Concrete Calculator

How Much Concrete Do I Need?

To calculate how much concrete you need, multiply the length by the width by the thickness to get cubic feet. Then divide cubic feet by 27 to convert the result to cubic yards. Add extra concrete for waste, uneven excavation, form variation, and spillage.

Most ready-mix concrete is ordered by the cubic yard. Bagged concrete is usually estimated by dividing the required cubic feet by the yield of each bag.

How to Calculate Concrete for a Slab

A concrete slab is calculated by multiplying slab length by slab width by slab thickness. This method works for patios, walkways, shed pads, garage slabs, driveways, and other flat rectangular concrete pours.

Common Slab Thicknesses

For a driveway or garage slab, do not estimate based on volume alone. Subbase preparation, reinforcement, control joints, drainage, and expected loads can affect the final slab design.

How to Calculate Concrete for Footings

Footings are calculated by multiplying footing length by footing width by footing depth. This is useful for strip footings, wall footings, and rectangular footing trenches.

Footing excavations often vary slightly in width and depth. For that reason, a 10% to 15% waste factor is often more realistic than ordering the exact calculated amount.

How to Calculate Concrete for Post Holes

Post holes are calculated as cylinders. For a more precise estimate, subtract the volume of the post from the volume of the hole. This is useful for fence posts, deck posts, mailbox posts, pole footings, and similar round holes.

If you do not want to subtract post displacement, set the post diameter deduction to zero in the calculator. This gives a more conservative concrete estimate.

How to Calculate Concrete for Columns and Piers

Round columns, piers, and sono tubes are calculated using the cylinder volume formula. Enter the diameter and height or depth of the pier, then multiply by the number of identical columns or piers.

How to Calculate Concrete for Walls

Concrete walls are calculated by multiplying wall length by wall height by wall thickness. If the wall has openings, such as door openings or large blockouts, subtract the opening volume from the total.

How to Calculate Concrete for Stairs

Concrete stairs are more complex than flat slabs because the volume depends on the rise, run, number of steps, stair width, and any landing area. A solid stair estimate combines the stair volume with the optional landing volume.

For irregular stair designs, field measurements and formwork details can significantly affect the final amount of concrete needed.

How Many Bags of Concrete Do I Need?

The number of concrete bags depends on the total concrete volume and the yield of each bag. The calculator divides the final cubic feet of concrete by the selected bag yield, then rounds up to the next full bag.

Always round up when buying concrete bags. Running short during a pour can create weak joints, visible seams, or delays while the concrete is already setting.

Concrete Bag Sizes and Yields

Bag yield varies by brand, mix, and water content, so always check the bag label before purchasing. These values are common planning yields used for estimating.

How Much Does Concrete Weigh?

Normal-weight concrete is often estimated at about 150 pounds per cubic foot for planning purposes. Actual weight can vary based on mix design, aggregate, moisture, reinforcement, and air content.

Concrete weight is useful when thinking about transport limits, trailer loads, disposal, demolition, elevated slabs, or temporary storage of bagged material.

How Much Extra Concrete Should You Order?

A waste factor helps account for uneven excavation, spillage, form movement, subgrade variation, and small measurement errors. A 10% waste factor is a practical default for many concrete projects, but irregular excavation may require more.

Bagged Concrete vs Ready-Mix Concrete

Bagged concrete is usually best for small jobs such as fence posts, deck footings, repairs, and small pads. Ready-mix concrete is usually more practical for larger pours because it reduces labor and provides more consistent mixing.

Concrete Reinforcement Estimating

Reinforcement needs depend on the slab use, thickness, support conditions, loads, soil conditions, and local requirements. For simple slab planning, a rebar or mesh estimate can help understand the approximate amount of reinforcement needed before ordering materials.

For more detailed takeoffs, use a dedicated rebar calculator to estimate bar spacing, lap lengths, grid layouts, and total linear footage.

Concrete Cost Estimate

Concrete cost depends on local ready-mix prices, bag prices, delivery fees, short-load fees, labor, reinforcement, subbase material, and finishing requirements. The calculator estimates material cost using your entered cost per cubic yard and cost per bag.

Site-Mixed Concrete Ratios

Some users mix concrete on site using cement, sand, and gravel rather than pre-mixed bags. A common basic reference ratio is 1 part cement, 2 parts sand, and 3 parts gravel by volume, but the proper mix depends on the required strength, exposure, aggregate, water content, and project requirements.

For structural work, use a specified concrete mix design rather than relying on a generic ratio.

Common Concrete Estimating Mistakes

• Entering slab thickness in feet instead of inches.
• Forgetting to divide cubic feet by 27 to convert to cubic yards.
• Ordering the exact calculated amount with no waste factor.
• Ignoring uneven excavation or subgrade variation.
• Using bagged concrete for a project that is too large to mix efficiently.
• Forgetting to subtract post displacement from post hole volume when precision matters.
• Forgetting that stairs, trenches, and irregular forms often need a higher waste factor.