Tank and Vessel Calibration to API and ISO Standards

A tank capacity table turns a measured level into a quantity, and itmad provides tank and vessel calibration: capacity table production to API MPMS Chapter 2 and ISO 7507 using manual strapping, EODR and 3D laser scanning, for shore tanks, ship tanks, LPG vessels and process equipment. The table is what gets bought, sold, taxed and reconciled, so an error of a fraction of a percent matters on every transfer. itmad operates across the Middle East, Asia and Europe. Last reviewed: October 2026.

Why the capacity table matters

The quantity read from a capacity table is the quantity that is traded, taxed and reconciled. On a large storage tank, an error of a fraction of a percent is a commercially significant volume of product on every transfer. That is why tables are produced to defined standards and recalibrated at defined intervals, and are not treated as permanent.

Methods

Manual strapping measures the circumference directly with a calibrated steel tape at each shell course, to API MPMS 2.2A. It is the reference method, still specified by many operators and terminal agreements, and the one against which other methods are validated, but it needs access to the external shell.

EODR (electro-optical distance ranging) measures from inside, from a single instrument position, using laser distance ranging to API MPMS 2.2D. It gives the full shell geometry without external access or scaffolding, which suits tanks that cannot be strapped safely or where out-of-service time is short. 3D laser scanning captures a dense point cloud of the whole internal geometry, producing a capacity table and a permanent digital record of the tank, including deformation, out-of-roundness and settlement, against which later surveys can be compared. Liquid calibration works by measured transfer of liquid through a certified meter, and is used for smaller and irregular vessels where geometric methods are impractical and the highest accuracy is needed.

Tanks and vessels covered

Vertical cylindrical shore tanks, fixed and floating roof; horizontal cylindrical tanks; ship and barge cargo tanks; pressurised vessels, LPG and LNG; bullets, spheres and process vessels; road and rail tankers; and wet and dry bulk cargo compartments.

What a capacity table has to account for

A calibration is more than a measurement of the shell. The table must allow for everything that displaces or affects volume at a given level: shell course dimensions, plate thickness and out-of-roundness; tank tilt and floor deformation and their effect on volume at low levels; the mass and displacement of a floating roof and the critical zone where it lands; internal structures such as heating coils, columns and mixers, deducted as deadwood; the bottom profile and dead stock below the outlet; the reference height and the condition of the datum plate, where reconciliation discrepancies usually begin; and temperature correction for the shell material.

Standards applied

itmad works to API MPMS Chapter 2 on tank calibration (2.2A manual strapping, 2.2B optical reference line, 2.2C optical triangulation and 2.2D EODR), the ISO 7507 series for vertical cylindrical tanks for petroleum and liquid petroleum products, and ISO 8311 for refrigerated and non-refrigerated tanks where it applies. API MPMS Chapter 3 applies where gauging is verified alongside calibration, and client, terminal and custody transfer agreement requirements apply where they are more stringent.

Recalibration

Capacity tables are not permanent. Tanks settle, floors deform, shells distort under repeated loading and repairs change the geometry. API and ISO practice sets recalibration intervals, commonly five to fifteen years depending on tank type, service and construction. Recalibration is also needed after any repair or modification that affects the shell or bottom, after significant settlement, and where reconciliation discrepancies suggest that the table no longer reflects the tank.

When calibration is commissioned

Operators commission it when a new tank is commissioned, before the first custody transfer; at the recalibration interval set by the standard or the terminal's procedures; after repair, plate replacement or modification of the shell or bottom; following measurable settlement or tilt; where stock reconciliation shows persistent unexplained variance; on a change of ownership or operator of a terminal; and before a tank enters custody transfer service.

Frequently asked questions

What is a tank capacity table?

A tank capacity table converts a measured level into a quantity, and that quantity is what is bought, sold, taxed and reconciled. A fraction of a percent of error on a large tank is a commercially significant volume on every transfer.

Which calibration methods does itmad use?

Manual strapping to API MPMS 2.2A, electro-optical distance ranging (EODR) to API MPMS 2.2D, 3D laser scanning and liquid calibration. The choice depends on access to the tank, the out-of-service time available and the accuracy required.

Which standards does itmad apply?

API MPMS Chapter 2, the ISO 7507 series for vertical cylindrical tanks, and ISO 8311 for refrigerated and non-refrigerated tanks where it applies. API MPMS Chapter 3 applies where tank gauging is verified alongside calibration.

How often must a tank be recalibrated?

API and ISO practice sets intervals commonly of five to fifteen years, depending on tank type, service and construction. Recalibration is also needed after repair or modification affecting the shell or bottom, after significant settlement, and where reconciliation discrepancies suggest the table is out of date.

What does a capacity table account for?

Shell dimensions and out-of-roundness, tilt and floor deformation, floating roof mass and displacement, internal structures deducted as deadwood, bottom profile and dead stock, reference height and datum plate condition, and temperature correction for the shell material. Each of these affects the volume held at a given level.

When is tank calibration commissioned?

When a new tank is commissioned, at the recalibration interval, and after repair or modification of the shell or bottom. Also after measurable settlement or tilt, where stock reconciliation shows persistent unexplained variance, and on a change of ownership or operator.