Buy 8,000 gallons, sell 8,000 gallons, and the tank should agree. It rarely does exactly, and that gap has a name: fuel shrinkage. Some of it is harmless physics, and some of it is a warning. Knowing which is which is one of the most useful skills in running a fuel site.
What it is
Fuel shrinkage is the difference between the fuel you bought and the fuel you actually sold or still have in the tank. Part of it is normal and physical; part of it can signal a real problem. The work is separating expected wiggle from genuine loss.
What causes it
- Temperature. Fuel expands and contracts, so warm and cool gallons differ.
- Meter inaccuracy. A dispenser slightly off true adds up over volume.
- Evaporation. Small but real, especially with gasoline.
- Delivery shorts. Receiving less than the invoice says.
- Leaks and theft. The causes you most want to catch.
Normal causes are small and steady; the worrying ones grow or appear suddenly.
How much is normal
A small variance is expected, mostly from temperature and measurement. The regulatory line gives you a ceiling: EPA's monthly inventory control standard flags a tank when the variance exceeds 1 percent of throughput plus 130 gallons. Pump 50,000 gallons in a month and that trigger sits at 630 gallons. Most well-run sites stay far inside that line, so watch your own trend against it. Steady and small is normal. Rising or sudden is a flag, even below the threshold. The same logic drives store shrink control on the inside.
How you measure it
Through wet stock reconciliation: comparing the fuel you bought and sold against what the tank actually holds, typically read by an automatic tank gauge. When purchases minus sales does not match the measured level, that gap is your variance, and watching it daily makes a real loss visible while it is still small. FastDragon runs that reconciliation automatically from your purchases, sales, and tank readings, so a normal variance stays quiet and an odd one stands out.
When to worry
When the variance grows, jumps, or trends one way over time. A creeping loss can mean a leak, which is an environmental and compliance risk, or theft. Track the variance daily and that pattern shows up in weeks instead of quarters.
Frequently asked
Does diesel shrink as much as gasoline?
No. Gasoline is more volatile, so it loses more to evaporation, and its volume swings more with temperature than diesel does. The same site will usually show a noticeably larger variance on its gasoline tanks than on its diesel tank, which is worth remembering before you compare the two.
Why do summer fuel deliveries seem short?
Fuel loaded warm at the terminal contracts as it cools in the underground tank, which holds product near the ground's steady temperature of roughly 55 degrees Fahrenheit. The gallons on the bill of lading were real at loading temperature; a chunk of them shrank on the way down. Temperature-compensated (net) billing exists for exactly this reason.
Can a faulty dispenser meter cause shrinkage?
Yes, and it is a common quiet cause. Meters drift with wear, and one dispensing slightly more than it registers gives fuel away on every fill. State weights-and-measures inspections catch gross errors, but operators who calibrate between inspections often find drift that was costing them gallons for months.
What is statistical inventory reconciliation (SIR)?
SIR is a leak detection method where a trained analyst or software applies statistical tests to your daily inventory, delivery, and sales records. EPA accepts it as a monthly release detection option for underground tanks, so the same daily data you collect for shrinkage tracking can also satisfy compliance.
What should you do when fuel shrinkage spikes?
Isolate it to one tank and product, recheck recent deliveries against their bills of lading, and have the meters tested before assuming the worst. If the loss persists, order a tank tightness test and report a suspected release to your state UST authority. Federal rules generally give you 24 hours once a release is suspected.