Calculators/ Concrete/Flowable Fill Calculator
Flowable Fill Calculator
CLSM volume for trenches, voids and abandoned pipes.
Flowable fill, or CLSM, is a very low strength concrete that pours like thick soup and needs no compaction. It is used where compacting backfill is impractical: narrow utility trenches, voids under slabs, and abandoned pipes and tanks.
How this is calculated
- Volume = length × width × depth, divided by 27 for cubic yards. It is ordered exactly like ready-mix concrete.
- Add 10 to 20% for overbreak. Trench walls slough, and CLSM finds every void it can reach, which is the point of using it.
- Typical delivered price runs $80 to $120 per cubic yard, below structural concrete because the cement content is far lower.
Frequently asked questions
What strength is flowable fill?
Deliberately low: usually 50 to 150 psi so it can be re-excavated with a backhoe later. Mixes above about 300 psi become difficult to dig out, which defeats the purpose in a utility trench.
Does flowable fill need compaction?
No. That is its main advantage over granular backfill: it is self-levelling and self-compacting, so it reaches under haunches and into voids a plate compactor never could.
How soon can I load it?
Light traffic after about 24 hours for a typical mix. Confirm with the supplier, because set time varies a lot with the specified strength and the weather.