Flowable Fill Calculator: CLSM Volume for Tanks, Trenches & Voids
A 1,000 gallon tank takes about 5 cubic yards of flowable fill. Pick your tank or enter trench dimensions for the yards to order and the truck count.
A tank closure pour that comes up half a yard short leaves a void at the top of the shell, the exact thing the closure exists to eliminate. A second truck for half a yard costs more than the 5% overage on the first order ever will.
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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 to somewhere between 50 and 300 psi. That strength target is the whole idea: 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
Closure-in-place regulations require the cleaned tank shell to be filled with an inert material, and three candidates dominate. 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, which is why most commercial closures specify it. Polyurethane foam wins where truck access is impossible (basement tanks, courtyards) because it arrives as two drums and a hose, at a premium price per filled gallon.
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, honestly banded
Plan on $85 to $150 per cubic yard delivered for standard excavatable CLSM in most of the country. Rural plants with cheap aggregate sit near the bottom of that band; metro markets with long truck cycles (Boston, the Bay Area, urban New Jersey) run $150 to $200. Two line items surprise first-time buyers. Short-load fees of $75 to $150 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, typically 5 minutes per yard.
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 30-ton 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
How much does flowable fill cost per yard?
Between $85 and $150 per cubic yard delivered in most markets, with rural plants at the low end and dense metro markets pushing $200. Compare that to about $130 to $180 for structural concrete and roughly $30 to $50 for delivered sand. The spread against sand looks bad until you price the labor: sand needs placement and compaction in lifts, while CLSM self-levels and self-compacts off the chute. On a tank closure, the material premium usually costs less than the extra hours of hand work sand demands.
Is flowable fill cheaper than concrete?
Per yard, yes, usually by $30 to $60. Flowable fill uses far less cement than structural concrete because it targets 50 to 300 psi instead of 3,000 plus. The real comparison is fitness for the job: concrete in a tank closure is a mistake, not an upgrade. It costs more, and if the site ever redevelops, a tank shell full of 4,000 psi concrete turns a routine excavation into demolition. Specify excavatable CLSM (under about 150 psi) and future you, or the future buyer, can dig it back out with ordinary equipment.
Can you dig flowable fill back up?
Excavatable mixes, yes: material under about 150 psi comes out with a standard excavator bucket, which is exactly why closure specs call for it. Higher-strength CLSM (200 to 300 psi) sits in the gray zone where a bucket struggles and a hoe ram gets mentioned. The strength is set by the mix design on the day of the pour and cannot be changed afterward, so tell the plant the intended use when ordering. One caution: CLSM keeps gaining strength slowly for months, so a mix that digs easily at 28 days digs harder at 2 years.
How many yards of flowable fill does a 1000 gallon tank take?
About 5 cubic yards, and the math is short: 1,000 gallons times 231 cubic inches per gallon is 231,000 cubic inches, divided by 46,656 cubic inches per cubic yard gives 4.95 yards. Add 5% overage and you order 5.5 yards, one ready-mix truck. The same arithmetic puts a 275 gallon heating oil tank at about 1.4 yards and a 10,000 gallon station UST at roughly 52 yards, which is a six-truck pour and the reason big closures get scheduled like concrete jobs.
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. Cost bands are planning figures that move with cement prices and hauling distance. 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, the 5% overage, and the cost banding were compiled by USTContractors.com from ready-mix industry practice and FHWA guidance. Last verified July 2026.
Authoritative references:
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