From measured space to purchasable material
A slab with a thickened edge contains more concrete than its floor area multiplied by the central thickness. Use this tool when a drawing specifies a uniform interior slab and a deeper band around all four sides. It calculates the perimeter band without counting the corner areas twice. The output shows the main slab, additional edge volume and the combined order quantity. All depths and edge dimensions must come from your project specification; this is a material takeoff rather than a foundation design.
Measure, select yield and round the order
- Measure the outside rectangle.
- Enter central thickness and the specified total edge depth.
- Check the extra perimeter volume before placing a ready-mix order.
Net requirement and purchase calculation
A worked example
A 20 × 12 ft slab at 4 in contains 80 ft³. A 1 ft-wide edge at 12 in total depth adds 40 ft³. The combined 120 ft³ becomes 4.80 yd³ with 8% allowance.
Concrete Slab: 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 |
|---|---|---|---|
| Outside slab length | 20 ft | 24 ft | Main slab volume: 80 → 96 cu ft Additional edge concrete: 40 → 45.3333 cu ft |
| Outside slab width | 12 ft | 14.4 ft | Main slab volume: 80 → 96 cu ft Additional edge concrete: 40 → 43.2 cu ft |
| Central slab thickness | 4 in | 4.8 in | Main slab volume: 80 → 96 cu ft Additional edge concrete: 40 → 36 cu ft |
| Thickened edge width | 12 in | 14.4 in | Additional edge concrete: 40 → 47.36 cu ft Combined geometric volume: 120 → 127.36 cu ft |
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
- An edge on only some sides requires a separate measured takeoff.
- Openings, internal beams and isolated footings are not part of this geometry.
- Take reinforcement, frost depth and bearing requirements from the project design.
Frequently asked questions
Is thickened edge depth additional to slab thickness?
Here it is the total depth. The calculator subtracts central thickness before adding the edge volume.
Why not multiply the perimeter by edge width?
That approximation counts the four corner squares twice. Subtracting the inner rectangle avoids that error.
Can this calculate an irregular garage slab?
Divide an irregular footprint into rectangles and account for shared thickened edges separately.
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.