Why You Always Order the Wrong Amount of Concrete (And the Math to Fix It)

A concrete calculator takes 30 seconds to run. Most homeowners skip it and either waste money on excess material or face the worst possible scenario: running short mid-pour with a truck on the way back to the plant and a half-finished slab beginning to set.

Both mistakes are avoidable. The fix is one calculation done before you call the supplier — not after you open the first bag.

The Formula Behind Every Concrete Order

Concrete volume is calculated in cubic yards, which is how suppliers price and deliver it. The formula is straightforward:

Cubic yards = (length × width × (depth ÷ 12)) ÷ 27

The depth gets divided by 12 to convert inches to feet. The result gets divided by 27 because one cubic yard contains 27 cubic feet.

On a 10×10 slab at 4 inches thick:

• 10 × 10 × (4 ÷ 12) = 33.33 cubic feet
• 33.33 ÷ 27 = 1.23 cubic yards

That's the base number. Before you call anyone, add 10% for overage. That brings the order to 1.35 cubic yards. The 10% rule is standard industry practice — it accounts for spillage, uneven subgrade, and the fact that your forms are never perfectly level.

For circular slabs (fire pit bases, column footings, round patios), the formula adjusts:

Area = π × (diameter ÷ 2)²

Then divide that area by 12 and divide by 27 as before.

The Real-World Numbers: What Common Slabs Actually Require

Common slab sizes at 4-inch thickness:

Coverage reference: 1 cubic yard covers 81 square feet at 4 inches thick. It covers 54 square feet at 6 inches thick. Every inch of additional depth increases your material need by 25%.

The number most people never run: a standard 20×20 slab requires 296 bags of 60-pound concrete. That's not a small hardware store run. That's a pallet-and-a-half, and it's exactly the scenario where readymix becomes faster and cheaper than bags.

Bags vs. Readymix — The Decision That Changes at 1 Cubic Yard

The crossover point is approximately 1 cubic yard. Below that threshold, bags are practical and often cheaper when you factor in the readymix delivery fee. Above it, readymix wins on cost, time, and quality.

Each 60-pound bag yields approximately 0.45 cubic feet of concrete — meaning 60 bags fill one cubic yard. Each 80-pound bag yields approximately 0.60 cubic feet — meaning 45 bags fill one cubic yard.

Mixing bags by hand or with a rented mixer is reasonable for a small shed foundation or fence post project. For anything approaching 2 cubic yards or more, you're looking at hours of mixing, inconsistent water ratios, and a finished product that will likely show variation. A readymix truck delivers uniform material in a single pour.

The practical breakdown:

• Under 1 cubic yard — bags are practical, especially for fence posts and small pads
• 1 to 2 cubic yards — bags are possible, readymix starts making sense
• Over 2 cubic yards — order readymix

For reference: fence posts require roughly 0.10 to 0.20 cubic feet each depending on hole size. A single 60-pound bag handles one standard fence post hole with room to spare.

How to Use the Calculator Before You Call

The calculation sequence takes under a minute:

1. Measure your project dimensions: length and width in feet, depth in inches
2. Run the formula or use the calculator to get your cubic yard figure
3. Add 10% for overage
4. Decide bags or readymix using the 1-yard threshold
5. If readymix: call with your cubic yard number plus overage, confirm delivery fee and minimum order

Calculate exactly how much concrete you need →

One detail first-time buyers miss: suppliers often charge a short load fee when you order under their minimum — typically 5 to 8 yards. If your project lands below that threshold, factor the fee into your bags-vs-readymix comparison.

Frequently Asked Questions

How do you calculate concrete for a slab?

Multiply length by width by depth in feet (convert inches to feet by dividing by 12), then divide by 27 to convert cubic feet to cubic yards. A 20×20 slab at 4 inches: 20 × 20 × 0.333 = 133.33 cubic feet ÷ 27 = 4.94 cubic yards. Always add 10% before ordering, bringing this example to 5.43 cubic yards.

How many bags of concrete do I need for a 10×10 slab?

A 10×10 slab at 4 inches thick requires 1.23 cubic yards of concrete. At 60 bags per cubic yard, that's 74 60-pound bags. If using 80-pound bags, you need 55 bags. Add 10% overage to either figure before purchasing.

Should I use bags or readymix concrete?

The crossover point is approximately 1 cubic yard. For projects below that threshold — fence posts, small footings, minor repairs — bags are practical. Above 1 yard, readymix is faster, produces more consistent results, and typically costs less per cubic foot once you account for your time and the cost of bag labor.

What does the 10% concrete overage rule mean?

Standard industry practice is to order 10% more concrete than your calculated volume. This accounts for spillage during the pour, slight variations in your form depth, and uneven subgrade. On a 4.94-cubic-yard project you'd order 5.43 yards. Running short mid-pour is a job-ending problem — a small excess is not.

How does slab depth affect how much concrete I need?

Every additional inch of depth increases your concrete requirement by 25% of the 4-inch baseline. A 20×20 slab at 4 inches requires 4.94 cubic yards. At 6 inches, the same footprint requires 7.41 cubic yards — 50% more material at 50% more depth. Thickness decisions have direct, proportional cost consequences.