Below is a link to the Armed Forces Pension Calculator which is regularly updated by the Ministry of Defence. The Ministry points out that:
"The Armed Forces Pension and Annual Allowance Calculator (AFPAAC) which includes a projection of benefits from the new Armed Forces Pension Scheme 2015 (AFPS 15). This calculator is designed to be used by Regular Service, MPGS and Reservist personnel including those with Transitional Protection. The video link below provides greater detail on how to get the best out of the calculator and Service personnel are encouraged to view the video and read the Terms and Conditions before proceeding further."
This is the MOD video referred to: