How should I calculate the running balance for banks that don't support it?

Calculating the running balance will only be truly possible for transactions that have a timestamp (most banks just return the date). If you are happy to have a running balance that’s correct to the day, you can act under the assumption that transactions are returned in time order and do the following:
 

1. Get the current balance from the /balance endpoint
2. Get transactions from the /transactions endpoint
3. Subtract the most recent transaction amount from the current balance, and assign this as the running balance for this transaction
4. Take the next transaction, subtract the amount from the previous transaction’s running balance and assign this as the running balance for this transaction
5. Repeat step 4 for the rest of the transaction dataset

 
Note that we don’t recommend considering pending transactions. That’s because these may not settle, or may settle with a different final amount. You would need to use the available balance if you want to factor in pending transactions.