Flowable fill (CLSM) prices and pours like concrete but works like perfect backfill, no compaction, no voids. Enter the trench or void dimensions for yards, weight and cost.
Flowable Fill Calculator
CLSM flows into every corner, so the waste factor is small, 5% covers irregular trench walls. Deduct large pipe displacement with the Tank & Trench Calculator first if the pipe is big.
You need (with waste)
CLSM is batched by the ready-mix plant like concrete, order by the yard, in truck increments, and confirm the mix type: ‘excavatable’ (low strength, diggable later) is the standard trench choice.
A flowable fill calculator is trench volume with a ready-mix mindset: length × width × depth, divided by 27, plus a small waste factor because the material self-levels into every void. A 50 ft trench, 24 in wide and 24 in deep, takes 7.41 yd³ exact, 7.78 yd³ to order. Enter your trench above for yards, weight and cost.
How to Calculate Flowable Fill
Flowable fill, controlled low strength material, CLSM, solves the trench problem that compaction cannot: it is a fluid, cement-lean mix that flows around pipes, fills every haunch void, and sets into a firm, diggable solid without a single pass of a compactor. Ordering it is concrete ordering, not dirt ordering: yards, trucks and slump-like flow. This flowable fill calculator gives the yardage, the approximate placed weight and a cost line from your plant’s rate.
The Formula
Cubic yards = ft³ ÷ 27
Order = yards × (1 + waste %), 5% typical · Weight ≈ 140 lb per ft³ placed
CLSM is not concrete and must not be specced like it: excavatable mixes target roughly 50–100 psi at 28 days (hand-diggable), structural fills go higher. It is delivered at high flow, placed by chute or pump, and it will find every leak in a form or bulkhead, dam the trench ends before the truck arrives.
Worked Example: 50 ft Trench, 24 in Wide, 24 in Deep
50 × 2 × 2 = 200 ft³ → 7.41 yd³ exact; at 5% waste, 7.78 yd³ to order, an 8 yd³ ticket. At $120/yd³ that is about $933 of material. Placed weight ≈ 200 × 140 = 28,000 lb in the ground. Note what did not happen: no compactor rental, no lift testing, no settlement call-back, the comparison every trench contractor runs.
Flowable Fill vs Compacted Backfill
| Property | Flowable fill (CLSM) | Compacted soil / stone |
|---|---|---|
| Placement | Poured, self-levelling | Placed in 8–12 in lifts |
| Compaction | None needed | Every lift, tested on spec work |
| Voids around pipe | Flows into all haunches | Hand-tamped, often incomplete |
| Later excavation | Excavatable mixes: yes, by hand/backhoe | Yes |
| Settlement | Essentially none | Depends on compaction quality |
| Cost basis | Per yd³ from the plant | Per ton + compaction labour |
CLSM’s classic home is the trench under pavement, where a settlement dip is a warranty and a headline. For open-field trenches, compacted native backfill is usually cheaper. Many specs name CLSM by strength class, order the class, not just ‘flowable fill’.
Why CLSM Waste Is Only 5%
A fluid finds the true volume of a trench, including the bellies and over-dig that rigid fill bridges over, which argues for more waste, not less. In practice the trench section you measured already averages those irregularities, and over-ordering a plant-batched material by 10% buys an expensive partial yard. Five percent is the working figure for a measured trench; use 10% on hand-dug, irregular excavations and voids of guessed dimensions, like old tank pits.
How to Measure and Calculate: Step by Step
- Measure the trench length and the average width and depth (probe several sections).
- Deduct big pipe displacement first with the Tank & Trench Calculator if the main is large.
- Enter dimensions, a 5% waste factor and your plant’s per-yard price.
- Order to the truck increment (round 7.78 up to 8 yd³ with the plant).
- Bulkhead the trench ends, place in one continuous pour, and screed or strike the top.
Worked Example 2: Abandoned Tank Pit, 10 × 10 × 6 ft
Volume = 600 ft³ = 22.22 yd³ exact. At 10% for the guessed pit walls: 24.4 yd³, three truck loads, scheduled back to back so the fill stays continuous. Filling a void like this with compacted soil would take days of lifts; with CLSM it is a morning, and the surface can be paved without a settlement watch period.
Common Quantities at a Glance: Cheat Sheet
| Job / quantity | Result |
|---|---|
| Per ft of 24 × 24 in trench | 0.30 yd³ |
| Per ft of 24 × 36 in trench | 0.44 yd³ |
| 50 ft trench, 24 × 24 in | 7.41 yd³ exact |
| 100 ft trench, 12 × 18 in | 5.56 yd³ exact |
| 1 yd³ CLSM | ≈ 1.9 tons placed |
| Excavatable strength | ≈ 50–100 psi at 28 days |
Practical Tips From the Job
Tell the plant the application when ordering, trench fill, void fill and structural fill are different mixes at different prices, and ‘excavatable’ is the word that protects your future self. Place against a bulkhead and work backwards; CLSM flows downhill eagerly and a 50 ft trench will happily drain into the manhole you forgot to dam. Keep people and pets off until it takes initial set, it looks like mud long after it stops behaving like it.
