Concrete Calculator

1. Choose the shape: a slab or footing (length by width by thickness) or a round column or pier (diameter by height).
2. Enter the dimensions. Keep length, width and height in feet, and thickness or diameter in inches, exactly as the fields are labelled.
3. Set the quantity of identical pours, add a waste allowance of 5 to 10 percent, and optionally a ready-mix price to estimate cost.
4. Read the result: cubic yards is what a supplier delivers against, while bags and cost help you compare ordering options.

Here is what each result line means:

A concrete calculator is a tool that converts the dimensions of a pour into the amount of concrete you need to buy. It answers the single most common question on any slab, footing or post project: how much concrete do I order?

It helps homeowners, DIY builders, landscapers and contractors turn a sketch into an order. Concrete is sold by the cubic yard for ready-mix delivery, or by the bag for small pours, so the calculator translates feet and inches into both units at once. It is an estimating aid, not a structural design tool: it tells you the volume to order, not the mix strength, reinforcement or footing size your project requires.

Volume is length times width times depth. Because concrete is priced in cubic yards and one cubic yard equals 27 cubic feet, the calculator finds the volume in cubic feet and divides by 27.

1. Convert thickness from inches to feet by dividing by 12 (a 4 inch slab is 0.333 feet).
2. Multiply length by width by that thickness to get cubic feet, then multiply by the quantity of identical pours.
3. Divide by 27 for cubic yards, and multiply by 0.0283168 for cubic meters.
4. Add the waste allowance, then divide cubic feet by the bag yield (0.60 cubic feet for an 80 pound bag) for a bag count.

For a round column the footprint is a circle, so the volume is pi times the radius squared times the height, with the radius taken as half the diameter.

Suppose you are pouring a patio slab 10 feet long, 10 feet wide and 4 inches thick, with a 10 percent waste allowance.

Calculation: thickness is 4 / 12 = 0.333 ft. Volume = 10 x 10 x 0.333 = 33.33 cu ft. In cubic yards that is 33.33 / 27 = 1.23 cu yd. Adding 10 percent waste gives 1.36 cu yd, or about 36.7 cu ft. At an 80 pound bag yield of 0.60 cubic feet, that is 36.7 / 0.60 = 62 bags.

The lesson: a pour this size sits on the line between bagging and a small ready-mix order. Sixty-plus bags is a lot of mixing, so most people would price a short-load delivery against the bag cost before deciding.

Thickness

Thickness drives volume more than people expect because it multiplies the whole area. Moving a slab from 4 to 6 inches adds 50 percent to the concrete, so confirm the thickness your project actually calls for.

Subgrade and Over-excavation

A soft or uneven subgrade swallows concrete where the base dips below grade. This is the main reason real pours use more than the raw geometry suggests, and why a waste allowance exists.

Waste and Spillage

Some concrete is lost to the forms, the wheelbarrow, the chute and trimming. A 5 to 10 percent allowance covers it; add more for a complex pour or a long barrow run.

Shape and Edges

Thickened edges, footings under a slab, and round or stepped shapes all change the volume. Break an irregular pour into simple rectangles and columns, calculate each, and add them.

The volume decides which way of buying concrete makes sense. Bags suit small pours; ready-mix by the cubic yard suits large ones. The crossover is usually somewhere between one and two cubic yards.

An 80 pound bag yields about 0.60 cubic feet, so a single cubic yard (27 cubic feet) takes about 45 bags. Once a project needs dozens of bags, the ready-mix cost line in this calculator usually wins.

Ordering Ready-mix

Before you phone the supplier, size the order in cubic yards and add waste, so you neither run short mid-pour nor pay for concrete you cannot place.

Costing a Project

Enter a delivered price per yard to turn volume into a budget figure, then compare it against the equivalent cost in bags.

Planning Footings and Piers

Use the round-column shape for deck footings, fence posts and piers, multiply by the number of holes, and you have the total in one step.

1. Mixing Up Feet and Inches

Entering thickness in feet instead of inches shrinks the result twelvefold. Keep length and width in feet and thickness in inches, as the fields are labelled.

2. Forgetting a Waste Allowance

Ordering the exact geometric volume leaves no margin for an uneven base or spillage. Running short mid-pour means a cold joint or a second delivery.

3. Ignoring Thickened Edges and Footings

A slab with a footing or a thickened edge holds more concrete than a flat rectangle. Add those parts separately.

4. Rounding Down Bags

Always round the bag count up. A part bag still has to be bought, and being one bag short stops the pour.

5. Treating the Estimate as a Structural Spec

Volume is not strength. Thickness, mix and reinforcement for a load-bearing slab or footing come from a code or an engineer, not from this tool.

The arithmetic is exact; the usefulness of the answer depends on the dimensions and the waste allowance you give it.

A 10 by 10 foot slab at 4 inches thick needs about 1.23 cubic yards, or roughly 1.36 cubic yards once you add a 10 percent waste allowance. That is around 56 to 62 eighty-pound bags, which is why most people order ready-mix at this size.

About 45. An 80 pound bag of pre-mixed concrete yields roughly 0.60 cubic feet, and a cubic yard is 27 cubic feet, so 27 divided by 0.60 is 45 bags. A 60 pound bag yields about 0.45 cubic feet, which is about 60 bags per yard.

Divide the cubic feet by 27, because one cubic yard contains 27 cubic feet. For example, 54 cubic feet is 2 cubic yards. The calculator does this step for you and also shows cubic meters.

Between 5 and 10 percent for most pours. The allowance covers spillage, an uneven subgrade and concrete left in the barrow or chute. Add more for a complex shape, a long barrow run or a rough base.

Yes. Switch the shape to round column or pier and enter the diameter in inches and the height in feet. For several identical footings, set the quantity and the tool totals them in one step.

It depends on the load. A footpath or patio is often 4 inches, while a driveway that carries vehicles is usually 6 inches or more. Use the figure from your plans or local code; this tool estimates volume, not the required thickness.

Suppliers add short-load fees, delivery charges and tax, and they may round up to a whole or half yard. The calculator estimates the concrete volume and material cost, not the full invoice. Always confirm the final quantity with the supplier.

Yes. Running short mid-pour risks a cold joint or a second delivery, both of which cost far more than a small surplus. The waste allowance is there to give you that margin without guessing.

About 4,050 pounds, or roughly 2 short tons, for normal-weight concrete at 150 pounds per cubic foot. Lightweight mixes weigh less. The calculator shows the estimated weight so you can plan barrow loads, access and subgrade support.

For small pours under about a cubic yard, bags are convenient and often cheaper once you count delivery and short-load fees. Past one to two cubic yards, ready-mix usually wins. Enter both a price per yard and a price per bag and the verdict names the cheaper option.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• QUIKRETE Concrete Mix product data (QUIKRETE; bag sizes and coverage).
• QUIKRETE concrete and bag calculator (QUIKRETE; bag-count method).
• National Ready Mixed Concrete Association (NRMCA; ready-mixed concrete and ordering).
• American Cement Association (Portland Cement Association) (American Cement Association; what concrete and cement are).