CONCRETE
Concrete slab & patio planning: from dimensions to an order quantity
The core concrete calculation is simple. The useful part is knowing where thickness, unit conversion, waste and package yield enter the decision.
1. Start with geometric volume
For a rectangular slab, multiply length × width × thickness. Keep the units consistent. If length and width are in feet and thickness is in inches, divide thickness by 12 first.
Concrete is commonly ordered in cubic yards. Divide cubic feet by 27: 66.67 ÷ 27 = 2.47 yd³.
2. Thickness changes the answer fast
| Same 10×20 footprint | Geometric volume | With 10% planning allowance |
|---|---|---|
| 4 in | 2.47 yd³ | 2.72 yd³ |
| 5 in | 3.09 yd³ | 3.40 yd³ |
| 6 in | 3.70 yd³ | 4.07 yd³ |
The allowance is not a structural recommendation. It is a purchasing buffer for uneven subgrade, measurement error and handling loss. The required slab thickness itself depends on the project and local requirements.
3. A second example: 12×12 patio
At 4 inches thick: 12 × 12 × (4/12) = 48 ft³. Then 48 ÷ 27 = 1.78 yd³. Adding 10% produces 1.96 yd³. That arithmetic is why a small change in dimensions should be recalculated rather than guessed.
4. Bags versus ready-mix
Bag count must use the yield printed for the exact product. As a reference point, QUIKRETE's calculator/training material lists approximately 0.60 ft³ yield for an 80-lb bag and 0.45 ft³ for a 60-lb bag. At 0.60 ft³, one cubic yard (27 ft³) is about 45 bags before any allowance. Verify the current bag label before buying.
5. Cost planning without pretending local prices are universal
Separate quantity from price. First calculate the order quantity. Then multiply by a local delivered price per cubic yard or the current price per bag. Keep short-load, delivery, pumping, reinforcement, base preparation, forms, finishing and disposal as separate line items unless your quote explicitly includes them.
6. Where concrete estimates usually go wrong
Using nominal depth everywhere
A 4-inch design depth does not mean every point in an excavated base is exactly 4 inches. Low spots, edge thickening and an uneven subgrade increase placed volume. Measure several points after base preparation instead of relying only on the design number.
Mixing area and volume
Square feet describe surface area; concrete is purchased by volume. A 200 ft² slab has no usable concrete quantity until thickness is included. Convert inches to feet before multiplying.
Rounding too early
Keep the unrounded cubic-foot result through the conversion to cubic yards. Round only when converting the final requirement to a supplier order or whole bags.
Forgetting non-concrete costs
Concrete quantity is not the project budget. Base material, reinforcement, forms, delivery, pumping, finishing and disposal can be separate quote lines.
7. Verify the result before ordering
- Re-measure length and width at the prepared work area, not from an old sketch.
- Confirm the intended thickness and any thickened edges or footings separately.
- Recalculate cubic feet independently: area × thickness in feet.
- Divide by 27 and compare that result with the calculator.
- Ask the supplier how they handle minimum loads, short-load fees and order increments.
- For bagged mix, use the yield printed on the exact bag rather than a generic bag-count rule.
8. A three-scenario order check
| 10×20 slab | Calculated | Planning question |
|---|---|---|
| 4 in + 5% | 2.59 yd³ | Is the prepared base consistently close to design depth? |
| 4 in + 10% | 2.72 yd³ | Useful sensitivity check for ordinary measurement/base variation. |
| 4 in + 15% | 2.84 yd³ | Do not add a large buffer blindly; investigate why extra volume may be needed. |
The point of comparing scenarios is not to prescribe a universal waste percentage. It shows how much the purchasing decision changes when assumptions change, so the assumption can be checked before money is spent.
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Primary references
QUIKRETE Concrete Calculator — manufacturer quantity calculator.
QUIKRETE Mixing & Placing training PDF — bag-yield reference used above.