Concrete Calculator

Concrete Calculator Tool

Enter your values in the calculator above, then use the explanations below to understand the result, check assumptions, and compare scenarios.

About this calculator

Use this concrete calculator to estimate cubic yards of concrete for slabs, footings, walls, columns, fence-post holes, and other common pours. It also helps plan ready-mix concrete quantities and concrete by the bag after the required volume is known. The core calculation is volume, but a practical order should account for unit conversion, uneven subgrade, excavation variation, spillage, and waste. Measure each section carefully, calculate complex projects as separate shapes, and add a project-appropriate allowance before ordering.

For a rectangular placement, volume equals length × width × thickness. When dimensions are entered in feet, divide cubic feet by 27 to calculate cubic yards of concrete. Circular placements use V = πr²h. The same volume method works for a concrete footing, fence-post hole, pier, column, slab, or other formed placement: model the placement with the appropriate shape, total the sections, then convert the result to the supplier's ordering unit. Ready-mix concrete is normally ordered by the cubic yard in the United States, while smaller projects are often planned by the number of concrete bags required.

Treat the calculated cubic yards as the theoretical minimum. A clean, accurately formed slab may need about 5% extra, while footings, trenches, rough excavation, pumping, or difficult access may justify 8–12%. For bagged concrete, use the manufacturer's stated yield for the exact bag size and mix rather than assuming every 60 lb or 80 lb bag produces the same volume. For ready-mix concrete, confirm truck capacity, minimum-order rules, short-load fees, and the supplier's accepted ordering increment.

Worked examples

Practical tips

• Measure actual formed depth at several locations instead of relying only on the plan dimension.
• Calculate thickened edges, grade beams, piers, fence-post holes, and steps separately from the main slab.
• For concrete by the bag, divide required cubic feet by the yield printed on the selected bag and round up to a whole bag.
• For ready-mix concrete, round the final cubic-yard order to the supplier's accepted increment and verify short-load charges.
• Confirm access, pump setup, crew size, and finishing plan before the truck is dispatched.

Common mistakes

• Entering inches as feet, especially slab or footing thickness.
• Forgetting to divide cubic feet by 27 when converting to cubic yards of concrete.
• Using nominal excavation dimensions when a footing trench or post hole is wider or deeper in the field.
• Assuming a fixed bag yield without checking the concrete manufacturer's package instructions.
• Adding waste twice—once in dimensions and again as a percentage.
• Assuming a volume calculator determines mix design, reinforcement, footing design, or structural adequacy.

Key terms

Cubic yard

A volume measuring 3 ft × 3 ft × 3 ft, equal to 27 cubic feet.

Bag yield

The approximate volume of mixed concrete produced by one bag, as stated by the manufacturer.

Ready-mix concrete

Concrete batched at a plant and delivered to the project, commonly ordered by cubic-yard volume.

Yield

The volume of concrete produced by a batch relative to its stated quantity.

Short load

A ready-mix delivery smaller than the supplier's preferred truck quantity, often subject to an added fee.

Frequently asked questions

How many yards of concrete do I need?

Calculate the placement volume in cubic feet and divide by 27 to get cubic yards. Add an appropriate waste allowance for excavation variation, form tolerances, spillage, and site conditions before placing the order.

How do I use a concrete calculator by the bag?

First calculate the required concrete volume. Then divide that volume by the yield stated on the specific bag of concrete mix and round up to the next whole bag. Bag yield varies by product and bag size, so the manufacturer's label is the correct source.

How do I calculate concrete for a footing?

For a rectangular footing, multiply length by width by depth using consistent units. Convert the resulting cubic feet to cubic yards by dividing by 27, then add a field allowance appropriate to the excavation and forming conditions.

How do I calculate concrete for fence posts?

Treat each round post hole as a cylinder: measure diameter and depth, calculate πr²h, and multiply by the number of holes. For bagged mix, convert the total volume using the manufacturer's stated bag yield.

What is the difference between ready-mix concrete and bagged concrete estimates?

The geometric volume calculation is the same. Ready-mix is typically ordered in cubic yards, while bagged concrete is purchased as whole bags whose coverage depends on the manufacturer's stated yield.

How much extra concrete should I order?

A 5–10% allowance is common, but the appropriate amount depends on form accuracy, excavation quality, placement method, access, and supplier rules. Rough trenches or pumped placements may justify more.