A waste percentage is a planning decision, not a universal constant. The right allowance depends on how accurately the base was prepared, how many shapes are being poured, and how the concrete will be delivered and placed.
What the allowance covers
An allowance can cover small depth variation, concrete left in a pump or chute, handling losses, and the practical difficulty of ordering a fractional supplier unit. It does not repair a bad measurement or replace a structural takeoff.
Use the site to choose the range
A simple, level rectangle measured at several points may need only a modest allowance. A rough subgrade, many footings, narrow forms, wheelbarrow placement, or uncertain access argues for more review. Record why you selected the percentage so the next person can challenge or refine it.
Do not add the allowance to each footing and then add another allowance to the total. Calculate the theoretical total first, then apply one allowance unless a supplier or engineer specifies otherwise.
Compare the order with the calculation
Ready-mix is commonly discussed in cubic yards, while bagged mixes use bag yield. Convert the theoretical volume and the planned volume, then ask the supplier how they round deliveries. A small difference can be a delivery increment rather than a mathematical error.
When an allowance is not enough
If the pour has structural beams, steps, voids, or a varying thickness, model those sections separately. For structural work, the project drawings and responsible professional control the quantity and specification.
Worked examples
Flat, well-prepared slab
- Measured volume is 2.00 yd³.
- The base is level, the form is simple, and delivery is direct.
- Compare a 5% allowance (2.10 yd³) with the supplier’s rounding increment.
Result: The percentage is visible and can be adjusted without changing the geometry.
Uneven base with multiple pads
- Measured total is 3.40 yd³ across six pads and a strip footing.
- The subgrade has several depth readings and access is difficult.
- Compare 10% (3.74 yd³) with the supplier’s recommended order.
Result: The higher allowance is a planning conversation, not a guarantee that the pour will be sufficient.