Extra concrete should be based on project uncertainty, not a universal percentage. Start with accurate geometry, then identify the reasons your actual placement could require more material than the theoretical volume. The cubic-yard calculation guide helps establish the base volume before any extra quantity is added.
Calculate your base quantity first
Enter the dimensions of the uniform slab in the concrete volume and allowance calculator. Keep known thickened edges and low spots in a separate quantity line, then use the worked example below to avoid counting the same concrete twice.
Record the base volume, separately measured additions, allowance and supplier rounding in the free project planning workbook. Its direct PDF download does not require an email or a marketing subscription.
Reasons a concrete order may need an allowance
- Subgrade is not perfectly level
- Forms bow or dimensions vary
- Thickened edges or footings were not included in the first calculation
- Some material remains in the chute, pump or buggy
- Spillage or placement losses occur
- Supplier order increments require rounding
Compare allowance scenarios
| Theoretical volume | 3% extra | 5% extra | 10% extra |
|---|---|---|---|
| 2.0 yd³ | 2.06 yd³ | 2.10 yd³ | 2.20 yd³ |
| 5.0 yd³ | 5.15 yd³ | 5.25 yd³ | 5.50 yd³ |
| 10.0 yd³ | 10.30 yd³ | 10.50 yd³ | 11.00 yd³ |
These are comparison scenarios, not universal recommendations. A simple, accurately formed slab may justify less uncertainty than an irregular excavation with variable depth.
Measure thickened sections separately
A specific 20-by-20 slab worked example shows how thickness and edge volume change the order without counting the same concrete twice.
Edges, beams, footings and steps can add substantial volume. Calculate them as separate shapes and add them to the slab volume before applying any allowance.
How much extra concrete for uneven subgrade and a thickened edge?
First measure the actual shapes rather than hiding missing concrete inside a generic waste percentage. Suppose a hypothetical 12-by-16-foot patio has a uniform 4-inch slab: 12 × 16 × (4 ÷ 12) = 64 cubic feet, or 64 ÷ 27 = 2.37 cubic yards. If one 16-foot edge has an additional 4 inches of depth over a 1-foot-wide strip, count only that extra depth: 16 × 1 × (4 ÷ 12) = 5.33 cubic feet, or 0.20 cubic yard. The geometric total is about 2.57 cubic yards before allowance.
If measured low spots across 48 square feet average an additional half inch of concrete, the extra volume is 48 × (0.5 ÷ 12) = 2 cubic feet, or about 0.074 cubic yard. The revised measured total is about 2.64 cubic yards. An illustrative 5% placement allowance then produces 2.64 × 1.05 = 2.77 cubic yards before supplier rounding. Do not apply an allowance to compensate for a low spot you already measured and added. Do not multiply the edge by its full depth if the base slab volume already covers the first 4 inches.
This calculation is an order-planning example, not a prescribed patio design. A professional should confirm subgrade, thickness, reinforcement, drainage and any footing or permit requirements for the actual site. Verify the ready-mix supplier’s delivery increment and short-load terms before the final order.
Supplier increments can matter more than the percentage
If the supplier only accepts certain order increments or applies short-load minimums, the practical order may be driven by those rules. Ask about minimums, increments and the policy for adding material on pour day.
Running short is not the same as ordering too much
A shortage can interrupt placement and create construction-joint or finishing problems. Too much concrete can create disposal and washout issues. The right order balances measurement confidence with the cost and risk of shortage.
Better ordering workflow
- Measure every slab, edge, footing and step.
- Calculate theoretical volume.
- Check form dimensions and subgrade.
- Add only a justified allowance.
- Confirm supplier minimums and increments.
- Review placement access and pump/chute losses.
Keep an order record you can update
Before calling the supplier, keep four numbers together: measured geometric volume, known extra geometry, the chosen placement allowance and the supplier-rounded order. Write down who checked each assumption. Return to the calculator if the dimensions change; do not silently add a second allowance to an already adjusted figure.
If you need a longer planning workflow for delivery access, material choices and quote review, preview the concrete project handbook before deciding whether it fits your project. The free calculator, article and workbook remain available on their own.
Use the concrete calculator
Run the geometry first in the Concrete Calculator, then compare allowance scenarios before placing the order.