Flowable Fill Calculator: CLSM Volume for Tanks, Trenches & Voids

What flowable fill (CLSM) actually is

Controlled low strength material is a soupy blend of sand, a little cement, fly ash, and water, batched at a ready-mix plant and delivered in the same trucks as concrete. It pours like a thick milkshake, flows into every corner of a void without vibration or compaction, and sets strong enough to never settle, weak enough to excavate later if the mix is specified that way.

Flowable fill is not concrete, and that is a feature.

The industry splits mixes into excavatable (under about 150 psi, removable with a backhoe) and non-excavatable (up to 300 psi and beyond, for permanent structural fills). Tank closures and utility trenches almost always want excavatable. The tradeoff is set in stone, literally, on pour day: nobody ever un-specifies a hot mix after it cures.

Flowable fill vs sand vs foam for tank closure

Sand is the cheapest per ton but must be placed, rodded, and compacted through tank openings, and it can bridge over voids around internal baffles that show up as settlement years later. CLSM self-levels into all of that geometry off the truck chute. Polyurethane foam wins where truck access is impossible (basement tanks, courtyards) because it arrives as two drums and a hose.

The decision usually makes itself from access and volume. Yard tank a truck can reach: CLSM. Basement 275 with no chute path: foam. Deep-rural site where the nearest ready-mix plant is 90 minutes away and sand is on the farm: sand, carefully placed. The closure-in-place guide walks the whole fill-material decision, including the state rules that sometimes take the choice away from you.

Flowable fill cost

Two line items surprise first-time buyers. Short-load fees hit orders under about 4 yards, which makes small-tank closures cost more per yard than the material deserves. And standby time bills by the minute once the truck waits on site past its included unload window.

Small jobs have a workaround: some plants sell a U-cart or allow will-call pickup of a yard or two, and a closure contractor folding your pour into a route beats a dedicated short load. Get the per-yard rate, the short-load threshold, and the standby clock in writing when you call.

What to tell the ready-mix plant when you order

Five things, in the order dispatchers ask. The yardage, from the calculator above with the overage already in it. That you want excavatable CLSM or flowable fill (plants also know it as controlled density fill or sand slurry). The intended use. The pour method. And the site access. Use matters because a tank closure wants a wetter, flowier mix than a trench cap. Access matters because a truck needs a path; measure gates and check septic and shallow-utility locations before the truck is idling at the curb.

Ask two questions back. What is the actual mix strength (get a number, not a product name), and what does the plant charge for standby. On regulated UST closures, the fill step sits inside a permitted process with sampling and documentation around it; the decommissioning guide covers where the pour fits in the closure sequence, and your contractor usually handles the ordering anyway. Homeowners closing a small tank themselves should check whether their state allows owner-performed closure at all before dialing a plant.

Frequently Asked Questions

Volumes are ideal geometry; internal baffles, piping, and residual sludge displace some fill, and a tank pitched off level pours differently than the math assumes. Closure-in-place of a regulated UST is permitted work with cleaning, sampling, and documentation requirements before any material goes in; this calculator sizes the order, not the project.

CLSM strength classes and the 5% overage were compiled by USTContractors.com from ready-mix industry practice and FHWA guidance. Last verified July 2026.

Authoritative references:

• EPA UST Closure requirements
• FHWA on flowable fill / CLSM in construction

Pricing a closure-in-place job?

The fill is one line on a closure bid that also covers cleaning, sampling, and the paperwork. Request free decommissioning quotes or browse contractors by state.