What is a concrete calculator?
A concrete calculator is a free estimation tool that converts your project dimensions into the amount of concrete needed — expressed in cubic yards or bags — so you can order the right quantity before the pour. Use this concrete calculator to calculate concrete volume for slabs, footings, columns, driveways, patios, and poured walls without manual conversion errors.
The tool supports both imperial and metric inputs — enter dimensions in inches, feet, or meters — and outputs volume in cubic feet, cubic yards, and cubic meters alongside a bags of concrete count for small pours. It also covers mortar, mortar mix, and concrete repair estimates so you have one place to handle all your concrete project quantities.
How the concrete calculator works
The calculator runs shape-specific volume formulas and then converts the result into practical ordering units. Here are the three core calculations:
Volume (ft³) = Length × Width × Thickness
Use this for a concrete slab, garage floors, patios, small patio pads, sidewalks, and square footings. The length of your concrete slab or length of your concrete footing drives the result. Divide cubic feet by 27 to get the number of cubic yards.
Volume (ft³) = π × radius² × Thickness
Enter the diameter and the thickness for a circular slab or round footings. The calculator converts radius automatically. Use this mode for round piers, post bases, and circular patio pours.
Volume (ft³) = π × radius² × Height
For a concrete column, enter the diameter and height of your concrete column to calculate how much concrete you need. The column to calculate how much volume is needed uses the same cylinder formula as round footings.
Cubic yards to bags conversion
One cubic yard of concrete ≈ 45 × 80 lb bags
Standard 80 lb bags yield approximately 0.60 ft³ of mixed concrete. The calculator divides your total volume by the yield per bag and rounds up to give you the number of bags needed to complete your project. For larger jobs, switch to ready mix concrete — ordering by the cubic yard of concrete is more practical than counting 80-pound bags.
How to use this concrete calculator
Use our concrete calculator to estimate the concrete needed to complete any pour in five steps:
- 1
Choose the project type
Select slab, footing, column, or known volume mode. Each mode pre-loads the correct input fields. Use slab mode for a concrete slab to calculate area-based pours; use footing mode for a concrete footing to calculate strip and pad foundations; use column mode for a concrete column to calculate pier and post volumes.
- 2
Enter the dimensions
Enter the depth, width and length of your concrete shape in feet or inches. For circular shapes, enter the diameter and height. The tool accepts mixed units and converts automatically between US customary or metric.
- 3
Set the thickness of concrete
Thickness of concrete is the single biggest variable in slab volume. A standard residential slab runs 4 inches thick; driveways and garage floors typically use 5–6 inches. The slab depends on load requirements — always verify with your local building code.
- 4
Add waste factor and bag size
Add 5–10% overage for standard pours and 10–15% for footings or columns with complex form shapes. Select your bag size — 60 lb or 80 lb bags — to get a concrete bags needed to complete your project count alongside the cubic yard figure.
- 5
Review estimated material and costs
The calculator returns volume in cubic yards, bag count, estimated concrete weight, and optional material costs per square foot. Use the results to compare ready mix concrete delivery against bagged concrete products for your project scale.
Common use cases
Garage floors & driveways
Enter the slab dimensions and 5-inch thickness to get cubic yards needed for a concrete slab. Add 8% overage for garage floors to account for edge forms and spillage.
Footings & foundations
Use footing mode for the length of your concrete footing, width, and depth. Square footings and continuous strip footings both use the rectangular volume formula.
Fence post & pier bases
Round footings for fence posts are small-volume pours where bags of concrete make more sense than ready mix. Enter the diameter and depth to get the many bags needed per hole.
Block wall & mortar
Use the block wall and mortar mix estimation mode to calculate concrete needed for a rectangular or block wall core fill alongside mortar quantities for the joints.
Rebar & reinforcement
Toggle rebar estimation on to estimate the amount of rebar for a slab based on bar spacing and slab dimensions. The calculator outputs bar lengths and lap splice counts alongside the concrete volume.
Concrete repair & patching
For concrete repair and patching with precast concrete products or bagged mixes, enter the patch dimensions to calculate how much concrete you need in bags without committing to a full cubic yard order.
Frequently asked questions
- How much concrete do I need for a rectangular slab?
- To calculate concrete needed for a rectangular slab, multiply the length, width, and thickness in feet, then divide by 27 to convert to cubic yards. For example, a 10 × 10 ft slab at 4 inches thick equals (10 × 10 × 0.333) ÷ 27 = 1.23 cubic yards. Use this concrete calculator to get results in US customary or metric units instantly.
- How many bags of concrete do I need?
- The number of bags depends on bag size and total volume. One cubic yard of concrete requires approximately 45 × 80 lb bags. For smaller pours — fence posts, repair patches, and small patio sections — enter the dimensions and the calculator returns the concrete bags needed to complete the pour at your chosen bag weight.
- What overage percentage should I add?
- Add 5–10% for standard rectangular slabs and garage floors. Use 10–15% for footings or columns, circular pours, or any project with complex formwork. The slab and the gravel needed beneath it should both be estimated with overage — running short mid-pour is a costly problem.
- Can I use this for footings or columns?
- Yes. Select footing mode and enter the length of your concrete footing, width, and depth for strip or pad footings. For columns and piers, enter the diameter and height of your concrete column to calculate how much concrete is needed. The calculator handles both square footings and round footings.
- When should I use ready mix instead of bags?
- Ready mix concrete becomes more economical above roughly 1 cubic yard. For garage floors, driveways, and larger jobs, ordering by the cubic yard of concrete from a ready mix supplier is faster and often cheaper than mixing 80-pound bags on site. The calculator shows both options so you can compare.
- Does the calculator estimate rebar?
- Yes — toggle the rebar option to estimate the amount of rebar for a slab based on your slab dimensions and bar spacing. The calculator outputs total bar length, the number of bars per direction, and lap splice allowances alongside the concrete volume estimate.
Calculate how much concrete you need
Use our free concrete calculator to estimate the concrete needed to complete your project — whether that is a garage slab, a set of round footings, or a block wall core fill. Enter the dimensions, set your waste factor, and get cubic yards, bag count, and material costs in seconds so you can order with confidence and avoid a mid-pour shortage.