards (0 × 80-lb bags)
0 cu ft exact · 10% added for uneven subgrade and spillage ·
Also buy
What it will cost
- Bagged mix
- $0 at $7/bag — change below
- Ready-mix delivered
- $0 typical, short-load fees extra under 4 yd
- Poured and finished by a pro
- $0 slab, typical range
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Calculator last reviewed 2026-09-03. How we calculate.
How this calculator works
Volume is length × width × thickness (converted from inches to feet), or π × r² × depth for a round pier, times the number of pours. Concrete is ordered in cubic yards, so we divide cubic feet by 27. We add 10% because a gravel subgrade is never perfectly flat and some mix is always lost in the wheelbarrow. Bag counts use the yield printed by the manufacturers: an 80-lb bag makes 0.60 cu ft, 60-lb makes 0.45, 50-lb makes 0.375. Above about one cubic yard (45 eighty-pound bags) a ready-mix truck is usually cheaper and far easier on your back; the calculator tells you when you cross that line.
cu_yd = cu_ft × 1.10 ÷ 27
bags = ⌈ cu_ft × 1.10 ÷ bag_yield ⌉
Frequently asked questions
How many 80-lb bags of concrete are in a yard?
45. An 80-lb bag yields 0.60 cu ft and a cubic yard is 27 cu ft. For 60-lb bags it is 60, and for 50-lb bags it is 72.
How thick should a patio slab be?
Four inches is standard for patios, walkways and shed floors. Driveways and anything a vehicle parks on should be 5–6 inches with rebar or wire mesh. Thickness is the biggest driver of volume, so measure it carefully.
When should I order a ready-mix truck instead of bags?
Around one cubic yard. Mixing 45 bags by hand is a full day and the joints between batches are weak spots. Most suppliers have a minimum load and charge a short-load fee under 3–4 yards, which the cost estimate flags.
How much concrete for a 4×4 fence post?
A 12-inch hole, 30 inches deep, with a 4×4 post takes about 1.5 cu ft of concrete after subtracting the post, which is two to three 60-lb bags. Choose "round pier" above and enter the hole diameter and depth.