Free construction math tool
Concrete Calculator
Estimate how much concrete you need for slabs, footings, post holes, and round forms. Get cubic feet, cubic yards, cubic meters, waste factor, bag counts, and a ready-mix order estimate without juggling unit conversions at the store.
By Chris Gaglardi Last Updated June 18, 2026
Concrete calculator
Choose a project shape, enter the dimensions, and add a waste factor. The calculator normalizes everything to feet before doing the volume math.
Results
Enter your dimensions and select Calculate concrete. Results will show volume, bag estimates, ready-mix rounding, and the formula used.
Concrete calculator guide
What this calculator assumes
- Finished dimensions: Enter the inside dimensions of the form, hole, trench, or tube you plan to fill.
- Simple shapes: Slabs and footings are treated as rectangles. Post holes and round forms are treated as cylinders.
- Waste factor included: The waste factor is applied before calculating bag counts and ready-mix rounding.
- Bag yields are estimates: The calculator uses common manufacturer yield values for general concrete mix. Specialty mixes, regional products, and moisture conditions can vary.
- Material quantity only: The tool does not choose slab thickness, footing size, reinforcement, base prep, drainage, frost depth, or load capacity.
How to calculate concrete
Concrete volume is just shape math. For a rectangular slab or footing, convert length, width, and thickness to the same unit, multiply them together, and then convert the result to the order unit you need.
Rectangular volume: cubic feet = length × width × thickness, with all dimensions in feet.
Cubic yards: cubic yards = cubic feet ÷ 27.
Round hole or column: cubic feet = π × radius² × depth.
The easy place to get burned is thickness. A 4-inch slab is 0.333 feet, not 4 feet. Convert inches before multiplying, or the estimate will go spectacularly sideways.
Concrete slab calculator
For a concrete slab, measure the form length and width, then enter the slab thickness. Common residential slab thicknesses vary by project, soil, load, reinforcement, and code requirements, so this tool does not tell you what thickness you should use. It only estimates material volume once you already know the design thickness.
For patios, sidewalks, garage pads, equipment pads, or shop slabs, confirm the required base, reinforcement, control joints, slope, drainage, and local requirements before pouring. This calculator is the tape measure and arithmetic part, not the engineering department.
Concrete footing calculator
For a rectangular footing, enter the footing length, width, and depth. Use the quantity field when you have multiple footings of the same size.
Footings can be code-sensitive because load, soil, frost depth, reinforcement, and local rules matter. If the footing supports a deck, wall, building, retaining structure, or anything else that would be expensive or dangerous to botch, use the calculator only after your dimensions come from plans, code requirements, or a qualified pro.
Concrete post hole calculator
For post holes, enter the hole diameter, hole depth, and number of holes. The calculator treats each hole like a cylinder:
Post hole volume: π × (diameter ÷ 2)² × depth × number of holes.
Real holes are rarely perfect cylinders. Augers wander, sides crumble, bell-shaped bottoms add material, and loose soil can push the actual volume above the neat estimate. A waste factor helps cover that messier jobsite reality.
Round form and column calculator
For a round pier, column, or form tube, enter the inside diameter, height, and number of forms. The calculator uses the same cylinder formula as the post-hole mode, but the label changes from depth to height.
This mode is useful for simple round concrete forms. It is still only a material estimate. If the form is structural, load-bearing, reinforced, or tied to a foundation detail, use dimensions from plans, local code requirements, or a qualified pro before ordering.
How many bags of concrete do I need?
The calculator estimates bag counts using common approximate yields for general concrete mix. Manufacturer data matters because product type, bag size, region, and mix design can change yield.
| Bag size | Approximate yield | Approx. bags per cubic yard before waste |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 bags |
| 50 lb | 0.375 ft³ | 72 bags |
| 60 lb | 0.45 ft³ | 60 bags |
| 80 lb | 0.60 ft³ | 45 bags |
For example, one cubic yard is 27 cubic feet. If an 80 lb bag yields about 0.60 cubic feet, 27 ÷ 0.60 = 45 bags before waste. That is a lot of lifting, which is one reason bigger jobs often shift from bags to ready-mix.
