How do I calculate how much concrete I need?

For a rectangular slab or footing: Volume = Length × Width × Depth. For a circular column or post hole: Volume = π × (Diameter/2)² × Height. This calculator handles all shapes and unit conversions automatically.

How many 60lb or 80lb bags of concrete do I need?

A 60lb bag yields about 0.45 cubic feet (0.0127 m³). An 80lb bag yields about 0.60 cubic feet (0.0170 m³). Divide your total volume by the bag yield and round up. This calculator does that automatically — switch between 60lb and 80lb with one click.

How much does concrete for a slab cost?

Enter your price per bag in the Cost section and the calculator multiplies bag count by price. For ready-mix concrete delivered by truck, costs vary by region — the volume result (in m³ or yd³) is what you give to your supplier for a quote.

How much concrete do I need for fence post holes?

Use the Post Hole tab. Enter the hole diameter (typically 3–4 times the post width) and depth (at least 1/3 to 1/2 of the post height above ground). Multiply the result by your number of posts to get the total.

Should I add extra concrete for waste?

Yes. It is standard practice to order 5–10% extra to account for spillage, uneven sub-base, and over-excavation. Round your bag count up to the nearest full bag or delivery increment.

Concrete Calculator

Calculate concrete volume for slabs, footings, columns, and post holes. Instant results in cubic yards, cubic feet, and bags.

Length

Width

Depth / Thickness

Volume

0.000m³

Bags Needed

0bags

80lb bags (rounded up)

Estimated Cost

Price per bag

$0.00

$6.50 × 0 bags

Building a fence?

Use our Fence Calculator to get post count, rails, and materials before calculating concrete for the post holes.

Fence Calculator

How Concrete Volume Is Calculated

Concrete volume depends on the shape you're pouring. Use the formula that matches your project, then convert to bags or ready-mix as needed.

Slab / Footing: Volume = Length × Width × Depth
Column: Volume = π × Radius² × Height

Example: a 10 × 10 ft patio at 4 in (0.33 ft) thick needs 10 × 10 × 0.33 = 33 cubic feet, or about 1.22 cubic yards.

Bagged Concrete Yield Guide

Bag Weight	Yield (ft³)	Yield (m³)	Bags per m³
40lb (18 kg)	0.30	0.0085	~118
60lb (27 kg)	0.45	0.0127	~79
80lb (36 kg)	0.60	0.0170	~59

Yields are approximate and vary slightly by brand. Always check the bag label for exact coverage.

Recommended Slab Thickness

Walkway / Path

3–4 in

Light foot traffic only

Patio / Driveway

4–5 in

Vehicle-rated driveways need rebar or mesh

Footing / Foundation

8–12 in

Depends on local frost line and load

Frequently Asked Questions

How much concrete do I need for a slab?

Multiply length × width × depth (in feet) to get cubic feet, then divide by 27 to get cubic yards. Add 5-10% extra for waste and uneven subgrade.

How many bags of concrete do I need?

An 80lb bag yields about 0.60 cubic feet. Divide your total cubic feet by 0.60 to estimate bag count, then round up.

Should I order ready-mix or use bags?

For pours over 1 cubic yard (about 40+ bags), ready-mix delivery is usually cheaper and less labor-intensive than mixing bags by hand.

How thick should a concrete slab be?

Walkways need 3-4 inches, patios and driveways need 4-5 inches (with rebar for vehicle loads), and footings depend on local building code and frost depth.

Need to Measure Your Project Area?

Use LandLens to measure slab dimensions and project boundaries with GPS precision.

Download LandLens Free

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