Free Concrete Calculator — Yards, Bags & Cost Estimator

How to Use the Concrete Calculator

How to Calculate Concrete Volume

Concrete volume is calculated with the formula Length × Width × Depth, with the result converted from cubic feet to cubic yards by dividing by 27. One cubic yard equals 27 cubic feet — a cube measuring 3' × 3' × 3'. For example, a 10' × 10' slab poured 4" thick contains 10 × 10 × 0.333 = 33.3 cubic feet, which is 33.3 ÷ 27 = 1.23 cubic yards. Add a 10% waste factor and you need approximately 1.36 cubic yards — about 62 eighty-pound bags if you are mixing by hand instead of ordering a truck.

This calculator handles the unit conversions automatically. Enter your dimensions in any format — feet-inches-fractions, decimal feet, decimal inches, or metric — and the calculator returns the total volume in both cubic yards and cubic meters, the number of bags needed for your chosen bag size, and the adjusted total with your selected waste factor applied.

Calculating Concrete for Footings

Continuous footings use the same Length × Width × Depth formula as slabs, but the proportions are different — footings are typically narrow and deep rather than wide and thin. Common footing sizes depend on the load they carry: 12" wide × 6" deep for garden walls and small sheds, 16" wide × 8" deep for standard residential foundations, and 24" wide × 12" deep for two-story homes or heavy loads.

Footing depth is driven by your local frost line — the bottom of the footing must sit below the maximum frost penetration depth to prevent heaving. Frost line depth varies dramatically by region: 12" in the southern United States, 36" or more in northern states, and up to 72" in parts of Alaska. Always check your local building code for the required footing depth before ordering concrete.

Column and Sonotube Calculations

Round columns and Sonotubes use the cylinder formula: π × (diameter / 2)² × height. Common Sonotube diameters are 8" for lightweight deck posts, 12" for standard structural columns, and up to 24" for heavy-load piers and commercial footings.

A single 12" diameter Sonotube poured 4' deep requires approximately 0.116 cubic yards of concrete. That is a small volume per column, but it adds up quickly — a deck with 12 piers needs nearly 1.4 cubic yards before adding waste. Enter the diameter, height, and quantity into this calculator and it computes the total volume for all columns at once.

Concrete Steps Volume

Concrete steps are poured as a solid mass, not as individual treads sitting on a frame. Each step adds its own rectangular volume on top of the step below, so the total volume increases with each additional step. Standard residential steps have a rise of approximately 7" and a tread depth of about 11", with the width of the staircase determining how much concrete each step consumes.

A three-step entry staircase 4' wide can easily require 0.5 to 0.75 cubic yards depending on the platform depth and landing slab. Steps are one of the most commonly underestimated concrete pours because people forget that the lower steps support the full volume of every step above them. This calculator computes the stepped volume correctly, accounting for the cumulative geometry of each riser and tread.

Understanding Waste Factor

You should always order more concrete than the calculated volume. Subgrade is never perfectly flat, forms flex slightly under the weight of wet concrete, some material spills during the pour, and over-excavation creates pockets that need to be filled. A 10% waste factor is the industry standard for most projects on reasonably level ground with well-built forms.

Increase to 15% for irregular shapes, sloped ground, stone or gravel subgrade that may have voids, or if this is your first concrete project. For large commercial pours on carefully prepared subgrade with precision forms, experienced contractors sometimes reduce the factor to 5%. When in doubt, round up — leftover concrete can fill post holes or small pads, but running short during a pour creates a cold joint that weakens the finished slab.

Bags vs Ready-Mix Truck

The break-even point between bagged concrete and a ready-mix truck is roughly 1 cubic yard. One cubic yard requires approximately 45 eighty-pound bags — that is 3,600 pounds of material you have to open, dump, mix, and pour by hand. For projects under 0.5 cubic yards (about 22 bags), bagged concrete is practical and cost-effective. Between 0.5 and 1 cubic yard, consider renting a portable mixer. Above 1 cubic yard, order a ready-mix truck.

Most ready-mix plants have a minimum delivery of 1 cubic yard, and some charge a short-load fee for orders under 3 to 5 yards. A cubic yard of ready-mix weighs approximately 4,000 pounds, so plan your access route — the truck itself can weigh over 60,000 pounds fully loaded. Have the site prepared, forms set, and all reinforcement in place before the truck arrives. Once the drum starts turning, you typically have 60 to 90 minutes to place and finish the concrete.

Pro Tips

• Always round up your order — running short in the middle of a pour is far worse than having a little leftover. Excess concrete can fill post holes, make a small pad, or be used for landscape edging.
• Order a minimum of 10% extra; use 15% for irregular shapes, sloped subgrade, or if this is your first concrete project. The waste factor covers form flex, uneven ground, and spillage.
• Pre-wet the subgrade before pouring to prevent dry soil from absorbing water out of the fresh concrete. Rapid moisture loss weakens the slab surface and can cause premature cracking.
• Have enough help on site — concrete waits for no one once the truck arrives. For a typical driveway pour, plan on at least three people: one to direct the chute, one to screed, and one to float behind.
• Check the weather forecast before scheduling your pour. Avoid placing concrete when temperatures are below 40°F (cold slows curing and risks freeze damage) or above 90°F (heat accelerates setting and can cause plastic shrinkage cracks).