Why add a waste factor?
A waste factor accounts for the gap between neat math and messy dirt reality. Uneven subgrade, over-excavation, spills, form flex, measurement errors, and imperfect hole shapes can all increase concrete demand. The calculator defaults to 10 percent because it is a practical starting point, not a guarantee.
Use a higher waste factor for rough excavations, multiple small forms, hand-dug holes, irregular shapes, or any job where running short would be a disaster.
When to order ready-mix instead of bags
Bagged concrete is handy for small jobs, scattered post holes, tight access, and repairs. Ready-mix often becomes more practical when the job approaches a cubic yard or more, when a continuous pour matters, or when mixing dozens of bags would slow the work or create consistency problems.
The calculator rounds the ready-mix estimate up to your selected increment. Your supplier may have minimum orders, short-load fees, delivery windows, slump requirements, and access rules, so treat the rounded number as a conversation starter, not a purchase order.
Common mistakes when estimating concrete
- Mixing inches and feet: Convert thickness before multiplying. Four inches is 0.333 feet.
- Forgetting waste: Perfect-volume orders leave no room for uneven subgrade, form flex, spills, or measuring mistakes.
- Ignoring subgrade variation: A low spot under a slab can eat more concrete than expected.
- Using post size instead of hole size: A 4-inch post in a 12-inch hole needs concrete for the 12-inch hole, not the post.
- Treating the estimate as structural design: Volume math does not determine thickness, reinforcement, frost depth, or load capacity.
Construction training and related skilled trades
Concrete estimating is basic jobsite math, but it touches a lot of skilled trades. If you like practical layout work, materials, measurements, and building things that do not exist only inside a spreadsheet, these TSNET pages are worth a look:
Concrete calculator FAQ
How do I calculate how much concrete I need?
For a slab or footing, multiply length by width by thickness after converting all dimensions to the same unit. If you use feet, the result is cubic feet. Divide by 27 to convert cubic feet to cubic yards.
How many 80 lb bags of concrete are in a cubic yard?
Using an approximate yield of 0.60 cubic feet per 80 lb bag, one cubic yard takes about 45 bags before adding waste. Always check the yield printed on the specific product you are buying.
How much concrete do I need for post holes?
Measure the hole diameter and depth, not the post size. The calculator uses the cylinder formula: π × radius² × depth, multiplied by the number of holes.
Should I use bags or order ready-mix?
Bags are practical for small or scattered jobs. Ready-mix is usually worth considering as the project gets closer to a cubic yard or when you need a continuous pour. Delivery fees, minimum orders, access, and timing can change the decision.
Can I use this for columns or round concrete forms?
Yes, for simple round forms. Choose the round form option, then enter the inside diameter, height, and number of forms. Use engineered or code-approved dimensions for structural piers, columns, and foundation work.
Does this calculator tell me how thick my slab or footing should be?
No. It only calculates material quantity from dimensions you provide. Thickness, reinforcement, base preparation, frost depth, load capacity, and footing design depend on the project and local rules.
Source and formula notes
- Rectangular volume: length × width × thickness, normalized to feet.
- Cubic yards: cubic feet ÷ 27.
- Circular volume: π × radius² × depth.
- Metric conversion: 1 cubic foot = 0.028316846592 cubic meters.
- Bag yields are approximate and product-specific. This calculator uses 40 lb = 0.30 ft³, 50 lb = 0.375 ft³, 60 lb = 0.45 ft³, and 80 lb = 0.60 ft³. These match QUIKRETE Concrete Mix No. 1101 yield references and closely align with common Sakrete yield references, but the specific bag you buy is the source of truth.
- QUIKRETE Concrete Mix data sheet lists approximate yields for 40, 50, 60, and 80 lb bags.
- Sakrete High-Strength Concrete Mix and Sakrete Crack-Resistant Concrete Mix provide additional product-yield references. Note that some manufacturers round 50 lb yield to 0.38 ft³, so always verify the product you actually buy.
- Sakrete estimating guidance is used as a sanity check for cubic-foot-to-cubic-yard math and the practical 10 percent waste-factor note.