Concrete Calculator
Figure out how much concrete — and how many bags — your slab, footing, or post holes actually need.
Bags Needed
About this tool
What this concrete calculator does
Tell it the shape you're pouring — a slab, a footing, a round column, or a wall — along with its dimensions, and it works out exactly how much concrete you need: in cubic yards, cubic feet, cubic meters, and liters. It then converts that volume into a bag count for the most common pre-mix bag sizes (40 lb, 50 lb, 60 lb, 80 lb, 25 kg, 50 kg), so you know exactly how many bags to buy.
Who it's for
DIYers pouring a patio slab or fence post footings, contractors quoting a small job, and anyone standing in the concrete aisle trying to figure out how many bags of concrete mix to load into the cart.
How to use it
- Pick your shape: slab/rectangular pour, footing, round column, or wall.
- Enter the dimensions — length and width in feet or meters, thickness or depth in inches or centimeters.
- Set how many of that shape you're pouring (e.g. 6 fence post footings) and a waste buffer percentage — 10% is a sensible default that covers uneven ground and spillage.
- Read off the total volume needed and, right below it, exactly how many bags of each common size that works out to.
Real-world use cases
- Pouring a patio or driveway slab: enter the length, width, and a 4-inch thickness to get cubic yards for a ready-mix truck order or bag count for a smaller pour.
- Setting fence posts: use the round column shape with the post hole diameter and depth, multiplied by how many posts you're setting.
- Building a retaining wall: use the wall shape with length, height, and thickness to estimate the concrete needed for the footing or wall pour.
- Comparing ready-mix vs. bagged concrete: cubic yards tells you what to order from a ready-mix truck (usually sold by the 1/4 yard), while the bag counts tell you what to buy off the shelf for smaller jobs.
Frequently asked questions
How much concrete is in one bag?
It depends on the bag size: an 80 lb bag yields about 0.60 cubic feet of mixed concrete, a 60 lb bag about 0.45 cubic feet, a 40 lb bag about 0.30 cubic feet, and a 25 kg bag about 0.35 cubic feet. This calculator does that math for all common sizes at once.
Why include a waste buffer?
Real pours almost never use exactly the theoretical volume — uneven subgrade, form leakage, and spillage during mixing all eat into your material. A 10% buffer is the standard rule of thumb; bump it up for a rough or uneven site.
How many bags of concrete do I need for a fence post?
Use the round column shape with the diameter of your post hole and its depth, set the quantity to your number of posts, and the bag count for your preferred bag size is calculated automatically.
Should I order ready-mix or use bags?
As a rough guide, anything over about 1 cubic yard is usually cheaper and faster as a ready-mix delivery; smaller pours like footings or a single slab are typically more practical with bagged mix.
Does this account for rebar or gravel base?
No — this calculates the concrete volume for the pour itself. Rebar and a compacted gravel base are recommended for most structural pours but don't reduce the concrete volume in any meaningful way for estimating purposes.