How Much Concrete Do I Need?

Quick answer: Multiply your length × width × depth (all in feet), then divide by 27 to get cubic yards. A standard 10×12 foot patio that's 4 inches thick needs about 1.48 cubic yards — or roughly 34 bags of 80 lb concrete mix.

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The Formula (Step by Step)

Here's how to calculate concrete by hand:

Measure your area — length and width in feet
Convert depth to feet — divide inches by 12 (4 inches = 0.333 ft)
Calculate cubic feet — length × width × depth
Convert to cubic yards — divide cubic feet by 27
Add 10% extra — for waste, spillage, and uneven ground

Real Example: 10×12 Patio, 4 Inches Deep

10 × 12 × 0.333 = 40 cubic feet
40 ÷ 27 = 1.48 cubic yards
With 10% extra: 1.63 cubic yards

How Many Bags of Concrete Do I Need?

If you're mixing bags instead of ordering a truck, here's the math:

80 lb bag = 0.6 cubic feet → about 45 bags per cubic yard
60 lb bag = 0.45 cubic feet → about 60 bags per cubic yard
40 lb bag = 0.3 cubic feet → about 90 bags per cubic yard

Pro tip: For anything over 1 cubic yard, seriously consider ordering a ready-mix truck. Mixing 45+ bags by hand is brutal — and the concrete quality is more consistent from a truck.

Common Concrete Depths

Sidewalks and patios: 4 inches
Driveways (cars): 4–5 inches
Driveways (heavy vehicles): 6+ inches
Footings: 8–12 inches (check local code)
Garage slabs: 4–6 inches

Common Mistakes to Avoid

❌ Forgetting to convert inches to feet

This is the #1 mistake. If your slab is 4 inches deep, you use 0.333 — not 4. Using 4 gives you a number 12× too high.

❌ Not ordering extra

Always add 10% for waste. Ground is never perfectly level, forms aren't perfectly straight, and you'll spill some. Running short mid-pour is a disaster.

❌ Ignoring the sub-base

If you're pouring over soft or uneven soil, you may need a gravel sub-base. That changes your excavation depth but not your concrete calculation.

❌ Mixing too many bags at once

Concrete sets fast. Mix 1–2 bags at a time unless you have a large mixer and a helper.

Pro Tips

Round up on bags. You can return unopened bags. You can't return to the store mid-pour.
Check the weather. Don't pour if rain is expected within 24 hours or temps are below 40°F.
Use fiber mesh or rebar for slabs over 4 inches or any slab that'll bear vehicle weight.
Ready-mix minimum is usually 1 cubic yard. Below that, bags are more economical.

Frequently Asked Questions

How much does a cubic yard of concrete cover?

At 4 inches thick, one cubic yard covers about 81 square feet. At 6 inches thick, it covers about 54 square feet.

How much does concrete cost?

Ready-mix concrete runs $125–$165 per cubic yard delivered. An 80 lb bag costs $5–$7 at most home centers, which works out to about $225–$315 per cubic yard — more expensive but no delivery minimum.

Can I pour concrete in sections?

Yes, but you need to plan your joints. Cold joints (where new concrete meets cured concrete) are weaker. Use a bonding agent and pour within 24–48 hours if possible.

How long does concrete take to cure?

You can walk on it in 24–48 hours. It reaches about 70% strength in 7 days and full strength in 28 days. Don't park vehicles on it for at least a week.

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