Wet-stock loss at a gas station: how to detect and reduce it.

What is wet-stock loss?

Wet-stock loss is fuel that left the underground tank without showing up in dispensed-and-sold gallons. It's measured by comparing how much fuel the tank should contain (based on starting inventory + deliveries received − POS-dispensed gallons) against how much actually remains.

Some wet-stock loss is normal. Evaporation through tank venting accounts for ~0.05–0.10% of throughput. Measurement error in tank reads and dispensed-volume counts adds another 0.05–0.10%. Combined acceptable baseline: about 0.1–0.2% of gallons throughput.

Anything sustained above that benchmark indicates a real issue: pump miscalibration, undocumented sales, theft, or (rarely but serious) underground tank leak.

0.1–0.2%

Normal wet-stock loss as % of throughput

Evaporation + measurement error baseline. Above 0.5% sustained warrants pump calibration check; above 1% triggers UST regulatory escalation.

The wet-stock reconciliation formula

expected_inventory = previous_day_inventory
                   + deliveries_received_today
                   − POS_dispensed_gallons_today

actual_inventory = end_of_day_ATG_reading
                   (or manual tank-stick reading)

wet_stock_variance = expected_inventory − actual_inventory

variance_pct = wet_stock_variance / POS_dispensed_gallons_today

dollar_exposure = wet_stock_variance × wholesale_cost_per_gallon

Positive variance (expected > actual) means fuel disappeared, the worry case. Negative variance (expected < actual) means there's more fuel than expected , usually a measurement or delivery-record error, less urgent but still worth investigating.

Common causes of wet-stock loss

1. Pump miscalibration

Most common cause of sustained wet-stock loss above the baseline. Mechanical wear in the dispenser's flow meters drifts over time. A 0.5% over-dispense, the pump delivers 1.005 gallons for every gallon it bills the customer, on 30,000 gallons/month equals 150 gallons of unbilled fuel = $600/month at $4 wholesale.

Detection: per-pump dispensed-volume vs. POS-reported volume. A pump consistently over-dispensing flags first; calibration scheduled with the dispenser service vendor.

2. Evaporation

Tank ventilation allows some fuel evaporation, especially in summer at warm-climate stations. Modern Stage 2 vapor-recovery systems limit this. Typical evaporation loss: 0.05–0.10% of throughput, higher in summer than winter.

3. Measurement error

Tank-stick reads vary by who's measuring and how the stick is held. Even ATG sensors have ±0.1% accuracy , small at low volumes, meaningful at high volumes. Measurement error contributes to baseline variance, not large excursions.

4. Undocumented sales

Fuel dispensed but not rung up. Common patterns: a manager fueling a personal vehicle without ringing it (or ringing at a discount that doesn't cover cost), maintenance crew filling equipment from the station tank without documentation, friends/family sales not entered.

5. Theft

Real but rare in retail fuel. Common patterns: thieves rigging the dispenser to dispense without billing (siphoning), tank-tap theft (drilling and draining a tank, rare and detectable), or insider theft via deliberate POS bypass.

6. Underground storage tank (UST) leak

The serious case. EPA-regulated single-walled tanks (mostly retired) leak through corrosion; modern double-walled tanks have leak-detection sensors. A leak appears as persistent wet-stock loss that doesn't correlate with any pump or operational cause.

EPA regulations require investigation of suspected releases within 7 days and confirmation within 30 days. Persistent wet-stock loss above 1% of throughput is a regulatory escalation trigger.

Detection workflow

1. Daily ATG read (or tank stick)

End-of-day tank inventory per grade. ATG (Veeder-Root TLS, Gilbarco SmartTLS) reads every 15 minutes and provides the cleanest data. Manual tank-stick reads work but require disciplined timing (same time daily, same person ideally).

2. POS-dispensed gallons per grade

Sum of all dispensed fuel transactions for the day, per grade. POS feed provides this where integrated; Z-report total where not.

3. Delivery records

Any BOLs received during the period get added to expected inventory. Match the BOL gallons against the tank-rise observed during the delivery window.

4. Compute and tag variance

wet_stock_variance = expected − actual. Tag the variance: balanced (under 0.2% of throughput), warning (0.2–0.5%), error (0.5–1.0%), critical (above 1.0%).

5. Per-pump diagnostic when warning band hits

Most ATG systems report dispensed-volume per pump. Cross-reference: per-pump dispensed-volume vs. per-pump POS-reported. The pump that's over-dispensing is the calibration suspect.

6. Escalation thresholds

Sustained variance above 0.5% (3 consecutive days): schedule pump calibration check. Above 1.0% (single day or sustained): EPA UST escalation, call your environmental consultant or the state UST regulator.

