Concrete Calculator: How Many Bags Do I Need?
Concrete bags are heavy, and mixing them is hard work — the worst outcome is discovering at bag forty that you are four bags short. This free concrete calculator works out exactly how many 40, 60 or 80 lb bags your slab, footing or post holes need, and shows the volume in cubic yards so you know when it is smarter to order ready-mix. Enter your dimensions, pick your bag size, and get a reliable number before you load the cart.
Concrete Bag Quantity Calculator
How to Use This Concrete Calculator
Measure the length and width of your slab in feet and its depth in inches — 4 inches is standard for patios and walkways, 6 inches for driveways that carry vehicles. If you are working out post holes instead, measure one hole's diameter and depth, work out its volume in cubic feet (π × radius² × depth, with the radius in feet), and enter that as a "slab" with those dimensions, or just multiply the result for one hole by the number of holes.
Choose your bag size. The 80 lb bag is the most economical per pound, but 60 lb bags are far easier to lift and mix for one person. Enter a price per bag if you want the total cost, then click Calculate. The tool shows the cubic feet and cubic yards of concrete, the number of bags, and a warning if your project is big enough that ready-mix becomes the smarter option.
The Concrete Calculation Formula
The volume is length × width × (depth ÷ 12) in cubic feet — depth in inches is divided by 12 to convert to feet. Bag yields are fixed by the mix: a 40 lb bag makes about 0.30 cubic feet, a 60 lb bag 0.45 cubic feet, and an 80 lb bag 0.60 cubic feet. So bags = cubic feet ÷ yield per bag, rounded up.
Worked example: a 10 × 10-foot slab at 4 inches deep is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, which is 33.3 ÷ 27 = 1.23 cubic yards. With 80 lb bags: 33.3 ÷ 0.60 = 55.6 — so you buy 56 bags. Mixing 56 bags by hand is a long day; at this size, many people price a ready-mix delivery as well.
Pro Tips for Working with Bagged Concrete
Buy about 5% extra — spills, over-dug forms and slightly low bags happen, and a return trip mid-pour is the last thing you want. Water matters more than effort: follow the water amount printed on the bag exactly. Too much water weakens concrete dramatically; too little makes it unworkable.
For post holes, use fast-setting mix and pour it dry into the hole, then add water — no mixing tub needed, and the post stands up on its own within minutes. For slabs, reinforce with wire mesh or rebar and keep the surface damp for a few days after pouring; concrete cures, it does not just dry, and slow curing is what makes it strong.
Frequently Asked Questions
How much does an 80 lb bag of concrete cover?
An 80 lb bag yields about 0.60 cubic feet of mixed concrete. A 10x10-foot slab 4 inches thick needs 33.3 cubic feet, which works out to 56 bags of 80 lb mix.
How many bags of concrete do I need for a 4x4 post hole?
A typical 4x4 post hole is about 10 to 12 inches in diameter and 24 to 30 inches deep — roughly 1.3 to 2 cubic feet of concrete. That means 3 bags of 80 lb mix per hole. Use fast-setting mix so you do not have to brace the post while it cures.
How many 80 lb bags of concrete are in a cubic yard?
One cubic yard is 27 cubic feet. An 80 lb bag yields 0.60 cubic feet, so a cubic yard takes 45 bags. In 60 lb bags it takes 60 bags, and in 40 lb bags it takes 90.
Should I use bags or order ready-mix concrete?
Bagged mix is economical up to about 2 cubic yards. Beyond that, mixing dozens of bags by hand becomes slow and inconsistent, and ordering ready-mix from a concrete truck is usually cheaper per cubic yard.