Calculate the volume of concrete needed for slabs, columns, and footings, plus the number of bags required.
Concrete Required
About the Concrete Calculator โ Concrete Calculator Online
This free concrete calculator online helps contractors, builders, and DIY homeowners determine exactly how much concrete they need for any project. Enter your project dimensions and the tool instantly calculates concrete volume in cubic yards, cubic feet, and cubic meters, plus the number of 60 lb and 80 lb pre-mixed bags required โ including a built-in 10% waste factor to account for spillage and uneven ground. Supported project types include rectangular slabs and footings, circular columns and round posts, and stair steps.
Getting the concrete quantity right before you start a pour is critical. Too little concrete means stopping mid-pour to mix more, which can cause cold joints and structural weaknesses. Too much concrete wastes money and creates a disposal problem. This concrete calculator online takes the guesswork out of the ordering process by giving you accurate volume and bag counts based on your actual dimensions, supporting both imperial (feet) and metric (meters) inputs.
How to Use the Concrete Calculator
Select your Shape / Project Type from the dropdown: Rectangular Slab / Footing for flat pours, Circular Column for round posts or pier footings, or Stair Steps for a set of cast-in-place stairs.
Select your preferred unit of measurement โ Feet/Inches or Meters. All dimension fields will use the selected unit.
Enter the dimensions for your selected shape. For a slab, enter length, width, and depth/thickness. For a column, enter diameter and height. For stairs, enter rise, run, width, and number of steps.
Click Calculate to see the results. The tool displays volume in cubic yards, cubic feet, and cubic meters, plus the number of 60 lb and 80 lb bags needed and the equivalent number of ready-mix truck loads.
Click Clear to reset all fields and start a new calculation for a different section of your project.
Volume Formulas and Bag Yield Calculations
For a rectangular slab or footing, volume in cubic feet equals Length ร Width ร Depth. The tool divides by 27 to convert to cubic yards. For a circular column, volume equals ฯ ร (Diameter รท 2)ยฒ ร Height. For stair steps, each step is treated as a rectangular block accumulating in a staircase pattern and the total volume is summed across all steps. Bag estimates use standard yields: a 60 lb bag yields approximately 0.45 cubic feet of mixed concrete, and an 80 lb bag yields approximately 0.60 cubic feet. Both bag counts include a 10% waste factor applied automatically.
Tips for Getting the Best Results
Convert inches to feet before entering thickness: Slab depths are often specified in inches. Divide by 12 to get feet: 4 inches = 0.333 ft, 6 inches = 0.5 ft, 8 inches = 0.667 ft. Enter the decimal foot value in the Depth field for an accurate calculation.
Order at least 10% more than the calculated volume: The bag estimate already includes a 10% waste factor, but for ready-mix orders you should ask your supplier to round up to the nearest quarter-yard or add 5โ10% manually to account for uneven subgrade, form deflection, and spillage during the pour.
Use 80 lb bags for large projects to save money: An 80 lb bag yields 33% more concrete than a 60 lb bag and typically costs less per cubic foot. Choose 60 lb bags only if you are working alone and need manageable bag weights, or when mixing in small batches with limited equipment.
Consider ready-mix for anything over 1 cubic yard: Mixing bagged concrete by hand or with a small mixer is practical for small projects, but projects requiring more than 1 cubic yard (about 27 cubic feet) often benefit from ordering ready-mix delivery. Ready-mix is priced per yard and eliminates the labor of mixing dozens of bags.
Calculate each section separately for complex projects: For a driveway with different thickness zones, or a project that combines a slab and footings, use the Clear button and run separate calculations for each section, then add the results together to get the total order quantity.
Why Use a Concrete Calculator Online
A concrete calculator online removes the need to perform multiple unit conversions and volume calculations by hand on a job site where mistakes are costly. This tool runs in any browser, works on a phone or tablet, and requires no app download or account. Because it runs entirely in JavaScript locally, it works offline once the page is loaded โ useful on construction sites with limited connectivity. Results are instant, letting you check calculations multiple times as dimensions change during planning.
Professional contractors use this concrete calculator to estimate material costs and create accurate quotes for clients. DIY homeowners use it to plan backyard projects like fence post footings, garden paths, and shed pads without needing to hire an estimator. Civil engineering students use it as a reference to verify manual calculations and understand the relationship between dimensions, volume, and material quantities.
Frequently Asked Questions about the Concrete Calculator
Divide the depth in inches by 12 to get the decimal foot value. Common slab depths: 3.5 inches = 0.292 ft (typical residential slab), 4 inches = 0.333 ft, 6 inches = 0.5 ft, 8 inches = 0.667 ft. For driveway slabs, 4 inches is standard for light passenger vehicles; 6 inches is recommended for driveways that will carry heavy trucks or RVs.
Yes. The bag count already includes a 10% waste factor built in to this calculator, but for ready-mix orders from a batch plant you should specify the volume and ask the dispatcher to round up to the nearest quarter-yard minimum. Experienced contractors add 5โ10% to ready-mix orders to account for subgrade irregularities, form bulging under pressure, and normal spillage during the pour. Running short mid-pour creates a cold joint that can weaken the finished slab.
Ready-mix concrete delivered by truck is more economical than bagged concrete when your project requires more than approximately 1 cubic yard (27 cubic feet). At that volume, the cost per cubic foot of ready-mix is typically lower than bagged concrete, and you save significant labor from mixing. Most batch plants have a minimum order of 1โ2 cubic yards. For projects under 0.5 cubic yards, bagged concrete mixed with a rented electric mixer is usually the most practical and economical option.
A 60 lb bag of pre-mixed dry concrete yields approximately 0.45 cubic feet of mixed concrete. An 80 lb bag yields approximately 0.60 cubic feet. Both bags contain the same concrete mix โ the difference is just the quantity. The 80 lb bag is more economical per cubic foot but weighs 33% more, which matters when you are mixing many bags by hand. For overhead or strenuous pours where bag handling is a concern, 60 lb bags are easier to manage.
A standard front-discharge or rear-discharge ready-mix concrete truck holds 8โ10 cubic yards per load. Many batch plants have a minimum delivery quantity โ often 1โ3 cubic yards โ and charge a short-load fee for orders below the truck's capacity. If your project is close to 4โ5 cubic yards and you have flexibility in timing, ordering a full truckload may be more cost-effective than two smaller deliveries.
Residential walkways and patios typically use 4 inches (0.333 ft) of concrete thickness. Driveways for passenger cars are typically 4โ5 inches thick. Driveways that will support heavier vehicles such as RVs, trucks, or heavy equipment should be 6 inches thick. Commercial slabs bearing forklifts or heavy machinery are often 6โ8 inches or more. Always consult local building codes and a structural engineer for load-bearing applications.
Use the Circular Column calculator mode for cylindrical fence post holes. Enter the diameter of the augered hole (typically 10โ12 inches, which is 0.83โ1.0 ft) and the depth of the hole (typically one-third of the post length plus 6 inches for drainage). Multiply the per-hole volume by the number of holes for the total quantity. A standard 10-inch diameter hole, 30 inches deep, requires approximately 1.36 cubic feet โ which is about three 60 lb bags or two 80 lb bags per hole.
No. This concrete calculator calculates concrete volume and bag quantities only. It does not account for rebar, wire mesh, fiber reinforcement, forms, footings, or compacted base material. For structural projects, the concrete volume is just one component of the total material estimate. Rebar quantity depends on the structural design, spacing requirements, and local building codes. Consult a structural engineer for reinforced concrete applications such as retaining walls, foundations, or load-bearing columns.