Concrete Yard Calculator
How Many Cubic Yards Do You Need?
Calculate cubic yards of concrete for slabs, footings, columns, or walls. Select your pour type, enter dimensions, and get an order-ready yard count with the correct overage applied for your application.
Get your yard count before you call the plant. Select your pour type below and enter dimensions to get an order-ready cubic yard figure in seconds.
Enter thickness in inches
ACI recommends 7–10% for slabs, 15–20% for trench footings
Select pour type and enter dimensions
to get your cubic yard count
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The cubic yard formula
Concrete is sold by the cubic yard in the United States. One cubic yard equals 27 cubic feet — a 3 ft × 3 ft × 3 ft cube. Every concrete calculation converts your project dimensions to cubic feet first, then divides by 27.
Rectangular Slab, Footing, or Wall
CY = (Length ft × Width ft × Thickness in ÷ 12) ÷ 27
Round Column or Pier
CY = (π × (Diameter in ÷ 24)² × Height ft) ÷ 27
The thickness-in-inches division by 12 is the step most calculation errors come from. If your slab is 4 inches thick, you divide by 12 to convert to 0.333 feet before multiplying. Getting this wrong by even a quarter-inch across a 1,000 sq ft slab produces a shortfall of approximately 0.75 cubic yards — enough to cause a cold joint on the final section. See the concrete volume calculator for multi-section pours that combine different pour types.
Magic number shortcuts by thickness
The magic number shortcuts are field-proven tools for quick yard estimates. They are mathematically exact — not approximations — and can be used to double-check calculator results before calling the plant.
| Thickness | Magic number | Formula | Example: 20×20 ft |
|---|---|---|---|
| 3.5" | 92.6 | Sq ft ÷ 92.6 | 400 ÷ 92.6 = 4.32 CY |
| 4" | 81 | Sq ft ÷ 81 | 400 ÷ 81 = 4.94 CY |
| 5" | 64.8 | Sq ft ÷ 64.8 | 400 ÷ 64.8 = 6.17 CY |
| 6" | 54 | Sq ft ÷ 54 | 400 ÷ 54 = 7.41 CY |
| 8" | 40.5 | Sq ft ÷ 40.5 | 400 ÷ 40.5 = 9.88 CY |
| 12" | 27 | Sq ft ÷ 27 | 400 ÷ 27 = 14.81 CY |
The magic number is simply 27 (cubic feet per yard) divided by the thickness in feet. For 4 inches: thickness = 4/12 = 0.333 ft, so 27 ÷ 0.333 = 81. For 6 inches: 27 ÷ 0.5 = 54. The arithmetic always checks out — these are not round-number approximations. Apply your overage factor after the magic number calculation before calling the plant. Also see the concrete cost per yard guide for how to apply regional pricing to your yard count.
Overage by pour type
The standard 10% overage figure is correct for many pours, but research and ACI guidance support a tiered approach based on pour complexity and site conditions. Under-specified overage on complex pours is a leading cause of cold joints.
| Pour type | Recommended overage | Primary risk factors |
|---|---|---|
| Simple flatwork (patio, walkway) | 5–10% | Spillage, minor grade variation |
| Residential slab (garage, basement) | 7–10% | Form deflection, sub-base compression |
| Driveway or heavy slab | 10% | Edge thickening, large-scale grade variation |
| Foundation or trench footing | 15–20% | Earth irregularities in unformed trenches |
| Column or deck pier | 10% | Tube form tolerance, minor spillage |
| Structural stairs | 15% | Complex formwork, geometry waste |
| Pumped concrete | 10% (fixed) | Residual volume in pump line and hopper |
The higher overage for trench footings reflects the physical reality of unformed earth walls. Even carefully dug trenches have irregularities of 0.5 to 1 inch across their width. On a 100 linear foot continuous footing at 12 inches wide, a half-inch average variance adds over 0.5 cubic yards of unexpected volume. Budget for 15% minimum on any unformed footing work.