Common Mistakes to Avoid
- Ordering structural-strength CLSM where future excavation is likely.
- Forgetting to bulkhead trench ends and openings.
- Measuring the trench at its narrowest section only.
- Treating CLSM like dirt and ordering by the ton, plants batch by the yard.
- Paving over it the same hour; give it its specified set time.
What It Costs and How to Order Smart
The example trench is about $933 of material at $120/yd³, more than stone per yard, less than stone plus compaction labour plus risk on most paved crossings. Your plant’s rate varies with cement content and haul; excavatable mixes are the cheaper end. Compare honestly: backfill stone (Gravel Cost Calculator) + compactor + testing versus one CLSM ticket.
Who Uses This Calculator?
Builders filling utility trenches under finished surfaces use the yardage daily. Contractors weigh CLSM against compaction crews on every bid. Engineers specify the strength class and check quantities on pay items. Architects meet it in urban details where settlement cannot be tolerated, and homeowners meet it once, filling an old pool or tank pit, and are glad the guesswork is this small.
Flowable Fill Calculator FAQs
How much flowable fill do I need for a trench?
Trench volume, minus large pipe displacement, plus about 5%. A 50 ft trench, 24 in wide and 24 in deep, is 7.41 yd³ exact and 7.78 yd³ to order. A 24 in deep × 36 in section runs 0.44 yd³ per foot. Measure the average section, fluid fill finds every extra inch you averaged away.
What is flowable fill (CLSM)?
Controlled low strength material: a fluid mix of cement, fly ash or slag, fine aggregate and water that is poured like thin concrete and hardens to a weak, soil-like solid, typically 50–100 psi for excavatable grades. It is backfill that places itself: no lifts, no compaction, no voids, and no settlement.
How much does flowable fill cost per cubic yard?
It varies with region, cement content and haul, this page’s $120/yd³ is a placeholder for the arithmetic, and your ready-mix plant’s quote is the real figure. Excavatable mixes sit at the lower end. Compare the ticket against stone plus compaction labour and testing, not against stone alone; that is the comparison CLSM usually wins under pavement.
Can flowable fill be dug up later?
Excavatable grades, yes, that is their design point: at roughly 50–100 psi they can be removed with a backhoe, and small amounts by hand, while still supporting pavement without settling. Higher-strength structural CLSM cannot be hand-dug. If the trench may ever be reopened, the word ‘excavatable’ must be on the order and the spec.
Does flowable fill need compaction?
No, that is its entire advantage. It is placed at high flow, consolidates under its own weight and fills haunches and voids that tamping cannot reach. It does need containment: bulkheads at trench ends, sealed penetrations, and care around structures it could float or push against while fluid.
How much does a cubic yard of flowable fill weigh?
About 3,700–3,900 lb placed, roughly 140 lb per cubic foot, similar to lean concrete and lighter than compacted stone per volume. The figure matters for load on underlying structures and for checking truck payloads on big void fills; this calculator shows the placed weight beside the yards.
How long does flowable fill take to set?
It typically takes an initial set within a few hours and can often be covered or paved the next day, per the mix and the spec, but ‘set’ is not ‘full strength’, which develops over 28 days like other cement materials. Follow the plant’s and spec’s stated times before loading it with pavement or traffic.
Can flowable fill replace concrete?
No. CLSM is deliberately weak, it is engineered backfill, not a structural material. It cannot serve as a footing, slab or wall. Its strength ceiling is a feature: strong enough to support and never settle, weak enough to excavate. Anything carrying a designed structural load gets real concrete from the other calculators on this site.
How accurate is this flowable fill calculator?
The volume math is exact; the 5% factor suits measured trenches because the material finds the true section. For irregular voids use 10% and probe the depth in a grid, an old tank pit is rarely the box it was drawn as. Plant batching and ticket increments govern the final order.
Related Calculators
Sources used for standards and conversions: National Ready Mixed Concrete Association (NRMCA) guidance on ordering and yield; American Concrete Institute (ACI) practice for mix and placement; Portland Cement Association (PCA) design and control guidance; US manufacturers’ printed yields (80 lb bag = 0.60 ft³). Depths, sizes and spacings set by your local building code govern wherever they differ from the general figures here.
Disclaimer: Results are estimates for planning and ordering. Confirm quantities with your supplier before ordering, and have structural work, footings, walls, columns, beams and anything load-bearing, checked by a licensed engineer.