Measuring database IO for AWS RDS costings

We have a test Postgresql AWS RDS instance running with a view to
transferring our Live physical Postgresql workload to AWS.

Apart from the cost of the instance, AWS also bill for IO requests per
month.

We are trying to work out how to estimate the IO costs our Live workload
would attract.
So if we can confirm metrics x+y measured from within our test Postgresql
instance on RDS maps to z billable IO requests, then we can measure the
same metrics from our Live Postgresql server and estimate costs.

I believe in the AWS world an IO request is each 16kb read or written to
disk.
How would I go about measuring 16kb blocks read or written to disk from
within Postgresql?

I was hopeful of pg_stat_database which has blks_read (which I believe are
8kb blocks), but there doesn't seem to be an equivalent for blks_written?

Can anyone give us any pointers?

Regards,
--
David