You should start by putting in an amount that will give you a full employer match. Experts recommend that you save 10% to 15% of your income. The actual figure will depend on your age and income level.

Typically, an employer will contribute 50% of your contributions up to 6% of your income. However, this will depend on your company plan. You should consult your company's HR department for information on your company plan.

Most calculators give a pre-tax income figure. However, some may give an after-tax income figure based on the income tax rate you choose for your retirement income.

Your existing 401(k) plan will continue growing based on your investment options. However, if you stop working and do not have another source of income, your employer will not continue matching your contributions.

You are allowed to withdraw your 401(k) plan by the time you are 59½ years old without any penalties. However, if you wish to withdraw earlier than this, a penalty of 10% will be charged on your account.

The calculator is not exactly accurate but gives a good estimate based on the inputs provided and the expected return on investment. The calculator cannot predict the future but is a good starting point for planning your savings better.