How Much Concrete Do I Need? A Practical Estimating Guide

The Basic Formula

Concrete volume is length times width times depth, converted to cubic feet, then divided by 27 to get cubic yards (since 1 cubic yard = 27 cubic feet). The tricky part is that depth is usually measured in inches, so you need to convert: divide inches by 12 to get feet before multiplying.

A 20-foot by 10-foot slab that's 4 inches thick: 20 x 10 x (4/12) = 66.7 cubic feet. Divide by 27 = 2.47 cubic yards. Add 10% for waste and you need about 2.7 cubic yards.

Bags vs Ready-Mix Truck

An 80-pound bag of concrete yields 0.6 cubic feet of mixed concrete. To fill 2.7 cubic yards (72.9 cubic feet), you'd need 122 bags. At 80 pounds each, that's 9,760 pounds of concrete to unload, open, pour, add water, and mix by hand. It would take two people an entire day.

The tipping point is around 1 cubic yard (45 bags). Below that, bagged concrete is cheaper and practical. Above that, order a ready-mix truck. Most concrete plants have a 1-yard minimum with a short-load fee ($50-$100) for orders under 5 yards. Ready-mix costs $130-$170 per yard delivered, and they pour it directly where you need it in minutes.

Post Holes and Footings

Round post holes use the formula: pi times radius squared times depth. A 10-inch diameter hole that's 36 inches deep: 3.14 x (5/12)^2 x 3 = 1.64 cubic feet. That's about 3 bags of 80-pound concrete per hole. For a fence with 20 post holes, that's 60 bags.

For fence posts, fast-setting concrete (like Quikrete Fast-Setting) is a game-changer. You pour the dry mix into the hole around the post, add water, and it sets in 20-40 minutes. No mixing needed. You can set 20 posts in an afternoon.

Slab Thickness Guidelines

Sidewalks and patios: 4 inches minimum. Driveways: 4-6 inches (6 inches if heavy vehicles park on it). Garage floors: 4-6 inches with welded wire mesh or rebar for crack resistance. Footings: depth and width per local code, typically 8-12 inches deep and 16-24 inches wide.

Thicker isn't always better. A 6-inch slab costs 50% more in concrete than a 4-inch slab for the same footprint. For a patio that only supports foot traffic and furniture, 4 inches with proper base preparation is more than adequate.

Don't Forget the Base

Concrete poured directly on soil is asking for cracks. A proper base consists of 4-6 inches of compacted gravel (crushed limestone or road base) that provides drainage and a stable foundation. Compact it with a plate compactor (rent for $50-$75/day). This step is not optional for any slab that matters.

For post holes, no gravel base is needed if you're pouring concrete around a fence post. Some people put 2-3 inches of gravel at the bottom for drainage, which is good practice but not required for most residential fences.