Ready-mix ordering guide
Once you have your cubic yard calculation, there are three practical steps before placing the order. Getting these right prevents the most common ready-mix ordering mistakes.
| Step | What to confirm | Notes |
|---|---|---|
| Round up | Order to the nearest 0.25 CY, always rounding up | Plants quote in quarter-yard increments |
| Check minimum | Plants typically accept min. 1 CY orders | Orders under 3–5 CY trigger $40–$150 short-load fee |
| Confirm mix design | Specify PSI and any admixtures (air, fiber, retarder) | Standard residential: 3000–4000 PSI depending on application |
| Confirm truck access | Can the truck reach within 18 ft of the pour site? | Beyond 18 ft may require pump rental ($300–$800/day) |
| Book a delivery window | Saturday and peak summer bookings fill weeks ahead | After-hours delivery often carries a $30–$60 premium |
For the price side of your order, run your cubic yard total through the concrete cost calculator to get your expected material cost and short-load fee impact. If your project calls for a standard 4-inch residential slab and you want to verify bag count as an alternative, cross-check with the concrete slab calculator which includes bag count, PSI spec guidance, and rebar vs wire mesh recommendations for your specific application.
Frequently asked questions
What is the magic number shortcut for calculating concrete yards?
For a 4-inch slab, divide square footage by 81. For 6-inch: divide by 54. For 5-inch: divide by 64.8. For 8-inch: divide by 40.5. These are mathematically exact, not approximations. The magic number equals 27 ÷ (thickness in feet). Apply your overage factor after the magic number calculation before ordering.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27. There are exactly 27 cubic feet in one cubic yard (3 × 3 × 3 = 27). Example: 40 cubic feet ÷ 27 = 1.48 cubic yards. Always convert to yards before calling the plant — ready-mix is ordered and priced in cubic yards, not cubic feet.
Why is concrete measured in cubic yards instead of cubic feet?
The cubic yard became the US standard for ready-mix because it corresponds to the load capacity of mixer trucks, and the entire supply chain — batch plants, truck billing, pump specs — was standardised around it. The convention persists because changing it would require restructuring all plant operations and pricing systems simultaneously.
How accurate does my yardage estimate need to be when ordering?
Order to the nearest 0.25 CY, always rounding up. An error of 0.25 CY on a 5 CY order is 5%, which is within acceptable range. For pours over 10 CY, recalculate from your forming dimensions on the day of the pour — not from the original plan — to catch any field changes before the truck arrives.
What happens if I order too little concrete?
Running short forces a second delivery, and the gap creates a cold joint — a structural weak point where fresh concrete is placed against concrete already beginning to set. Cold joints can cause cracking and structural failure in load-bearing applications. The cost of a short-load follow-up ($40–$150 fee plus the second pour price) always exceeds the cost of ordering correctly the first time.
How do I calculate cubic yards for an L-shaped or irregular slab?
Break the shape into rectangles. Calculate cubic yards for each section separately using (L × W × thickness ÷ 12) ÷ 27, then add the totals. For an L-shaped driveway, split it at the bend. For curved edges, calculate the bounding rectangle and subtract approximate corner areas. Most irregular residential slabs can be estimated within 5% this way.
What is the minimum concrete order from a ready-mix plant?
Most plants accept orders as small as 1 cubic yard, but orders under their standard minimum — typically 3 to 5 CY — trigger a short-load fee of $40–$150. For pours under 1 CY, bagged concrete is almost always more economical. Call the plant before ordering to confirm their current minimum and fee structure.
References
American Concrete Institute. (2022). ACI 301-22: Specifications for Structural Concrete. ACI.
American Concrete Institute. (2019). ACI 318-19: Building Code Requirements for Structural Concrete. ACI.
Ready Mixed Concrete Association. (2024). Best Practices for Ready Mixed Concrete Ordering. RMCA.