How to Calculate Concrete (Step-by-Step Guide)
The formula for calculating concrete is **Length × Width × Thickness (all in feet) ÷ 27 = cubic yards**. For a 12×10 foot patio at 4 inches thick, that's 12 × 10 × 0.33 ÷ 27 = **1.47 cubic yards**.
How to Calculate Concrete (Step-by-Step Guide)
The formula for calculating concrete is Length × Width × Thickness (all in feet) ÷ 27 = cubic yards. For a 12×10 foot patio at 4 inches thick, that's 12 × 10 × 0.33 ÷ 27 = 1.47 cubic yards.
The math is simple, but unit conversion is where most mistakes happen. This guide walks you through the process step-by-step—or you can skip the math entirely with our concrete calculator.
The Basic Formula
Volume = Length × Width × Thickness
Then convert to cubic yards by dividing by 27 (because 1 cubic yard = 27 cubic feet).
Complete formula:
(Length in feet) × (Width in feet) × (Thickness in feet) ÷ 27 = Cubic Yards
The critical step most people miss: all measurements must be in the same unit before multiplying. Mixing feet and inches without converting produces wrong answers.
Step-by-Step Calculation Example
Let's calculate concrete for a 12 × 10 foot patio at 4 inches thick.
Step 1: Convert All Measurements to Feet
Length: 12 feet ✓ (already in feet)
Width: 10 feet ✓ (already in feet)
Thickness: 4 inches → 0.33 feet (4 ÷ 12 = 0.33)
Common thickness conversions:
| Inches | Feet |
|---|---|
| 3" | 0.25 ft |
| 4" | 0.33 ft |
| 5" | 0.42 ft |
| 6" | 0.50 ft |
Step 2: Multiply to Get Cubic Feet
12 × 10 × 0.33 = 39.6 cubic feet
Step 3: Convert to Cubic Yards
39.6 ÷ 27 = 1.47 cubic yards
Step 4: Add 10% for Waste
1.47 × 1.10 = 1.62 cubic yards
Final answer: Order 1.62 cubic yards (round to 1.75 or 2 yards depending on supplier minimums)
Why Add 10% for Waste?
Always add 10% to your calculated volume:
Uneven ground: Your excavation isn't perfectly level. Low spots use more concrete than calculated.
Form bulging: Forms flex slightly under concrete pressure, increasing actual volume.
Spillage: Some concrete ends up outside the forms during pouring and finishing.
Measurement error: Your tape measure reading might be slightly off.
Running short mid-pour is a disaster—you can't stop and wait for more. Running 5% over is a minor waste of money. The math is obvious: add the buffer.
Common Mistakes to Avoid
Mistake 1: Mixing Units
Wrong: 12 feet × 10 feet × 4 inches = 480 (meaningless number)
Right: 12 feet × 10 feet × 0.33 feet = 39.6 cubic feet
Always convert inches to feet before multiplying, or convert everything to inches and divide by 46,656 (inches per cubic yard). Feet are easier.
Mistake 2: Forgetting to Convert to Cubic Yards
Wrong: "I need 39.6 of concrete" (39.6 cubic feet? cubic yards?)
Right: "I need 39.6 cubic feet, which is 1.47 cubic yards"
Concrete is ordered by the cubic yard. Bags are sold by weight but list yield in cubic feet. Know which unit you're working with.
Mistake 3: Using Area Instead of Volume
Wrong: "My patio is 120 square feet, so I need 120 of concrete"
Right: "My patio is 120 square feet × 0.33 feet thick = 39.6 cubic feet = 1.47 cubic yards"
Square feet is area (length × width). Concrete is a volume (length × width × thickness). You need the third dimension.
Mistake 4: Skipping the Waste Buffer
Wrong: "Calculator says 1.47 yards, I'll order exactly 1.5 yards"
Right: "Calculator says 1.47 yards, plus 10% = 1.62 yards, I'll order 1.75-2 yards"
Concrete suppliers don't do refunds on unused portions from a pour. Order enough.
