Concrete & Masonry
Concrete Calculator
Pick the shape you are pouring, enter its size and how many you need, and the calculator returns cubic yards, bag counts and the approximate weight of the finished concrete.
Worked example
- Volume of one piece = 12 ft x 10 ft x (4 in / 12) = 40 cu ft.
- Plus 10% waste = 44 cu ft, divided by 27 = 1.63 cu yd.
- Bags: 44 cu ft / 0.6 cu ft per 80 lb bag = 74 bags (rounded up).
Common sizes at a glance
| Size | Concrete | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 10 x 10 ft slab, 4 in | 1.36 cu yd | 62 | 82 |
| 12 x 12 ft patio, 4 in | 1.96 cu yd | 88 | 118 |
| 20 x 20 ft garage, 4 in | 5.43 cu yd | 245 | 326 |
| 24 x 24 ft garage, 6 in | 11.73 cu yd | 528 | 704 |
| Four 12 in columns, 4 ft tall | 0.51 cu yd | 24 | 31 |
Choose the shape before you measure
Most pours fall into two groups. Flat work, like a pad or a patio, is a rectangle: you need length, width and thickness. Columns, piers and round holes are cylinders: you need the diameter and the height or depth.
If a job mixes both, run the calculator once for each part and add the results. A shed base with four corner piers, for example, is one rectangle plus four cylinders.
Inches and feet in the same job
The calculator asks for length and width in feet but thickness and diameter in inches, because that is how the tape is usually read on site. Do not convert them yourself; enter them as they come off the tape and we do the conversion.
A small slip here is the most common reason for a huge or tiny answer. If the result looks wrong, check that a 4 in slab was not entered as 4 ft.
Why the waste allowance matters
Concrete is never poured onto a perfectly flat, perfectly square base. Low spots in the gravel, forms that bow out a little and the mix left in the wheelbarrow all eat into your total. The default 10% covers that on most small jobs.
Running short in the middle of a pour is far worse than having a few bags left over. Leftover bags keep for a while if stored dry; a half-finished slab with a cold joint does not.
Bags or a truck?
The result shows both cubic yards and bag counts so you can decide how to buy. In the worked example, a 12 x 10 ft slab at 4 in comes to 1.63 cu yd, which is 74 bags of 80 lb. That is a lot of mixing for one person.
For bag math by size, see the concrete bag calculator. If you plan to order ready-mix, the concrete yard calculator rounds to the quarter yards suppliers sell.
Weight is part of the plan
The approximate weight helps you plan the haul. The 1.63 cu yd example weighs about 6,600 lb once mixed, and the dry bags weigh close to that too, so a small pickup will need several trips.
For pads, footings and steps, the local building code and a licensed professional have the final word on thickness and reinforcement. This tool counts material only.
Questions
How many 80 lb bags make a cubic yard of concrete?
An 80 lb bag yields about 0.6 cu ft, and a cubic yard is 27 cu ft, so it takes 45 bags to make one yard.
Can I use this for a round column?
Yes. Choose the round shape, enter the diameter in inches and the height, and set how many columns you are pouring.
Should I change the 10% waste?
Use less on a tight formed pour with a flat base and more on uneven ground or when digging holes by hand, where sizes vary.
What about a slab with a thickened edge?
Run the slab and the edge as separate pieces and add them, or use the concrete slab calculator for the main area.
How this works: formulas and typical values are listed on our method page; every data source and its license is on sources.
Last reviewed November 2026.