PROJECT WORKED EXAMPLE · Updated September 2026
10×20 Concrete Slab: Material and Cost Planning
A 10 by 20 ft slab is 200 sq ft. The number that changes the order most is thickness: four inches and six inches are very different concrete orders.
1. Measurements used in this example
10 ft × 20 ft; compare 4 in and 6 in thickness. Write measurements down before opening a calculator. The purpose of the example is to make every assumption visible so you can replace it with your own project numbers.
2. Work the quantity before the price
At 4 in: 200 × (4/12) = 66.7 ft³ = 2.47 yd³. Add 10% planning waste: 2.72 yd³. At 6 in: 100 ft³ = 3.70 yd³; with 10%: 4.07 yd³.
These numbers are planning quantities, not a contractor quote or structural design. Site conditions, local code, product specifications and supplier policies can change the final purchase.
3. Turn the result into a purchase plan
For ready-mix, compare the supplier minimum, short-load fee, delivery window and whether the truck can reach the placement area. For bags, verify the yield printed on the exact bag rather than assuming every mix has the same yield.
| Check | Why it matters |
|---|---|
| Measurement source | Dimensions should come from the actual project, not a generic house-size assumption. |
| Waste/rounding | State the allowance explicitly and round only when converting to purchasable units. |
| Product data | Package yield, coverage and installation requirements override generic examples. |
| Local quote | Delivery, labor, tax, access and minimum charges vary by location and supplier. |
4. Mistakes this example is designed to prevent
Forgetting to convert inches to feet; ordering exactly the geometric volume; ignoring base depth; assuming quoted concrete price includes delivery.
5. Recalculate with your own dimensions
Use the Concrete Cost Calculator to replace the example measurements with your project. Keep this page open beside the calculator so the result is checked against the same assumptions.
What this guide does — and does not — claim
BuildCalc Guide publishes transparent planning math. We do not present national averages as local quotes, and we do not substitute calculator output for manufacturer instructions, building codes, permits, engineering or contractor judgment. Our methodology explains units, rounding and verification.