Free Online Tool
Concrete Calculator: Know Exactly How Much You Need
Enter the dimensions of your slab, footing, column, or post hole and get an instant, accurate estimate — in cubic yards, cubic feet, cubic meters, and ready-mix bags. No account required.
Formula: Concrete volume (cubic yards) = Length (ft) × Width (ft) × Thickness (ft) ÷ 27. For a standard 10 × 10 ft slab at 4 in thick, that's 10 × 10 × 0.33 ÷ 27 ≈ 1.23 cubic yards — about 56 bags of 60 lb concrete once you add a 10% waste allowance. Use the calculator below to run this for your own dimensions and shape.
The Tool
Concrete Volume & Bag Calculator
Select a shape, enter your dimensions, and calculate. Works for rectangular slabs and footings, circular slabs, columns/cylinders, and post holes.
Estimates use standard ACI yield assumptions and manufacturer-published bag yields (40 lb = 0.30 ft³, 60 lb = 0.45 ft³, 80 lb = 0.60 ft³). Always confirm quantities with your supplier before ordering.
Need reinforcement too? Use the rebar calculator and gravel base calculator to complete your material list.
Methodology
How to Use the Concrete Calculator
Four steps, no measuring-tape math required.
Choose your shape
Rectangular slab or footing, circular slab, column/cylinder, or post hole — pick whichever matches your pour.
Enter dimensions
Type length, width, diameter, thickness or height. Switch to metric if you work in meters and centimeters.
Set quantity & waste
Multiple footings or posts? Set the quantity. Add a 5-10% waste factor for spillage and uneven ground.
Read your results
Get cubic yards, cubic feet, cubic meters, and bag counts by size — instantly, no page reload.
Reading Your Numbers
Understanding Cubic Yards, Bags & Waste Factor
Concrete is sold two ways: by volume for ready-mix trucks, and by bag for small or DIY pours. A cubic yard is 27 cubic feet — the standard unit ready-mix suppliers quote and bill by, and most trucks carry between 8 and 10 cubic yards per load. A cubic meter (used across the UK, EU, Australia, and most of Asia) equals roughly 1.31 cubic yards. When your job is small, converting that same volume into bagged concrete is more practical: a 60 lb bag yields about 0.45 cubic feet once mixed, an 80 lb bag about 0.60 cubic feet, and a 40 lb bag about 0.30 cubic feet.
Every concrete calculator, including this one, should build in a waste factor — extra material to cover an uneven sub-grade, formwork that isn't perfectly square, spillage during placement, and minor measurement error. Industry guidance generally recommends 5% for a straightforward rectangular slab poured on flat, well-prepared ground, and up to 10% for irregular shapes, sloped sites, or hand-mixed small pours. Ordering short means a second delivery or an emergency bag run mid-pour — always round up rather than down.
Explore Further
Concrete Calculators by Shape
Each shape has its own dedicated calculator with deeper guidance for that pour.
Slab Calculator
Square and rectangular slabs for patios, garages, and floors.
ShapeCircular Slab Calculator
Round pads for fire pits, hot tubs, and light poles.
ShapeFooting Calculator
Strip and pad footings that support walls and columns.
ShapeSonotube Calculator
Cylindrical concrete piers poured inside cardboard forms.
ShapePoured Wall Calculator
Foundation and retaining walls measured by length, height, thickness.
ShapePost Hole Calculator
Fence, deck, and mailbox post footings, per hole or by the batch.
Explore Further
Concrete Calculators by Project
Prefer to start from your project instead of a shape? These calculators come pre-set with typical dimensions for the job.
Driveway
Standard 4-6 inch driveway slabs sized to your vehicle count.
ProjectPatio
Outdoor living slabs from small pads to full backyard patios.
ProjectGarage Slab
1, 2, and 3-car garage footprints with standard thickness presets.
ProjectShed Foundation
Slab-on-grade foundations for storage sheds and workshops.
ProjectFence Posts
Bag counts per post, scaled to your total fence line.
ProjectHouse Foundation
Full foundation volume estimates for new-build planning.
Technical Reference
Concrete Mix Ratio & Strength Quick Reference
Concrete strength is specified by compressive strength (PSI in the US, MPa or grade elsewhere) and matched to a cement:sand:aggregate mix ratio. Choosing the right grade matters as much as getting the volume right — under-strength concrete cracks and spalls under load it wasn't designed for.
| Strength | Typical Mix Ratio | Common Use |
|---|---|---|
| 2,500-3,000 PSI | 1 : 3 : 4 (approx.) | Non-structural fill, walkways |
| 3,500-4,000 PSI | 1 : 2.5 : 3.5 | Driveways, patios, sidewalks |
| 4,000-4,500 PSI | 1 : 2 : 3 | Garage slabs, footings, foundations |
| 5,000+ PSI | 1 : 1.5 : 2.5 | Structural columns, heavy-load slabs |
See the full mix ratio calculator for metric grade equivalents (M20, M25, M30) and admixture dosage guidance.
Direct Answers
Frequently Asked Questions
How much concrete do I need for a 10x10 slab?
A 10 ft × 10 ft slab poured at the standard 4-inch thickness needs about 1.23 cubic yards of concrete, or roughly 1.35 cubic yards once a 10% waste allowance is added — about 56 bags of 60 lb pre-mixed concrete.
How many 80 lb bags of concrete are in a cubic yard?
One 80 lb bag yields about 0.6 cubic feet, so it takes roughly 45 bags of 80 lb concrete to make one full cubic yard (27 cubic feet).
How thick should a concrete slab be?
Most residential slabs are poured 4 inches thick. Driveways carrying vehicles typically use 5-6 inches, and slabs supporting heavy equipment or structural loads may need 6 inches or more, with reinforcement.
How much concrete do I need per fence post?
A typical 8-inch diameter hole, 24 inches deep, needs roughly one 50 lb or 60 lb bag per post. Larger posts or deeper holes may need two bags — use the Post Hole tab above with your quantity set to your total post count.
What is a good waste factor to add to a concrete order?
Add 5% to 10% extra beyond the calculated volume to cover spillage, uneven subgrade, and formwork variance. Irregular shapes or hand-mixed pours often warrant the higher end of that range.
Is concrete the same as cement?
No. Cement is a powdered binder — one ingredient within concrete. Concrete is the finished material made by mixing cement with water, sand, and coarse aggregate (gravel or crushed stone).
How many cubic yards are in a cubic meter of concrete?
One cubic meter equals approximately 1.308 cubic yards.
Can I use this calculator for both bagged and ready-mix concrete?
Yes. It returns cubic yards for ordering ready-mix by the truckload, and converts that same volume into 40 lb, 60 lb, and 80 lb bag counts for DIY bagged-concrete projects.
Terminology
Key Concrete Terms
New to concrete work? These core terms come up throughout the calculators and guides on this site.
See the full A-Z concrete glossary for all definitions.
About This Tool
Built on Published Engineering Standards
Every formula behind this calculator is grounded in publicly documented references: volume math follows standard geometry used across ACI (American Concrete Institute) guidance, and bag-yield figures match values published on manufacturer spec sheets (Quikrete, Sakrete, and equivalent producers). This tool is provided as a planning estimate — always confirm final order quantities with your ready-mix supplier or a licensed contractor before a structural pour, since site conditions, sub-grade prep, and local code requirements can shift real-world needs.