Flowable Fill Calculator

1. Choose the shape: a rectangular trench or excavation (length by width by depth) or a round pipe or void (diameter by length).
2. Enter the dimensions. Keep trench length, width and depth in feet, and pipe diameter in inches, exactly as the fields are labelled.
3. Set the quantity of identical fills and a waste allowance of 5 to 10 percent for over-excavation and irregular voids.
4. Read the result: cubic yards is the ordering number. Add a unit weight for the tonnage, or a price per yard for the material cost.

Here is what each result line means:

Flowable fill is a self-levelling, low-strength concrete used as backfill instead of compacted soil. ACI 229R calls it a self-consolidating cementitious material used primarily as a backfill as an alternative to compacted fill, and it is also known as controlled low-strength material (CLSM), controlled density fill, lean concrete slurry and unshrinkable fill.

Because it flows into place and hardens in hours without layered compaction, it fills trenches, abandoned pipes and awkward voids that a plate compactor could never reach. By definition it is weak: ACI sets the ceiling at a 28-day compressive strength of 1200 psi (8.3 MPa) or less, and most applications stay at 300 psi (2.1 MPa) or less so the fill can be dug out again later. This tool estimates the volume to order for that fill. It is not a mix-design or strength tool; if you need the proportions of a conventional structural mix instead, use the concrete mix calculator, size the binder with the cement calculator, or check mix water against a target with the water cement ratio calculator.

The volume of a trench is length times width times depth. Because flowable fill is sold by the cubic yard and one cubic yard equals 27 cubic feet, the tool finds the volume in cubic feet and divides by 27, then adds the waste allowance.

1. Multiply trench length by width by depth, all in feet, to get the volume in cubic feet.
2. Multiply by the quantity of identical trenches, pipes or voids.
3. Add the waste allowance for over-excavation, then divide by 27 for cubic yards and multiply by 0.0283168 for cubic meters.
4. Multiply cubic feet by the unit weight for the tonnage, and divide cubic yards by about 10 for ready-mix truck loads.

For a round pipe or void the cross-section is a circle, so the volume is pi times the radius squared times the length, with the radius taken as half the diameter.

Suppose you are filling a utility trench 50 feet long, 2 feet wide and 3 feet deep, with a 10 percent waste allowance.

Calculation: volume = 50 x 2 x 3 = 300 cu ft. In cubic yards that is 300 / 27 = 11.11 cu yd. Adding 10 percent waste gives 300 x 1.10 = 330 cu ft, or 12.22 cu yd. At a unit weight of 120 pounds per cubic foot that is about 19.8 short tons, and at roughly 10 cubic yards a truck it is 2 truck loads.

The lesson: order the 12.22 cubic yards, not the bare 11.11, and line up continuous delivery so the two loads place without a cold joint.

Over-excavation

A trench is rarely cut to the exact plan size. Any over-dig below or beside the line fills with flowable fill, which is the main reason real volume beats the geometry and why a waste allowance exists.

Irregular and Collapsing Voids

Abandoned tanks, washouts and rock pockets hold more than their nominal shape. Because flowable fill self-levels, it finds every cavity, so round up and add waste for a rough excavation.

Pipe and Object Displacement

A pipe, duct or structure left in the trench displaces fill. Subtract its volume, or enter only the net space to be filled, so you do not over-order around large objects.

Fill Depth Versus Trench Depth

Sometimes only the lower part of a trench is flowable fill and the rest is soil or pavement. Enter the fill depth, not the full excavation depth, or the estimate runs high.

Mix and Unit Weight

Standard CLSM is considerably heavier than loose soil, while low-density foamed CLSM is much lighter. Volume is unchanged, but the tonnage and the load on the subgrade depend on which mix you use.

Flowable fill and compacted granular backfill both fill a trench, but they behave very differently. Flowable fill costs more per cubic yard yet often wins on in-place cost because it needs no compaction, testing or lift-by-lift labour.

Because it is ordered and placed like ready-mix, in conventional concrete trucks, you can size and schedule a job the same way you would a small pour.

Utility Trenches

The classic use is backfilling pipe and cable trenches in roadways, where compaction is slow and settlement later cracks the pavement. Flowable fill places fast and will not settle.

Abandoning Pipes and Tanks

Fill a decommissioned sewer, culvert or underground tank so it cannot collapse or float. Use the round shape for a pipe and enter the inside diameter.

Voids Under Slabs and Structures

Fill washouts, over-excavations and voids beneath slabs, footings and bridge approaches where hand compaction is impractical.

1. Using Trench Depth Instead of Fill Depth

If only the lower part of the trench is flowable fill, entering the full excavation depth over-orders. Enter just the depth you are filling.

2. Ignoring Over-excavation

Ordering the exact plan volume leaves nothing for over-dig and irregular walls. Flowable fill reaches every void, so under-ordering stops the placement short.

3. Forgetting to Subtract Large Objects

A big pipe or tank in the trench displaces fill. Not subtracting it buys concrete you cannot place.

4. Treating It as Structural Concrete

Flowable fill is deliberately weak. Do not use its volume estimate to size a load-bearing element, and do not substitute it for structural concrete.

5. Not Planning Continuous Delivery

On a larger fill, trucks arriving with gaps let an earlier lift stiffen. Order the full quantity and schedule the loads back to back.

The volume arithmetic is exact; the usefulness of the answer depends on the dimensions, the waste allowance and the unit weight you give it.

Flowable fill is a self-levelling, low-strength cementitious backfill used instead of compacted soil, known technically as controlled low-strength material (CLSM). ACI 229R defines it as a self-consolidating material used as an alternative to compacted fill, with a 28-day compressive strength of 1200 psi or less.

Multiply the trench length by width by depth in feet, divide by 27 for cubic yards, then add 5 to 10 percent for over-excavation. A 50 by 2 by 3 foot trench is about 11.1 cubic yards, or 12.2 cubic yards with 10 percent waste. Enter your dimensions above for the exact figure.

No. Both are cementitious, but flowable fill is deliberately weak and self-levelling, made to be dug out again, while structural concrete is strong and must be placed and finished. Flowable fill is ordered by the cubic yard like ready-mix and delivered in the same trucks.

Use the round shape and enter the inside diameter in inches and the length in feet. The tool finds the internal volume as pi times the radius squared times the length. For a 24 inch pipe 60 feet long that is about 7 cubic yards before waste.

Low by design. ACI 229R caps it at 1200 psi at 28 days, and most applications stay at 300 psi or less so the fill stays removable; mixes meant to be dug by hand are usually targeted under 100 psi long-term. It is backfill, not a structural material.

It depends on the mix. At a unit weight of 120 pounds per cubic foot a cubic yard weighs about 3,240 pounds, roughly 1.6 short tons. Low-density foamed CLSM weighs far less. Enter the unit weight of your mix for an accurate tonnage.

Between 5 and 10 percent for most trenches. The allowance covers over-excavation, irregular walls and slight subsidence as the fill settles into voids. Add more for a rough, collapsing or very irregular excavation.

No. It self-levels and consolidates under its own weight, filling voids that a compactor cannot reach, and hardens in a few hours. Eliminating compaction and density testing is the main reason it often beats compacted backfill on in-place cost.

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

• ACI 229R, Report on Controlled Low-Strength Materials (American Concrete Institute; CLSM definition, 1200 psi and 300 psi strength limits).
• FHWA, flowable fill (User Guidelines for Waste and Byproduct Materials) (Federal Highway Administration; self-levelling backfill, no layered compaction, ready-mix placement, alternate names).
• National Ready Mixed Concrete Association (NRMCA; ready-mixed concrete and CLSM delivery context).