From measured space to purchasable material
A concrete order starts with the shape of the space being filled. This calculator handles rectangular pads, solid round pads and hollow cylinders, so a patio and a circular equipment base do not have to be approximated in the same way. Enter finished dimensions rather than excavation dimensions, then add an allowance for an uneven base or handling loss. The result separates the exact geometric volume from the purchase allowance and compares a ready-mix order with bags using the yield printed on your chosen mix.
Measure, select yield and round the order
- Choose the actual project shape and complete its dimensions.
- Enter the placed thickness and the additional material percentage.
- Compare bulk volume against whole bags using the product-specific yield.
Net requirement and purchase calculation
A worked example
A 10 × 10 ft rectangle at 4 in thick contains 33.33 ft³. Adding 10% gives 36.67 ft³, equivalent to 1.36 yd³ or 62 bags at 0.6 ft³ each.
Thickness errors affect the whole slab
A 10 by 10 foot rectangle at four inches deep contains 33.333 cubic feet. At five inches it contains 41.667 cubic feet: one extra inch increases the volume by 25%. An allowance for handling losses is not a substitute for measuring an uneven excavation or intentionally thickened edges.
Check the depth model before refining a bag price. Split areas with different depths into separately measured quantities.
Concrete: what changes the result?
Compare dimensions and product yields without treating quantities as a design check. The comparisons below are calculated from this page’s default example. Each row changes only the named input; all other inputs stay at their displayed example values. These are mathematical scenarios, not measurements from a completed project.
| Changed assumption | Default input | Comparison input | Result: default → comparison |
|---|---|---|---|
| Rectangle length | 10 ft | 12 ft | Geometric concrete volume: 33.3333 → 40 cu ft Volume including allowance: 36.6667 → 44 cu ft |
| Rectangle width | 10 ft | 12 ft | Geometric concrete volume: 33.3333 → 40 cu ft Volume including allowance: 36.6667 → 44 cu ft |
| Concrete thickness | 4 in | 4.8 in | Geometric concrete volume: 33.3333 → 40 cu ft Volume including allowance: 36.6667 → 44 cu ft |
| Additional material | 10 % | 12 % | Volume including allowance: 36.6667 → 37.3333 cu ft Ready-mix quantity: 1.358 → 1.3827 yd³ |
Displayed values are rounded. Whole-pack results change in steps; continuous quantities may change before a new pack is needed. To compare your actual project, enter your dimensions above, save a scenario, then change one assumption.
Coverage, allowance and purchasing limits
- A round pad uses diameter, not radius. The hollow-cylinder option subtracts a complete central void.
- Do not use an allowance to compensate for a thickness that has not been designed.
- Delivery and placing costs can outweigh material differences on small orders.
Frequently asked questions
How many cubic yards of concrete do I need for a 10 by 10 slab?
At 4 inches thick the exact volume is 33.33 cubic feet, or 1.235 cubic yards. A 10% allowance brings that to about 1.36 cubic yards.
Can I calculate bags from an 80-pound label?
Use the package volume yield, not just its weight. Different mixes and products can produce different placed volumes.
Does this choose the right slab thickness?
No. It estimates quantities from the thickness you enter and does not evaluate loads, reinforcement or local requirements.
Read the project guides
- Concrete Bags vs Ready-Mix: Build a Quantity-Based Comparison
Work through slab volume, package yield, whole-bag rounding, ready-mix delivery charges and a complete materials comparison for a small concrete project.
Calculation method: JICWH methodology. Examples are planning assumptions, not product recommendations.