Concrete Calculator
Calculate cubic yards of concrete and the exact number of bags for slabs, footings, columns and post holes.
What you need
1.96cu yd
Concrete needed — Order this much ready-mix, waste allowance included.
- Same amount in cubic feet
- 52.8 cu ft
- Bags needed
- 88
- Approximate weight when cured
- 3.96 US tons
Rounded up. Mixing more than about 20 bags by hand is a long day.
Estimates for planning. Confirm quantities with your supplier before ordering — see the assumptions below for the figures used.
How to use the concrete calculator
Whether you are pouring a slab, a strip footing or a row of post holes, the arithmetic is the same and the risk is identical: running short mid-pour. Pick your form, enter the dimensions, and get cubic yards for ready-mix alongside a bag count for mixing on site.
What this calculator assumes
Every estimate depends on figures that vary by supplier and conditions. These are the ones used here, so you can sanity-check the result against your own quote.
- Bag yields are the manufacturer's published figures: 0.30 cu ft for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb.
- Cured concrete is taken as 150 lb per cubic foot (about 4,050 lb per cubic yard).
- A waste allowance is included in the volume shown. Ready-mix suppliers usually have a minimum order and charge for short loads.
Frequently asked questions
How do I calculate cubic yards of concrete?
Multiply length by width by thickness with every measurement in feet, then divide by 27. A 12 by 12 foot slab at 4 inches thick is 12 × 12 × 0.333 = 48 cubic feet, or 1.78 cubic yards. Add 10% for waste and you would order 2 yards.
How thick should a concrete slab be?
Four inches for patios, walkways and shed floors. Five to six inches for a driveway carrying cars, and 6 inches with reinforcement for anything heavier. Thickening the edges of a slab does more for crack resistance than adding an inch across the middle.
How much concrete do I need for a fence post?
A 4x4 post in a 10 inch diameter hole 3 feet deep needs about 1.4 cubic feet once you subtract the post — roughly three 60 lb bags. Set the form to a round column, diameter 10 inches, depth 3 feet, and the calculator does the rest.
How many 80 lb bags of concrete are in a cubic yard?
Forty-five. An 80 lb bag yields 0.60 cubic feet and a cubic yard is 27 cubic feet. That is around 3,600 pounds of bags to mix by hand — past roughly 1 cubic yard, ready-mix or a rented mixer stops being optional.
Should I order ready-mix or mix bags myself?
Bags make sense up to about half a cubic yard — post holes, small pads, repairs. Beyond that, ready-mix is cheaper per yard, far faster, and gives a consistent mix with no cold joints. Most suppliers have a 1 yard minimum plus a short-load fee.
How much extra concrete should I order?
Ten percent is the usual allowance. Subgrade is never perfectly flat, forms bow slightly, and running short mid-pour forces a cold joint you cannot undo. If your slab is deep or the subgrade is rough, go to 15%.
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