How much concrete do you need?
Buying the right amount of concrete saves money and avoids a stalled pour. The trick is to find the volume of what you are filling, then convert it into the units suppliers use, cubic yards in the US or cubic meters elsewhere. This concrete calculator handles slabs, walls, footings, round columns, tubes, and stairs, and it also estimates how many bags of pre-mix you would buy.
How does the concrete calculator work?
- Pick a shape. Slab, column, tube, or stairs.
- Enter the dimensions. Use any unit you like for each field.
- Set the quantity. Pouring several of the same size? Enter how many.
- Calculate. See the volume in cubic feet, yards, and meters, plus bag counts.
The formulas behind each shape
- Slab, wall, footing. length x width x thickness.
- Round column. pi x radius squared x height.
- Round tube. the outer circle minus the inner hole, times height.
- Stairs. the sum of each step block, plus an optional top platform.
How many bags of concrete per cubic yard?
A cubic yard is 27 cubic feet. Bag counts come from the yield printed on each bag: an 80 lb bag makes about 0.6 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.3. So one cubic yard is roughly 45 of the 80 lb bags, 60 of the 60 lb bags, or 90 of the 40 lb bags. For large pours, ready-mix delivery is usually cheaper than bags.
Tips before you order
- Add a buffer. Order 5 to 10 percent extra for waste and uneven ground.
- Check the subgrade. A dip in the base quietly adds volume.
- Round up bags. The tool rounds to whole bags, since you cannot buy part of one.
Frequently asked questions
How do you calculate how much concrete you need?
You work out the volume of the shape, then convert it to cubic yards or cubic meters, the units concrete is sold in. For a slab, multiply length by width by thickness. For a round column, use pi times the radius squared times the height. This calculator does the math for each shape and also estimates how many pre-mix bags you would need.
How many bags of concrete are in a cubic yard?
It depends on the bag size. A cubic yard is 27 cubic feet. An 80 lb bag yields about 0.6 cubic feet, so you need about 45 bags per cubic yard. A 60 lb bag yields about 0.45 cubic feet, which is about 60 bags per cubic yard. The calculator shows the bag count for 40, 60, and 80 lb bags.
Should I order extra concrete?
Yes. It is common to add about 5 to 10 percent extra to cover spillage, uneven subgrade, and waste. Running short in the middle of a pour is a real problem, so a small buffer is worth it. Add the extra to the volume this tool gives you before you place an order.
What units does the concrete calculator use?
You can enter each dimension in feet, inches, yards, meters, or centimeters, and mix units between fields if you like. The result is shown in cubic feet, cubic yards, and cubic meters, so it works whether you order in imperial or metric.
How do I calculate concrete for a round column?
Switch to the Column tab, enter the diameter and the height, and the tool uses the formula pi times the radius squared times the height. The radius is half the diameter. If you are pouring several identical columns, set the quantity and it multiplies the volume for you.
Is this concrete calculator accurate?
The volume math is exact for the shapes shown. Bag counts use the standard yields printed on common pre-mix bags, rounded up to whole bags. Real jobs vary because of waste and subgrade, so treat the bag count as a close estimate and add a small buffer before ordering.