Always a good idea to use the search engine for questions like this, as there have been specific threads on exactly this topic.
From memory - (and I've said this before) there isn't one. ISO only has one for "Flowchart symbols and their use in micrographics". I know Standards Australia has, or certainly had, one for flow charts, so other standards bodies may have similar guides.
These are only guides. Use whatever is easiest, e.g. WORD has a standard set of flowchart symbols to use (and do) or if you have a flowcharting computer program then that'll have a std set.
If you're looking to buy a flowchart program, check out past threads, or post in the quality software section.
There are standard flowchart symbols (international standards) and extended flowchart symbols that is used for specific needs. Below are the most commonly used ones.
Data object – The Data object, often referred to as the I/O Shape shows the Inputs to and Outputs from a process.
Rectangle – This is used to represent an event which is controlled within the process. Typically this will be a step or action which is taken.
Diamond – Used to represent a decision point in the process. Typically, the statement in the symbol will require a `yes’ or `no’ response and branch to different parts of the flowchart accordingly.
Document – The Document object is a rectangle with a wave-like base. This shape is used to represent a Document or Report in a process flow.
Rounded box – This is used to represent an event which occurs automatically. Such an event will trigger a subsequent action, for example, receive a telephone call, or describe a new state of affairs.
Stored data – This is a general data storage object used in the process flow as opposed to data which could be also stored on a hard drive, magnetic tape, memory card, of any other storage device.
Manual input – This object is represented by a rectangle with the top sloping up from left to right. The Manual Input object signifies an action where the user is prompted for information that must be manually inputted into a system.
Direct data – Direct data object in a process flow represents information stored which can be accessed directly. This object represents a computer’s hard drive.
Circle – Used to represent a point at which the flowchart connects with another process. The name or reference for the other process should appear within the symbol.
Internal storage – This is an object which is commonly found in programming flowcharts to illustrate the information stored in memory, as opposed to on a file.
Predefined process – This allows you to write one subroutine and call it as often as you like from anywhere in the code.