Ordering concrete is one of the few parts of construction where you get exactly one shot. You cannot run to the hardware store for another bag when the ready-mix truck is sitting in your driveway waiting to pour.
If you order too little, you end up with a cold joint (a weak seam where old concrete meets new) or a compromised structure. If you order too much, you are paying premium prices for concrete that will harden in a pile on your site. This is why a calculated waste allowance is critical for every estimate.
01. Why You Need a Waste Allowance
Even if your formwork is perfectly square and your subgrade is perfectly flat, the actual volume of concrete you need will always be slightly higher than the theoretical math. Why? Because real-world sites are not perfect.
- Uneven subgrade: Soil is never perfectly flat. If your excavation is 10mm deeper in one corner, that is extra concrete filling the void.
- Formwork deflection: The weight of the wet concrete causes wooden formwork to bow outwards slightly, increasing the volume.
- Spillage: Concrete splashes out of the chute, the pump, or the wheelbarrow during transport.
- Trapped in the pump: If you are using a boom pump, a significant amount of concrete remains in the pipeline and hopper at the end of the pour.
02. Standard Percentages by Project
The waste allowance you add to your calculated volume depends heavily on the type of pour. A simple slab on grade is very different from a complex foundation with grade beams.
| Project Type | Waste Allowance | Notes |
|---|---|---|
| Simple Slab on Grade | 5% – 7% | Assuming well-compacted, flat subgrade. |
| Footings & Trenchfill | 10% – 12% | Trenches are rarely perfectly straight or uniform in width. |
| Walls & Columns | 5% – 10% | Depends on the formwork type and height. |
| Pumped Concrete (Any) | + 5% extra | Add this to the base percentage to account for line loss. |
If you are ordering a very small quantity (e.g., a set of fence post footings), percentage-based waste is less helpful. In those cases, it is often better to round up to the next full bag of premix or add a flat minimum volume (like 0.1 cubic meters).