How to Figure Out How Much Concrete You Need (Without Getting It Wrong)

If you've ever stood in the concrete aisle at a hardware store staring at bags and trying to do math in your head, you know how this goes. You either buy too many bags (and haul them back) or too few (and make a second trip in the middle of a pour). Neither is fun.

Here's how to get the number right the first time.

The Core Calculation: Volume First

Concrete is measured by volume — specifically cubic yards for large pours and cubic feet for smaller ones. To find the volume, you multiply length × width × thickness. The key is keeping your units consistent.

For a 10×10 foot patio that's 4 inches thick: convert the thickness to feet first (4 inches ÷ 12 = 0.333 feet). Then: 10 × 10 × 0.333 = 33.3 cubic feet. Divide by 27 to get cubic yards: 33.3 ÷ 27 = 1.23 cubic yards.

How Many Bags Does That Equal?

Bags of pre-mixed concrete come in three common sizes:

• 40 lb bag: covers about 0.30 cubic feet
• 60 lb bag: covers about 0.45 cubic feet
• 80 lb bag: covers about 0.60 cubic feet

For our 33.3 cubic feet patio: 33.3 ÷ 0.45 = about 74 bags of 60-lb concrete. That's a lot of bags — which is why anything over about 1 cubic yard is usually worth ordering as ready-mix delivered by truck instead.

Always Add 5-10% for Waste

Concrete doesn't pour perfectly. There's spillage, uneven subgrade, and slight measurement errors. Add 5% to your calculation for simple rectangular forms. Add 10% for anything with curves, irregular shapes, or multiple levels.

For our 33.3 cubic feet: 33.3 × 1.05 = 35 cubic feet needed. Round up when ordering bags — it's much cheaper than making a second trip.

Fence Posts Are Different

Post holes are cylindrical, so the formula changes: Volume = π × r² × depth. For a 8-inch diameter hole (4-inch radius = 0.33 ft) that's 2 feet deep: 3.14159 × 0.33² × 2 = 0.69 cubic feet. That's about 1.5 bags of 60-lb mix per post hole.

When to Order Ready-Mix Instead of Bags

Ready-mix concrete (delivered by truck) runs $125-175 per cubic yard in most of the US. For anything over 1 yard, it's usually faster and often cheaper than buying bags, plus it's already mixed to the right consistency.

Calculate Exactly What You Need

Use our concrete calculator — enter your shape, dimensions, and preferred bag size, and it gives you the exact amount you need with a 5% waste buffer already included.