Concrete Calculator — How Much Concrete Do I Need?

Your Concrete Project Guide

Concrete is the backbone of countless home improvement projects — patios, sidewalks, driveways, shed foundations, and post footings all start with getting the right amount of concrete. Order too little and you'll have a cold joint in the middle of your slab. Order too much and you're paying for material you can't return. This concrete calculator takes your slab dimensions and tells you exactly how many cubic yards or bags you need, including a 10% waste factor.

For most residential slabs — patios, sidewalks, and shed pads — 4 inches of thickness is the standard. Driveways and garage floors that support vehicle traffic should be 5 to 6 inches thick. If you're pouring a footing for a wall or deck, local building codes typically require 8 to 12 inches of depth below the frost line. When in doubt, check with your local building department — under-building a concrete slab is an expensive mistake to fix.

The biggest decision for DIYers is whether to mix bags or order ready-mix from a truck. The math is simple: if your project needs less than about 1 cubic yard (roughly 45 eighty-pound bags), mixing your own is manageable. Between 1 and 3 cubic yards is a gray area where a concrete mixer rental starts to make sense. Anything over 3 cubic yards, and a ready-mix truck delivery is almost certainly faster, cheaper, and will give you a better result because you can pour the entire slab at once.

Site preparation makes or breaks a concrete project. The ground underneath should be compacted and level, with a 2- to 4-inch layer of compacted gravel as a base. This gravel base promotes drainage and prevents the slab from settling unevenly over time. Set your form boards to the exact height and slope you want — concrete goes where you tell it to, so crooked forms mean a crooked slab.

For slabs larger than 10 by 10 feet, reinforcement is strongly recommended. Welded wire mesh (6x6 W1.4/W1.4) or #3 rebar on 18-inch centers adds tensile strength that prevents cracking. Place the reinforcement in the middle third of the slab thickness — sitting on small supports called chairs — not on the ground where it does nothing.

Post holes are a common concrete project that this calculator can help with too. For a typical fence or deck post in a 10-inch diameter hole that's 36 inches deep, you need about 0.14 cubic yards per hole — roughly 2 to 3 eighty-pound bags. Multiply by your number of posts for the total. Fast-setting concrete mixes are ideal for post holes because they set in 20 to 40 minutes with no mixing required — just pour the dry mix in and add water.