Worked example: a 4.9¢/gal margin gap from wet-stock

Daily reconciliation, regular gasoline:
  Tank inventory yesterday end:    11,420 gal
  Deliveries today:                 0 gal
  POS dispensed today:           +/− 1,200 gal
  Expected tank inventory:       10,220 gal

  Actual ATG reading end-of-day: 10,205 gal
  Wet-stock variance:                15 gal (positive: 15 gal "missing")
  Variance %:                       1.25% of throughput
  Verdict:                        critical, investigate

Per-pump diagnostic:
  Pump 1 dispensed (ATG):         320 gal
  Pump 1 dispensed (POS):         320 gal, match
  Pump 2 dispensed (ATG):         435 gal
  Pump 2 dispensed (POS):         420 gal (15 gal over-dispense)
  Pump 3 dispensed (ATG):         445 gal
  Pump 3 dispensed (POS):         445 gal, match

  Pump 2 is over-dispensing 3.5%, well beyond normal.

Action:
  Service vendor called for calibration check on pump 2.
  Pump 2 taken out of service until calibration confirmed.
  Wet-stock variance resolved within 48 hours.

Margin impact if undetected for 30 days:
  Pump 2 throughput ~13,000 gal/month
  Over-dispense at 3.5% = 455 gal/month unbilled
  At $4 wholesale = $1,820/month margin loss
  Annualized = $21,840/year (single pump)

Common wet-stock mistakes

Weekly tank-stick reads instead of daily.A 0.5% leak compounds for a week before detection. Daily ATG read or daily manual stick is the discipline.
Ignoring positive variance."Extra fuel showed up" usually means a delivery record or measurement error. Investigate the data even if dollars aren't at risk, process integrity matters.
Not tracking per-pump dispensed volumes.Aggregate tank-vs-POS variance flags the total but not the source. Per-pump tracking identifies the specific dispenser drifting.
Allocating wet-stock loss to “wholesale cost” generically. Realized margin per gallon needs to subtract wet-stock loss as a per-gallon cost. Margin numbers without this allocation overstate profitability.
Not escalating EPA-threshold events.Persistent wet-stock loss above 1% triggers UST regulatory escalation in most states. Operating without investigation = regulatory exposure on top of margin loss.
Trusting customer complaints as the leak detector.Customers don't notice 0.5% over-dispense. By the time someone complains, the calibration drift has compounded for weeks or months.

Frequently asked questions

What is wet-stock loss at a gas station?

Wet-stock loss is fuel that left the underground tank without showing up in dispensed-and-sold gallons. Measured by comparing expected tank inventory (previous + deliveries − dispensed) against actual end-of-day inventory. Causes: pump miscalibration, evaporation, measurement error, undocumented sales, theft, or leak.

What's a normal wet-stock loss percentage?

0.1–0.2% of throughput from evaporation and measurement error combined. Above 0.5% sustained warrants a pump calibration check. Above 1.0% triggers UST regulatory escalation in most states under EPA suspected-release rules.

What's the most common cause of wet-stock loss?

Pump miscalibration. Mechanical wear in dispenser flow meters drifts over time, causing the pump to deliver more fuel than it bills. A 0.5% over-dispense on 30,000 gallons/month equals $600/month of unbilled fuel at $4 wholesale, compounds quickly across multiple pumps.

How do you detect wet-stock loss?

Daily ATG (automatic tank gauging) reads. Veeder-Root TLS or Gilbarco SmartTLS, provide 15-minute inventory readings. Manual tank-stick reads work but lag detection. Per-pump diagnostic identifies which dispenser is drifting when total wet-stock variance crosses the warning band.

When does wet-stock loss trigger EPA reporting?

EPA UST regulations require investigation of suspected releases within 7 days and confirmation within 30 days. Persistent wet-stock variance above 1% of throughput is a typical regulatory escalation trigger. State UST regulators (CA SWRCB, similar in other states) provide specific guidance.

Does wet-stock loss affect fuel margin calculations?

Yes. Realized fuel margin per gallon should subtract wet-stock loss as a per-gallon cost: realized = retail − wholesale − card fee − (wet_stock_loss × cost ÷ gallons). Margin calculations that skip wet-stock allocation overstate profitability, particularly for high-variance stores.

How often should I reconcile wet-stock?

Daily. Weekly reconciliation lets a 0.5% leak compound for 7 days before detection. Monthly reconciliation aggregates the loss without preserving the per-pump or per-day attribution needed to find the cause. Daily reconciliation, automated where possible via ATG, is the gold standard.