The One-Line Answer
API 650 builds the tank. API 653 keeps it running.
That distinction covers most of what people need, but it is worth understanding properly, because the two standards get used interchangeably in conversation and they are not interchangeable in practice. Quoting the wrong one in a scope of work leads to the wrong inspection being carried out, or a repair being signed off against criteria that do not apply.
What API 650 Covers
API 650 is a construction code. It governs the design, materials, fabrication, erection and testing of new welded above-ground storage tanks, and it applies from the drawing board through to the point the finished tank is handed over.
It sets out how thick each shell course has to be for the tank's diameter, height and stored product. It specifies acceptable materials, weld procedures and joint details. It defines the testing a new tank must pass before it goes into service, including hydrostatic testing of the completed structure.
Once that tank is commissioned, API 650 has largely done its job. It does not tell an owner how often to inspect the tank, how to evaluate corrosion, or when the tank is no longer fit for service.
What API 653 Covers
API 653 picks up where the construction code stops. It governs inspection, repair, alteration and reconstruction of tanks that are already in service, and it is the standard an owner works to for the whole operating life of the asset.
It defines the inspection types and their intervals, sets out how measured thickness is evaluated against the minimum the tank needs to retain, and provides the method for calculating corrosion rate and remaining life. It also governs what an acceptable repair looks like.
If you are being asked how a tank's condition is being managed, whether by an insurer, an auditor or a regulator, API 653 is almost always the standard being referred to. Our guide to what API 653 requires goes through the inspection types and intervals in detail.
The Handover Point
The line between the two is commissioning. Before it, the tank is a construction project governed by API 650. After it, the tank is an operating asset governed by API 653.
In practice the handover is not always clean. A tank that has sat unused for years, or one that was built without complete documentation, can arrive at its first API 653 inspection with no reliable record of what it was built to. That matters, because the in-service evaluation refers back to the construction code for acceptance criteria. Establishing the construction basis is often the first task on an older tank, and it is not an academic exercise.
Why API 12C Still Matters
API 650 has been the construction standard for welded steel storage tanks for decades, but it was preceded by API 12C, and a significant number of tanks still in service in Australia were built to that earlier code.
API 653 accounts for this. It recognises tanks built to API 12C and provides for their assessment, which is why a competent inspection report identifies the original construction code rather than assuming API 650. A tank built to API 12C is not deficient because of it. It simply has to be evaluated against the code it was actually built to.
Repairs Return the Tank to Its Construction Code
This is the point where the two standards intersect most directly, and it is the reason the construction code stays relevant for the tank's entire life.
When a repair is carried out under API 653, it must return the tank to the requirements of the code it was originally built to. Replacing a section of shell plate means matching the material, thickness and weld requirements that code specified. Laying new floor plate means meeting those same criteria.
So an inspection carried out under API 653 will routinely reference API 650 or API 12C when setting out what a repair must achieve. If a report proposes repairs without identifying the construction code, that is worth questioning. Repairs of any significance may also require a hydrostatic test before the tank returns to service.
Where the Australian Standards Fit
Both API codes are American, and both are used widely in Australia because they are the recognised international reference for above-ground storage tanks. They do not operate alone here.
AS 1692 covers the construction of steel tanks for flammable and combustible liquids. AS 1940 covers the storage and handling of those liquids on site, including bunding, separation distances and operational controls.
The practical consequence is that a tank can be entirely sound under API 653 and still be non-compliant on a site matter governed by AS 1940, or the reverse. The two frameworks answer different questions. A complete assessment reads them together, and any report worth having will state which codes it has been written against.
Which One Applies to Your Tank
If the tank is being designed or built, the answer is API 650, alongside AS 1692 for Australian construction requirements.
If the tank is in service, in almost every case the answer is API 653, with API 650 or API 12C referenced for repair acceptance criteria and AS 1940 governing how the tank is operated on site.
If you are not certain which code your tank was built to, that is a normal starting point rather than a problem, and establishing it is part of a first inspection. You can read more about our storage tank inspection services, or request a quote to talk through a tank you are responsible for.
