Free Concrete Calculator: How Much Concrete Do I Need? (Slabs, Footings, Columns)

A reliable concrete calculator takes the guesswork out of ordering mix for slabs, footings, columns, and other pours. That matters because concrete is expensive to under-order and frustrating to over-order. If you are a contractor, remodeler, or DIY homeowner, the fastest way to stay on budget is to measure once, calculate the volume correctly, and add a realistic waste factor before the truck or pallet shows up.

The good news is that concrete math is simple when you keep everything in the same units. Most suppliers sell by the cubic yard, while bag mix is measured by yield in cubic feet. Once you convert thickness from inches to feet and volume from cubic feet to cubic yards, the decision becomes straightforward. Our free tool at /trades/calc/concrete-calculator/ handles the math instantly, but it helps to understand the logic behind the numbers.

How to Calculate Concrete Yardage

Concrete volume is length × width × depth. The only trick is converting the result into cubic yards because one cubic yard equals 27 cubic feet. Use this three-step method for nearly every flat or rectangular pour:

1. Measure length and width in feet.
2. Convert thickness in inches to feet by dividing by 12.
3. Multiply length × width × thickness, then divide by 27.

Example formula: 10 × 12 × 0.333 ÷ 27 = 1.48 cubic yards. That result tells you a 10-foot by 12-foot slab at 4 inches thick needs about 1.48 yards before waste. Add 10% and the order becomes roughly 1.63 yards. On a bag-mix job, that same volume is about 39.96 cubic feet of concrete.

Slabs: Length × Width × Thickness ÷ 27

Slabs are the most common use for a concrete calculator because patios, shed pads, garage aprons, and sidewalks all use the same formula. Start with the finished dimensions, not the form lumber measurement. If the slab changes thickness or includes thickened edges, split the pour into separate rectangles and calculate each section individually.

1. For a 10×12 slab at 4 inches thick, convert 4 inches to 0.333 feet.
2. Multiply 10 × 12 × 0.333 to get 39.96 cubic feet.
3. Divide by 27 to get 1.48 cubic yards.
4. Add 10% waste and plan for 1.63 cubic yards.

Footings and Foundation Walls

Footings are still just rectangles, but they are usually long and narrow. Multiply the footing length by its width and depth, all in feet. A 40-foot footing that is 16 inches wide and 8 inches deep works like this: 40 × 1.33 × 0.667 = 35.5 cubic feet. Divide by 27 and you get roughly 1.32 cubic yards.

Foundation walls are similar, but now the wall height becomes one of your dimensions. If a wall is 30 feet long, 8 inches thick, and 4 feet high, the volume is 30 × 0.667 × 4 = 80.04 cubic feet, or about 2.96 cubic yards. For stepped foundations or wall sections with different heights, calculate each run separately so you do not accidentally short the truck.

Round Columns and Sono Tubes

Round concrete needs a cylinder formula instead of a rectangle formula: π × radius² × height. For a 12-inch-diameter column, the radius is 6 inches, or 0.5 feet. If that column is 10 feet tall, the math is 3.1416 × 0.5 × 0.5 × 10 = 7.85 cubic feet. Divide by 27 and you need about 0.29 cubic yards.

That is why a few sonotubes often make sense with bag mix instead of ready-mix. Small isolated footings, deck piers, and fence post bases typically use less than a yard total. The bigger the diameter and the taller the column, the more important it becomes to total everything before choosing bags versus truck delivery.

How Many 60lb vs 80lb Bags Do I Need?

Bag count depends on yield, not the printed bag weight alone. A standard 60-pound bag yields about 0.45 cubic feet, while an 80-pound bag yields about 0.60 cubic feet. Convert your project to cubic feet first, then divide by the bag yield.

Should I Order Ready-Mix or Bag Concrete?

As a rule of thumb, bag concrete makes sense for small repairs, deck footings, fence posts, stair landings, and one-off pours under about 1 yard. Ready-mix starts to win when the project is larger, the pour needs to happen continuously, or finish quality matters. Patios, driveways, garage slabs, and foundation work usually benefit from truck delivery.

Also think about access. If a truck cannot get close and you have no buggy or pump plan, the labor can erase the convenience of ready-mix. On the other hand, carrying 80-pound bags across a muddy backyard is not free either. The best estimate weighs material cost, labor hours, placement speed, and waste. A concrete calculator gets you the quantity. Good planning decides the delivery method.

Frequently Asked Questions