Pension is calculated on the basis of an estimate of expenses you will incur on a regular basis once you retire from paid work.
An online retirement calculator takes into consideration age, expected retirement age, current income and savings details. This enables it to get an estimate of the savings you will tote up once you opt for retirement. It will also be able to tell you whether it will be enough to cover your expenses in your golden years.
Based on these details, the online pension plan calculator will make an assessment about your estimated monthly expenses after your retirement.