Methodology

How Fast Retirement Calculator works

The calculator creates deterministic, year-by-year projections from the values you enter. It separates saving years from retirement years, applies constant annual return and inflation assumptions, and compares projected savings with inflation-adjusted spending needs. It does not use live market data or predict actual investment performance.

Projection timing

Current age is the starting point today. The first projection row covers the next full year, so an input age of 35 produces a first row ending at age 36. That row includes twelve monthly contributions, one annual inflation adjustment, and one annual investment return.

The row ending at the planned retirement age is the final saving row. The first retirement withdrawal appears in the following full-year row. Contributions are added before investment growth, and retirement withdrawals are subtracted after that year's growth is applied.

Inflation, contributions, withdrawals, and investment returns are modeled in whole-year steps. The calculator does not estimate partial years, monthly market returns, or the visitor's exact birthday within the year.

Core formulas

Inflation-adjusted return

(1 + nominal retirement return) / (1 + inflation) - 1

This real return is used to estimate the savings needed in today's dollars to support retirement spending.

Required retirement savings

annual spending x [1 - (1 + real return)^(-retirement years)] / real return

This is the present value of a fixed series of annual withdrawals. If real return is zero, required savings equals annual spending multiplied by retirement years.

Future-dollar conversion

today-dollar amount x (1 + inflation)^years to retirement

The main target is shown in future dollars for the year retirement begins.

Year-end balance

max(0, (starting balance + contribution) x (1 + return) - withdrawal)

Contributions are zero during retirement. Withdrawals are zero during saving years. The balance is not allowed to fall below zero.

How the three calculator goals differ

How much do I need?

The calculator estimates the required savings at the planned retirement age, projects the entered savings and contributions to that age, and reports the estimated gap or surplus.

When can I retire?

Candidate retirement ages are tested one year at a time from the current age plus one through age 100. The first age where projected savings meet the required target is returned.

How long will it last?

Savings are projected to the planned retirement age and then drawn down through age 100. The first retirement row ending at zero is reported as the estimated depletion age.

Estimate how much I need

Worked example using the default inputs

The default scenario starts at age 35 with $50,000 in savings, a monthly contribution of $500, a 7% pre-retirement return, and 2.5% inflation.

The first row covers age 35 to 36.
Twelve monthly contributions are adjusted for one year of inflation, producing $6,150 in future-dollar contributions.
The annual return is applied after those contributions, producing $3,931 of investment growth.
The projected year-end balance is $60,081.

Target-savings result from the same inputs

A 5% retirement return and 2.5% inflation produce a real return of 2.44%. For a planned retirement at age 65 lasting 25 years, the calculated future-dollar target is $2,335,037. Projected savings at retirement are $1,187,044, leaving an estimated future-dollar gap of $1,147,993.

Model assumptions

Returns and inflation remain constant within a scenario.
The monthly contribution input is annualized, adjusted for inflation each year, and added during saving years only.
Annual retirement spending is adjusted for inflation and withdrawn once per retirement year.
Current and future dollar figures are connected using the inflation assumption entered by the visitor.
Results are deterministic: the same inputs always produce the same estimate.

Important limitations

The model does not simulate market volatility or sequence-of-returns risk. It also excludes taxes, fees, contribution limits, healthcare costs, Social Security and pension payments, required minimum distributions, account-specific rules, and other region-specific retirement requirements.

Results are educational scenario estimates, not guarantees or personalized financial, tax, legal, or investment advice. Consider testing more than one set of assumptions and seeking qualified advice for decisions that depend on your individual circumstances.

References used for context

These official sources provide general context for compound growth, inflation, Social Security estimates, and retirement-account rules. The formulas above come from the calculator implementation and do not pull live values from these references.

Reference links

Open the retirement calculator Read the retirement guides