The complete answer with built-in formula, project-by-project examples, and the 10% buffer most calculators forget.
The Universal Concrete Formula
Every concrete calculation reduces to one formula. The volume of concrete (in cubic yards) for any rectangular pour is:
Cubic Yards = (Length × Width × Depth in inches) ÷ 324
The 324 in the denominator comes from converting feet to yards and inches to feet (27 × 12 = 324). For non-rectangular shapes, break the pour into rectangles, calculate each, and sum the results.
For circular pours like sonotubes:
Cubic Yards = (π × radius² × depth in inches) ÷ 324
How Much Concrete By Project -- Quick Reference
Most common DIY pours and the concrete amount needed (with 10% buffer included):
| Project | Cu Yards | 80lb Bags | Buy Method |
|---|---|---|---|
| 4×4 ft patio (4 in) | 0.22 | 10 | Bags |
| 10×10 ft patio (4 in) | 1.36 | 62 | Ready-mix or bags |
| 10×20 ft slab (4 in) | 2.72 | 123 | Ready-mix |
| 12×20 ft driveway (4 in) | 3.26 | 148 | Ready-mix |
| 20×20 ft garage (4 in) | 5.43 | 247 | Ready-mix |
| 3-ft sonotube × 4 ft deep | 0.04 | 2 | 1 bag of 80lb |
| 12 fence posts (36 in deep) | 0.25 | 12 | Bags (Fast-Set) |
Why You Always Order Extra
Every professional contractor adds 10% to the raw math. Here is what that buffer actually covers:
- Spillage during pour: 1-3% of every pour ends up on the ground around the forms, in the wheelbarrow, or on tools.
- Form bulging: Wet concrete weighs 150 lbs per cubic foot. Forms always bulge slightly under that pressure, increasing actual volume needed by 2-4%.
- Sub-base settling: The gravel base compacts slightly under the weight of fresh concrete, requiring more concrete to reach the form top.
- Measurement error: Tape-measure error of even 1 inch over a 20-foot pour adds 1-2% to true volume.
- The wheelbarrow that always tips: Murphy's Law of concrete -- one wheelbarrow will tip during every pour. Plan for it.
Running short on a pour is a disaster. The first batch starts setting at 60-90 minutes; if you do not have the second batch ready by then, you get a cold joint -- a permanent weak point in the slab that will crack at that exact line within 1-2 years.
Ordering Method by Project Size
The right way to buy concrete depends on the volume. Here is the practical decision matrix:
- Under 0.5 cu yd: Buy bags from Home Depot or Lowe's. Mix in a wheelbarrow with a mortar hoe.
- 0.5 to 1 cu yd: Buy bags + rent an electric mixer ($35-55/day). Allows you to mix consistent batches without waste.
- 1 to 3 cu yd: Order ready-mix delivery -- pay the $50-150 short-load fee. Still cheaper than 50+ bags of cement plus the time and back pain.
- 3 to 8 cu yd: Sweet spot for residential ready-mix delivery. No short-load fee. Best per-yard pricing.
- Over 10 cu yd: Coordinate 2 trucks. Second truck arrives 15-30 minutes after first. Have enough labor to keep pouring continuously.