Hello finance experts,

I'm a website programmer without any knowledge of finance. I'm creating a personal finance website which allows its users to enter their incomes and expenses and generate reports. I'm about to add a feature to my website which allows users to enter their incomes and expenses in any desired currency. So if a user lives in USA and his currency is USD and he goes to a trip to London and buys something and wants to enter that in his account, he may use GBP for that specific expense.

Now the main question. When generating reports with multiple currencies as inputs, what reference currency can I use to convert the amounts. Suppose we call the reference currency GRC (Global Reference Currency) so I can store in my database as an input that at the time this expense was added as 4 GBP, 1 GBP was equal to 1.456 GRC. Is there any special reference that is being used or may I mark every currency as reference myself?

I appreciate your help.
Best wishes,
Shakib

I'm a website programmer without any knowledge of finance. I'm creating a personal finance website which allows its users to enter their incomes and expenses and generate reports. I'm about to add a feature to my website which allows users to enter their incomes and expenses in any desired currency. So if a user lives in USA and his currency is USD and he goes to a trip to London and buys something and wants to enter that in his account, he may use GBP for that specific expense.

Now the main question. When generating reports with multiple currencies as inputs, what reference currency can I use to convert the amounts. Suppose we call the reference currency GRC (Global Reference Currency) so I can store in my database as an input that at the time this expense was added as 4 GBP, 1 GBP was equal to 1.456 GRC. Is there any special reference that is being used or may I mark every currency as reference myself?