Calculating Bags Needed
If you're using bagged concrete instead of ready-mix, convert cubic feet to bags:
| Bag Size | Yield (cu ft) | Bags per Cubic Yard |
|---|---|---|
| 40 lb | 0.30 | 90 bags |
| 50 lb | 0.375 | 72 bags |
| 60 lb | 0.45 | 60 bags |
| 80 lb | 0.60 | 45 bags |
Example: Your patio needs 39.6 cubic feet
- Using 80-lb bags: 39.6 ÷ 0.60 = 66 bags
- Plus 10% buffer: 66 × 1.10 = 73 bags
For more on bag calculations, see how many bags of concrete you need.
Calculating Different Shapes
Rectangular Slabs (Most Common)
Formula: Length × Width × Thickness ÷ 27
Example: Driveway 20 feet × 12 feet × 5 inches
20 × 12 × 0.42 ÷ 27 = 3.7 cubic yards (+ 10% = 4.1 yards, order 4.5)
Circular Slabs
Formula: π × Radius² × Thickness ÷ 27
Example: Round fire pit pad, 8 feet diameter, 4 inches thick
- Radius = 4 feet (diameter ÷ 2)
- 3.14 × 4² × 0.33 ÷ 27 = 0.61 cubic yards
Cylindrical Holes (Post Footings)
Formula: π × Radius² × Depth ÷ 27
Example: 10-inch diameter post hole, 36 inches deep
- Convert to feet: 10" diameter = 0.83' diameter, radius = 0.42'
- Depth: 36" = 3 feet
- 3.14 × 0.42² × 3 ÷ 27 = 0.06 cubic yards per hole
For post holes, it's easier to figure bags: this hole needs about 2-3 bags of 80-lb concrete.
Use our concrete slab calculator or driveway calculator to skip the math.
Quick Reference: Common Project Sizes
| Project | Typical Size | Cubic Yards |
|---|---|---|
| Small patio | 10×10 ft @ 4" | 1.2 yards |
| Medium patio | 12×12 ft @ 4" | 1.8 yards |
| Large patio | 16×20 ft @ 4" | 4.0 yards |
| Sidewalk | 3×20 ft @ 4" | 0.75 yards |
| Single-car driveway | 10×20 ft @ 5" | 1.5 yards |
| Two-car driveway | 20×20 ft @ 5" | 3.1 yards |
| Shed pad | 8×10 ft @ 4" | 1.0 yards |
These estimates include rounding but not the 10% buffer. Always add your waste factor.
For quick answers to common sizes, see how much concrete do I need.
Why Use a Calculator Instead
The formula is straightforward, but calculators eliminate:
- Unit conversion errors
- Math mistakes
- Forgetting the waste buffer
- Confusion about bag yields
Our concrete calculator handles rectangular slabs, circular pads, post holes, steps, and walls—and shows results in cubic yards, cubic feet, and number of bags. For pricing context, see concrete cost per yard.
Frequently Asked Questions
How do I calculate concrete in cubic yards?
Multiply Length × Width × Thickness (all in feet), then divide by 27. Example: A 10×10 slab at 4" thick = 10 × 10 × 0.33 = 33 cubic feet ÷ 27 = 1.2 cubic yards. Add 10% for waste.
How much concrete do I need for a 10×10 slab?
At 4 inches thick: 10 × 10 × 0.33 ÷ 27 = 1.2 cubic yards, plus 10% = 1.35 cubic yards or approximately 55 bags of 80-lb concrete.
Should I order extra concrete?
Yes, always order 10% more than calculated. Uneven ground, form bulging, and measurement errors mean you'll typically use more than the exact calculation. Running short mid-pour forces a cold joint that weakens the slab.
How do I calculate concrete for a round area?
Use π × Radius² × Thickness ÷ 27. For an 8-foot diameter circle at 4" thick: π × 4² × 0.33 ÷ 27 = 0.61 cubic yards. The radius is half the diameter.
Key Takeaways
- Formula: Length × Width × Thickness (in feet) ÷ 27 = cubic yards
- Always convert inches to feet before calculating (4" = 0.33 ft)
- Add 10% for waste—never order exactly what you calculate
- 1 cubic yard = 27 cubic feet = 45 bags of 80-lb concrete
- Use our concrete calculator to eliminate math errors
For volume context, see what is a yard of concrete. Browse all concrete guides.