Tank Strapping Chart Generator

A strapping chart, also called a tank chart or gauge chart, converts a liquid level measurement into a volume. The operator drops a gauge stick or reads an ATG probe, finds that height in the chart, and reads off the gallons. This generator produces that chart for horizontal cylinders, vertical cylinders, and oval tanks, at increments from a full inch down to one eighth of an inch, with columns for gallons, percent full, and the weight of the selected fuel at each level.

The audience for this tool is professional. Fuel site operators need charts for inventory control. Compliance managers need them behind statistical inventory reconciliation submissions. Fuel haulers can use them to confirm room in a receiving tank before pumping. Inspectors can use them to spot the mismatch between a site's chart and its actual tank, which is a surprisingly common finding.

Each chart can carry a tank ID label, so a four tank site can generate four labeled charts and post them at the console. Presets cover the standard commercial UST sizes from 6,000 to 20,000 gallons, or you can enter exact dimensions for anything nonstandard.

How the math works

For a horizontal cylinder, the generator computes every chart row with the exact circular segment integral, not a lookup approximation. At liquid height h in a tank of radius r, the filled cross section equals r squared times the inverse cosine of (r minus h) over r, minus (r minus h) times the square root of (2rh minus h squared). Multiply by the tank's straight shell length, divide by 231 cubic inches per gallon, and that is the row's gallon figure. The same formula runs at every increment you select, so a 96 inch diameter tank charted at eighth inch steps gets 768 exact computations.

This is the formula that matters for horizontal USTs because volume per inch changes continuously with height. Near the bottom and top of the tank, an inch of level is relatively few gallons. At the midline of a 10,000 gallon tank, a single inch represents more than 100 gallons. A chart built on a linear assumption misstates inventory at almost every level, overstating low readings and understating midrange ones.

Vertical cylinders are linear by geometry, so each row is simply the cross sectional area times the height. Oval tanks use a scaled version of the circular segment math fitted to the elliptical cross section, which is a close approximation appropriate for the residential and small commercial sizes where that shape appears.

Weight columns multiply each row's gallons by your selected fuel's density, drawn from ASTM specification values, 7.10 pounds per gallon for diesel #2 for example. The density background lives in the companion fuel weight reference and calculator if you need conversions beyond the chart.

When to use this

Standing up inventory control at a fuel site. An operator taking over a site with no documentation can measure the tanks, generate labeled charts, and have a defensible inventory basis the same day. Stick readings become gallons, and gallons reconcile against meter totalizers and delivery tickets.

Replacing a chart that does not match the tank. Charts migrate between sites, get photocopied from a different manufacturer's manual, and outlive the tank they described. If month-end reconciliation shows a persistent bias that is largest near half full and shrinks toward empty and full, the chart geometry is a prime suspect, and regenerating from measured dimensions is the cheapest diagnostic available.

Receiving fuel. Before a 7,500 gallon drop, the driver sticks the tank and needs to know the remaining ullage. A chart at quarter inch increments answers that without arithmetic at the fill point.

Verification during an inspection or audit. An inspector or consultant can independently chart a tank from its registered dimensions and compare it against the site's posted chart. Disagreement between the two is a finding worth running down before it becomes a reconciliation problem. Operators who would rather hand that off can request quotes from tank testing contractors in New Jersey and every other state through the directory at no charge.

Strapping charts and compliance

Two references govern this territory, one regulatory and one technical.

The regulatory anchor is 40 CFR 280.43, the federal release detection rule for USTs. Its inventory control provision requires that product level be measured with equipment capable of reading to the nearest one eighth of an inch over the full height of the tank. That requirement is the reason this generator offers an eighth inch increment: a chart can only be as fine as the measurement standard it supports. The same section defines statistical inventory reconciliation, commonly run as a monthly analysis of daily inventory, delivery, and dispensing records. SIR vendors evaluate those records against quantitative performance standards, and the volume conversions feeding the analysis come from the tank chart. A chart that misrepresents tank geometry degrades the analysis before any statistics run.

The technical reference is API's Manual of Petroleum Measurement Standards, Chapter 2, which covers tank calibration, the formal version of what the trade calls strapping. The MPMS procedures derive a tank's true capacity table from physical measurement, accounting for shell deformation, tilt, deadwood from internal hardware, and end head geometry. That is the standard custody-transfer terminals work to, and it is the ceiling this generator does not claim to reach.

The honest positioning: a generated chart from accurate dimensions is a major upgrade over a borrowed or mismatched chart, and it is well suited to operational inventory control. For custody transfer, tax measurement, or a dispute where gallons equal dollars, an MPMS-grade calibration of the physical tank is the defensible instrument. Knowing which tier your application requires is itself a compliance skill.

What this estimate doesn't account for

The generator assumes flat end caps. Real commercial USTs usually have dished or hemispheric heads, which hold additional volume distributed nonlinearly across the height range. The tool lets you flag the end type, but version 1 applies the flat-end formula in every case and says so in the output. For a 10,000 gallon tank, head volume is a real number; if your reconciliation tolerance is tight, correct against the manufacturer's capacity table.

It assumes a level tank. A tank pitched along its axis, which is common and sometimes intentional for water management, reads differently depending on gauge point location. MPMS procedures correct for tilt; this chart does not.

It assumes nominal geometry. Buried tanks deform slightly under load, shells are rolled to tolerances rather than perfection, and internal hardware displaces product. Each effect is small, but they accumulate, and they are exactly what physical calibration captures that arithmetic cannot.

Manway and fitting locations, thermal expansion of stored product, and the difference between gross and net standard volume are likewise outside scope. For a quick capacity check rather than a row-by-row chart, the dimension-based capacity tool covers the same shapes with less ceremony.