Wet Stock Variance: What Is Normal and When to Worry
Every tank is a little off. The question is how much is normal, what is a warning sign, and what the regulator expects you to detect. Here is how to read your variance.
Wet stock variance is the difference between the fuel your records say should be in a tank and the fuel that is actually there. A small variance is normal, because fuel changes volume with temperature and no measurement is perfect. A variance that is large, or that is negative month after month, is a warning that fuel is going somewhere you cannot see. This guide explains how to calculate it, what level is normal, and when to act.
Key takeaways
- Variance = closing inventory minus book inventory (opening plus deliveries minus sales). Negative means fuel is missing.
- Many operators treat under 0.5% of gallons sold as acceptable, but that is a rule of thumb, not a regulation.
- The EPA inventory-control detection standard is 1.0% of flow-through plus 130 gallons on a monthly basis. Your state may be stricter.
- A steady small shortage across months can matter more than one bad month.
How wet stock variance is calculated
For each tank, over a period:
- Book inventory = opening inventory + deliveries - gallons sold
- Variance = closing inventory - book inventory
- Variance percent = variance divided by gallons sold
A negative variance means you have less fuel than the records say. A positive variance means you have more. Both need explaining, though negative is the one that costs money.
For example, a tank that opens with 8,400 gallons, receives 8,000 and sells 9,120 should hold 7,280 gallons. If the closing reading is 7,260, the variance is minus 20 gallons, or about minus 0.22% of sales. That is a normal result.
What is normal?
There is no single legal "acceptable loss" number, and it helps to separate two ideas:
- A practical benchmark. Many operators treat variance under about 0.5% of gallons sold as acceptable, and variance above 1% as needing immediate investigation. This is a rule of thumb, not a regulation.
- The regulatory detection standard. Under the U.S. EPA underground storage tank rules, release detection by inventory control must be able to detect a release of 1.0% of flow-through plus 130 gallons on a monthly basis. It is a standard for how sensitive the method must be, not a loss you are allowed to ignore.
The same rule requires inventory records of fuel in, fuel out and fuel remaining, taken on each operating day, using measurement equipment that reads the product level to the nearest one-eighth of an inch. Your state may have its own, stricter requirements, so check with your state tank program.
| Result | What it usually means | What to do |
|---|---|---|
| Under 0.5% and within the limit | Normal measurement and temperature effects | Keep recording; watch the trend |
| 0.5% to 1%, within the limit | Worth watching | Re-check readings, deliveries and meters |
| Beyond 1% of sales plus 130 gallons | Possible leak, theft or a serious error | Investigate now; follow your state rules |
Why one number can mislead
A single month can hide a problem. A tank that is 0.4% short every month is losing fuel steadily, even though no month crosses a line. Look at the cumulative variance over several months and by grade. A pattern is more important than any single figure.
Common causes of a negative variance
- Reading errors. Stick or gauge readings taken at different times or by different people.
- Delivery errors. A delivery ticket that does not match what reached the tank.
- Meter calibration. Dispensers that deliver slightly more than they record put fuel out without a sale.
- Temperature. Fuel expands and contracts, especially on large deliveries.
- Unrecorded fuel. Drive-offs, pump-ups and employee use that never reach the POS.
- Water in the tank, which affects readings.
- A leak, in a tank, line or fitting, which needs urgent professional attention.
What to do when a tank is off
- Re-check the numbers. Re-take the reading and compare each delivery ticket with the tank increase.
- Check the meters. Look at the last calibration date.
- Look for a pattern by grade, day of week and delivery.
- Escalate. If the checks do not explain a large or repeating variance, stop selling from that tank if advised, and call a tank technician. Follow your state's reporting rules.
- Record what you did. Notes on your report help an inspector, a buyer or a lender.
Our free fuel reconciliation report generator works out book, variance and status for each tank, flags anything beyond your limit, and prints a signed report. For the full method, read our wet stock reconciliation guide, or try the wet stock variance calculator.
Calculate variance for every tank and print a signed report.
Frequently asked questions
What is an acceptable wet stock variance?
There is no single legal acceptable loss. A common benchmark is under 0.5% of gallons sold, and the EPA inventory-control standard is the ability to detect 1.0% of flow-through plus 130 gallons monthly. Check your state rules.
What does a negative variance mean?
You have less fuel than your records say you should. Causes range from reading and delivery errors to meter calibration, theft or a leak.
How often should I reconcile fuel?
The EPA inventory-control method expects records for each operating day. Many stations reconcile daily and review the monthly totals.
Does temperature affect variance?
Yes. Fuel volume changes with temperature, which is why deliveries and readings should be compared carefully and at consistent times.
When should I call a technician?
If re-checking readings, deliveries and meters does not explain a large or repeating negative variance, call a qualified tank technician and follow your state reporting requirements.
Sources
Last reviewed October 2026. This article is general information, not tax, legal, accounting or lending advice. Figures, rates and rules change, so confirm current details with the agency, lender or professional concerned.
