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Concrete Calculator

Estimate how many bags or cubic yards of concrete you’ll need for slabs, footings, or posts before you head to the store.

Project Type

Dimensions

Length

Width

Thickness / Depth

Post / Column Dimensions

Diameter

Height / Depth

Number of Posts

Known Volume

Volume

cu yd

Bag Size (optional)

Select bag size for estimate

Waste / Extra

Add extra for waste & spillage (%)

Estimated Concrete Volume

0 cu yd

How to Use the Concrete Calculator

This free concrete calculator helps you estimate the volume of concrete required for common DIY and construction projects. Enter your dimensions and get results in cubic yards, cubic feet, and the approximate number of bags.

Choose units Select Feet/Inches or Meters/cm so every measurement stays consistent.

Select project type Pick Slab/Pad, Footing, Round Post/Column, or enter a known custom volume.

Enter dimensions Measure length, width, and thickness (or diameter and height for posts). Double-check your numbers.

Choose bag size & waste factor Select the bag weight you plan to buy and add a small percentage for waste (5% is a good starting point).

Get your estimate Click Calculate to see cubic yards, cubic feet, and the recommended number of bags.

What Measurements Do You Need?

Accurate measurements are the foundation of a good concrete estimate. Here’s what to measure for each project type.

Slab or Pad Length × Width × Thickness (usually 3.5–6 inches for residential slabs).

Footing Length × Width × Depth of the trench or form.

Round Post Diameter of the hole × Depth of the pour × Number of posts.

How Concrete Volume Is Calculated

The calculator converts all inputs into cubic feet, then into cubic yards (1 cubic yard = 27 cubic feet). A small waste percentage is added so you don’t run short on pour day.

Slab / Footing Volume (cu ft) = Length × Width × Thickness (all in feet)

Round Column / Post Volume (cu ft) = π × (Radius)² × Height × Quantity

Cubic Yards Cubic Yards = Cubic Feet ÷ 27

Bags Needed Bags ≈ (Cubic Feet × 1.05) ÷ Bag Yield (cu ft per bag)

Concrete Bag Yield Guide

Bag yields vary slightly by brand and mix design. These are typical values used by most home centers and ready-mix bag manufacturers.

80 lb bag Approximately 0.60 cubic feet

60 lb bag Approximately 0.45 cubic feet

40 lb bag Approximately 0.30 cubic feet

Always check the bag label for the manufacturer’s stated yield. Weather, mixing method, and water content can also affect the final volume.

Practical Tips Before You Pour

🧱 Order a little extra Running short mid-pour is costly. A 5–10% buffer is standard practice.

🚚 Ready-mix vs bags For volumes over about 1 cubic yard, a ready-mix truck is often more economical and consistent.

🌡️ Temperature matters Hot or cold weather changes set time. Plan your pour and curing accordingly.

📏 Measure the forms Measure the actual form dimensions rather than the planned size for the most accurate result.

💡 Keep in mind

This calculator provides a reliable volume estimate based on the dimensions you enter. Actual bag counts can vary with mixing technique, moisture content, and the exact product you buy. For structural or large commercial pours, consult a contractor or engineer and follow local building codes.

Concrete Calculator FAQ

How many bags of concrete do I need for a 10×10 slab?

It depends on the thickness. A 4-inch thick 10×10 slab is about 1.23 cubic yards (roughly 55–60 eighty-pound bags including a small waste factor). Enter your exact thickness in the calculator for a precise number.

How many cubic feet are in a cubic yard of concrete?

There are exactly 27 cubic feet in one cubic yard. The calculator automatically converts between the two units.

Should I order ready-mix or buy bags?

Bags are convenient for small jobs (under ~1 cubic yard). For larger slabs or continuous pours, ready-mix concrete delivered by truck is usually cheaper, more consistent, and less labor-intensive.

How much extra concrete should I buy?

Most DIYers and contractors add 5–10% for waste, spillage, and slight over-excavation. Complex shapes or first-time pours often benefit from the higher end of that range.

Can I use this calculator for post holes?

Yes. Select “Round Post / Column,” enter the diameter of the hole, the depth of the concrete, and the number of posts. The tool calculates the total volume and bag count for all of them.

Does the calculator account for reinforcement or gravel base?

No. Volume is calculated from the concrete space only. Gravel base and rebar do not change the concrete volume itself, but they do affect preparation and total project cost.