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Flowable Fill / CLSM Calculator
Calculate flowable-fill quantity from the specified trench or void geometry. Optional pipe displacement is deducted so the result represents the space actually filled by CLSM.
Calculate flowable fill / clsm quantities
Use this calculator when the dimensions of the flowable-fill zone are known. It calculates the rectangular fill volume, subtracts the outside volume of identical pipes running through it and applies the allowance you enter.
Formula: Net CLSM = length × width × fill depth − [π × (pipe outside diameter ÷ 2)² × pipe length × pipe count].
Useful for flowable fill cubic yards, CLSM trench backfill quantity, controlled low-strength material volume and utility-trench flowable fill.
Step 1
Enter project dimensions
Use dimensions from the applicable drawing, specification or field takeoff. Switch between US / Imperial and metric at any time; entered dimensions are converted automatically.
Step 2
Quantity result
Use project-specified geometry and material requirements. Allowance is shown explicitly rather than hidden inside the dimensions.
Worked example
The example uses the calculator's starting dimensions so the arithmetic can be checked independently.
Example inputs
| Fill-zone length | 50 ft |
|---|---|
| Fill-zone width | 4 ft |
| Flowable-fill depth | 3 ft |
| Pipe outside diameter | 1 ft |
| Identical pipes through fill zone | 1 count |
| Pipe length within fill zone | 50 ft |
| Ordering allowance | 5 % |
Example results
| CLSM to order | 21.806 yd³ |
|---|---|
| Net geometric CLSM | 20.768 yd³ |
| Gross fill zone | 22.222 yd³ |
| Pipe displacement | 1.454 yd³ |
How the calculation works
Net CLSM = length × width × fill depth − [π × (pipe outside diameter ÷ 2)² × pipe length × pipe count].
FHWA describes flowable fill / controlled low-strength material (CLSM) as a self-leveling, self-compacting cementitious backfill used particularly in below-grade applications such as utility trenches and around pipes. Mix design, strength and placement requirements remain project-specific.
FHWA — Flowable Fill Application Description
Scope: verify dimensions, material specifications, structural/electrical requirements, reinforcement, excavation support and local requirements independently. JobsiteMeasure estimates quantities; it does not design the installation.
Flowable Fill / CLSM Calculator FAQs
How do I calculate flowable fill for a trench?
Calculate the specified fill-zone volume and subtract any structures or pipes occupying that same space. This calculator deducts identical cylindrical pipes when entered.
Is flowable fill the same as CLSM?
Flowable fill is commonly referred to as controlled low-strength material (CLSM), controlled density fill and other names. Use the material and mix specified for the project.
Does this calculator choose the CLSM mix or strength?
No. It estimates geometric quantity only. Material properties, strength, removability and placement requirements come from the project specification and supplier.