Flowable Fill Calculator

1. Choose the void shape: a rectangular trench or a round void or pipe.
2. Enter the dimensions in feet (round uses the diameter in inches and the length in feet).
3. Set the quantity and a small waste allowance.
4. Pick the fill type, excavatable or structural, for the strength note, then read the cubic yards to order.

Here is what each result means:

Flowable fill, known in the standard as controlled low-strength material or CLSM, is a self-leveling, self-compacting cementitious mixture used to fill trenches, voids, abandoned pipes and hard-to-reach spaces. It is placed like concrete but is much weaker, so it flows into every gap without compaction.

Because it flows and self-levels, you size it by the volume of the void, just like concrete, and order it by the cubic yard from a ready-mix supplier. What makes it different is the strength: it is deliberately low, and whether it should be excavatable or structural changes the mix, though not the volume.

The volume is the void dimensions multiplied together, divided by 27 for cubic yards. Because flowable fill reaches every corner, a small waste allowance for over-break is usually enough.

1. Multiply length by width by depth for a trench, or pi times radius squared times length for a round void.
2. Multiply by the quantity of identical voids.
3. Divide by 27 for cubic yards, and add a small waste allowance.

Suppose you are backfilling a 100 foot trench, 2 feet wide and 3 feet deep, with excavatable CLSM and a 5 percent waste allowance.

Calculation: volume = 100 x 2 x 3 = 600 cu ft = 22.2 cu yd. With 5 percent waste, order about 23.3 cu yd. As an excavatable mix, keep it under about 150 psi so the trench can be dug out later if the utility needs access.

The same trench filled with structural CLSM would be the same volume but a stronger, permanent mix that is not meant to be re-excavated.

The volume is the same either way; the difference is the strength, which decides whether the fill can be dug out later. ACI 229R defines CLSM as reaching 1,200 psi or less.

Excavatability is often specified at a late age, such as 90 days to a year, because the strength keeps rising. Set the target and test age in the project specification, not by this guide alone.

Over-break

A trench rarely cuts to the exact design width, so the real void is a little larger. The waste allowance covers this; raise it for rough or collapsing ground.

Existing Pipes and Obstructions

Pipes, conduits and bedding inside the void displace fill. Subtract their volume if they are significant, or you will over-order.

Shape and Corners

Break an irregular void into trenches and round sections, calculate each and add them, rather than guessing one average size.

Settlement and Bleed

Some mixes settle or bleed a little as they set. A modest allowance and a topped-up finish handle it.

Backfilling Trenches

Fill utility and pipe trenches quickly without compacting in layers, and with less settlement than soil.

Abandoning Pipes and Tanks

Fill an abandoned pipe, culvert or tank so it cannot collapse, using the round shape for the volume.

Filling Voids Under Slabs

Support a slab or pavement over a void or soft spot with a structural mix that will not be dug out.

1. Ordering Structural When You Need Excavatable

A fill that is too strong cannot be dug out, which is a problem over a live utility. Match the type to future access.

2. Ignoring Over-break

The real void is larger than the design trench. Add a waste allowance or you will run short.

3. Forgetting Pipes Inside the Void

Large pipes and bedding take up space. Not subtracting them over-orders the fill.

4. Judging Excavatability at 28 Days

CLSM keeps gaining strength. A mix that is diggable at 28 days may not be at a year, so specify a late test age.

5. Treating It as Structural Concrete

Flowable fill is low strength by design. It is not a substitute for structural concrete or a designed footing.

The volume arithmetic is exact for the dimensions you enter; the strength note is a typical range, not a mix design.

Multiply the void dimensions for the volume and divide by 27 for cubic yards. A 100 by 2 by 3 foot trench is about 22.2 cubic yards, or 23.3 with a 5 percent waste allowance. CLSM is ordered by the cubic yard like concrete.

Flowable fill, or controlled low-strength material, is a self-leveling cementitious mixture for backfilling trenches, voids and abandoned pipes. It flows into every gap without compaction and is much weaker than structural concrete.

Excavatable CLSM stays weak, under about 150 psi, so it can be dug out later; structural CLSM is stronger, about 300 to 1,200 psi, for permanent support. The volume you order is the same; the mix differs.

ACI 229R defines CLSM as reaching 1,200 psi or less. Excavatable mixes are usually kept under about 150 psi, with hand-tool mixes under 100, while structural fill runs 300 psi and up.

A small one, often around 5 percent, for over-break where the trench is wider than designed. Because flowable fill self-levels into every gap, you rarely need the larger allowance a hand-placed material would.

Yes. Use the round shape with the pipe diameter and length to get the volume. Flowable fill is a common way to fill abandoned pipes and culverts so they cannot collapse.

Yes, if they are large. Pipes, conduits and bedding take up space in the void, so subtract their volume from the trench volume to avoid over-ordering.

No. It is a low-strength, high-flow material for filling and support, not a structural concrete. It uses less cement and much more fine material or fly ash, and is not compacted.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• National Ready Mixed Concrete Association (NRMCA; controlled low-strength material).
• American Cement Association (Portland Cement Association) (American Cement Association; CLSM and concrete basics).
• QUIKRETE Concrete Mix product data (QUIKRETE; coverage and yield).
• QUIKRETE concrete and bag calculator (QUIKRETE; volume and yield method).