MaterialScopeHow much concrete for a 20 × 20 slab?
Compare thicknesses, extra material and bag quantities for a 20-foot-square slab.
Thickness changes the order
These figures assume straight-sided, level forms and a uniform thickness across 400 square feet. The table compares quantities only; it does not select a structurally suitable thickness.
| Thickness | Net concrete | With 10% extra | 80-lb bags* |
|---|---|---|---|
| 4 in | 4.94 yd³ | 5.43 yd³ | 245 bags |
| 5 in | 6.17 yd³ | 6.79 yd³ | 306 bags |
| 6 in | 7.41 yd³ | 8.15 yd³ | 367 bags |
*Bag counts assume 0.60 ft³ per bag and include the same 10% allowance. Ready-mix and bags are alternative purchase methods.
Work through the dimensions
20 × 20 × (4 ÷ 12) = 133.33 cubic feet. Divide by 27 to get cubic yards. Multiply the unrounded volume by 1.10 to add the allowance. Round only at the end, and confirm the supplier’s order increments.
Plan the job around the delivery
A larger pour needs a delivery plan as well as a quantity. Discuss truck access, unloading position, crew capacity and timing with the supplier. At a 4-inch thickness, the allowance-adjusted 5.43 cubic yards would become 5.50 only if the supplier accepts quarter-yard increments.
Round to the supplier’s actual increment
The 4-inch example is 5.4321 yd³ after its 10% allowance. Rounding up to quarter-yard or half-yard increments gives 5.50 yd³; whole-yard increments give 6.00 yd³. Ask which increments the supplier accepts before turning an estimate into an order. A displayed value of 5.43 is rounded for reading and should not be rounded down again.
| Order increment | Order volume | Material at an example $180/yd³ |
|---|---|---|
| ¼ yd³ | 5.50 yd³ | $990 |
| ½ yd³ | 5.50 yd³ | $990 |
| 1 yd³ | 6.00 yd³ | $1,080 |
The $180 rate is an arithmetic example, not a local price quote. Delivery, tax, short-load charges, pumping and labor are excluded. Obtain a written quote that separates these items.
Account for additional sections
The square-slab table excludes thickened edges, footings and steps. Measure these from the project design and add only the volume outside the already-counted slab. Discuss the final volume, truck access and placement schedule with the supplier before booking the pour.
Check the base before the pour
An uneven excavation can consume more material than the nominal dimensions suggest. Measure several depths and confirm that the base and forms match the design. Do not use this table to choose load-bearing capacity, reinforcement or local code requirements.
Use the bag calculator to change product yield, or the cost calculator to enter a supplier quote. Compare the 4-inch and 6-inch quantities before discussing the design with your project professional.
Product reference: QUIKRETE concrete mix technical data. Confirm the current label for your chosen product.
Change the measurements and allowance to match your plan.
Open Concrete Slab CalculatorMaterialScope editorial · Updated September 29, 2026 · Calculation methodology