How to use it
- Enter the slab length in feet.
- Enter the slab width in feet.
- Enter the slab thickness in inches.
- Click Calculate to see cubic yards, cubic feet, and 80-lb bag count.
This monolithic slab calculator tells you exactly how much concrete you need for a one-pour slab foundation. It converts your length, width, and thickness into the three numbers that matter: cubic yards for ordering ready-mix, cubic feet for volume reference, and bags if you're mixing by hand. The bag count is based on a standard 80-lb bag yielding about 0.6 cubic feet, so you can plan a job of any size. Why does this matter? Ordering too little means a cold joint or a second delivery with extra labor costs. Ordering too much wastes money and creates disposal headaches. The output makes it easy to compare ready-mix vs. bagged concrete, talk confidently with suppliers, and budget accurately. Whether you're building a shed pad, garage floor, or small home foundation, this gives you a clear starting point before you break ground.
Pro tips
- Add 10% waste for spillage, uneven subgrade, and slight over-excavation — especially on monolithic pours.
- Round bag counts up to the next whole bag; you can't order a fraction of a bag.
- For large slabs, compare bag costs against a ready-mix delivery — buying 80-lb bags gets expensive fast when you pass 1 cubic yard.
- Don't forget to verify your thickness in inches matches the actual form depth; a common mistake is measuring the form height but ignoring the gravel base.
- Use the same units throughout — mixing feet and inches in the same field throws off the result.
Real-life examples
- A homeowner sizing a 10x12 shed slab needs to know how many 80-lb bags to buy.
- A contractor estimating a 30x40 garage foundation wants cubic yards for a ready-mix order.
- A DIYer comparing bagged concrete vs. truck delivery for a 20x24 monolithic slab.
Case study
Before, Mark estimated his 12x16 shed slab by guessing and bought 45 bags — 12 too many, wasting $90. After using this calculator, he entered 12, 16, and 4 inches. It showed 2.37 cubic yards and 107 bags. He ordered a ready-mix truck instead and saved time and money.