Free Concrete Calculator
Calculate how many bags of concrete you need for any slab, footing, or hole.
What is a concrete calculator?
A concrete calculator is a free online tool that helps you estimate how many cubic yards or bags of concrete you need for a slab, footing, or column. It runs entirely in your browser — no signup, no app, and no data is ever sent to a server.
I've lost count of how many times I've seen someone show up at the lumber yard with 80 bags of concrete for a job that needed 95 — or worse, with 140 when they only needed 90. A concrete calculator tells you exactly how many bags of concrete mix you need for your project — whether it's a slab, a footing, or setting posts. Instead of guessing or overbuying, enter your dimensions and bag size to get an accurate count. The calculator handles 60lb and 80lb bags, the two most common sizes available at Home Depot and Lowe's. It also includes a waste allowance (5-15%) because concrete projects almost always require a little extra for spillage, uneven subgrades, and the bell-shape at the bottom of post holes.
How it works
The calculator converts your dimensions into cubic feet. For slabs: length × width × depth (converted to feet). For post holes: π × radius² × depth × 1.33 (the extra factor accounts for the bell-shape bottom of a dug hole). It then divides the total cubic feet by the yield of the selected bag size — 0.45 cu ft for 60lb bags, 0.6 cu ft for 80lb bags. The waste multiplier increases the total by your selected percentage.
When to use a concrete calculator
Contractors use it for material estimates on bids. DIY homeowners use it before heading to the home improvement store. Anyone pouring a patio slab, setting fence posts, building concrete steps, or pouring a garage foundation needs to know how many bags to buy. Ordering too few means a trip back to the store mid-pour. Ordering too many means returning or storing heavy unused bags. The calculator eliminates both problems.
Common mistakes people make
The biggest mistake I see is forgetting that post holes aren't cylinders — they're bell-shaped at the bottom because the shovel blade creates a wider cavity. The calculator adds a 33% volume factor (1.33) automatically, but if you're doing the math by hand it's easy to undercount by a third. Another common error: mixing up depth units. A slab that's 4 inches deep is 0.33 feet, not 0.4 feet, and that small difference means you end up buying 3 fewer bags than needed. I also find that most people skip the waste allowance entirely, then end up one or two bags short because the subgrade wasn't perfectly level and the slab came out slightly thicker in spots.
How to use this calculator
- Choose your project type — Select slab, footing, post hole, or custom dimensions.
- Enter the dimensions — Enter length, width, and depth (or diameter and depth for post holes) in feet and inches.
- Choose bag size — Select 60lb or 80lb bags to see exactly how many you need, including waste allowance.
Frequently asked questions
How many 80lb bags of concrete do I need for a 10×10 slab?
For a 10×10 slab that's 4 inches thick, you need approximately 56-57 80lb bags of concrete. That's 100 sq ft × 0.33 ft = 33.3 cubic feet = 1.23 cubic yards. An 80lb bag yields about 0.6 cubic feet (0.022 cubic yards). The exact count depends on waste and whether you order extra for spillage.
How many 60lb bags of concrete per cubic foot?
A 60lb bag of concrete yields approximately 0.45 cubic feet when mixed. To fill 1 cubic foot, you need about 2.2 bags (round up to 3 for practical purposes). For a cubic yard (27 cubic feet), you need about 60 bags of 60lb mix.
How many bags of concrete for a 4×4 post hole?
For a 4×4 post set in a 12-inch diameter hole that's 24 inches deep, you need about 2-3 80lb bags of concrete. The formula is π × radius² × depth × 1.33 (the bell-shape factor). The calculator handles post hole calculations automatically.
What is the difference between 60lb and 80lb concrete bags?
A 60lb bag yields approximately 0.45 cubic feet of concrete. An 80lb bag yields approximately 0.6 cubic feet. The 80lb bag is more economical per cubic foot but heavier to handle. Most contractors prefer 80lb bags for slabs and 60lb bags for smaller projects like post holes or repairs.
How many square feet does a yard of concrete cover?
One cubic yard of concrete (27 cubic feet) poured 4 inches thick covers approximately 81 square feet. At 6 inches thick, it covers 54 square feet. To calculate: area = cubic yards × 27 ÷ depth in feet. A typical concrete truck carries 10 yards, covering about 810 sq ft at 4 inches.
How much concrete do I need for a 20×30 slab?
For a 20×30 slab (600 sq ft) at 4 inches thick, you need approximately 7.4 cubic yards of concrete. At 6 inches thick, you need 11.1 yards. The calculator handles this in one step: select Slab, enter 20 × 30 ft, choose your depth, and see the exact number of bags or yards required.