How the Early Retirement Calculator works
This calculator answers two related questions: how big a nest egg do you need to sustain your desired spending in retirement, and will your current savings rate get you there by your target age? It combines the standard 4% rule for a sustainable withdrawal rate with the standard future value of a series formula used to project compound growth of savings plus regular contributions.
The 4% rule (your target nest egg)
The 4% rule, associated with financial planner William Bengen's research and the Trinity study, is a widely cited starting point for how much you can withdraw from a portfolio each year without depleting it too quickly over a long retirement. Under this rule, your target nest egg is your desired annual spending divided by 4%, which is the same as multiplying annual spending by 25:
Target nest egg = Annual retirement spending ÷ 0.04
For example, a $40,000/year retirement budget implies a target of $1,000,000. This is a general planning guideline based on historical market data, not a guarantee — actual sustainable withdrawal rates depend on your investment mix, retirement length, and the sequence of market returns you experience.
Projecting your balance (future value of a series)
To project how your current savings and monthly contributions grow, the calculator uses the standard future value formula, compounded monthly:
FV = PV × (1 + r)n + PMT × [((1 + r)n − 1) / r]
where PV is your current savings, PMT is your monthly contribution, r is your expected annual return divided by 12 (the monthly rate), and n is the number of months between now and your desired retirement age.
Estimated financial independence (FI) age
The calculator also solves the same formula in reverse to estimate the age at which your projected balance first reaches your target nest egg, holding your current savings rate and return assumption constant:
n = ln[(Target + PMT / r) / (PV + PMT / r)] / ln(1 + r)
This gives the number of months until your balance is projected to cross the target; adding it to your current age produces the estimated FI age shown in the results, which can land earlier or later than the retirement age you entered.
Worked example
A 35-year-old with $50,000 saved, contributing $1,500/month, expecting a 7% annual return, and planning to spend $40,000/year in retirement has a target nest egg of $1,000,000. Projecting to age 55 (240 months) gives a balance of roughly $983,000 — a modest shortfall — with an estimated FI age of about 55.2, just a few months later than the original goal.
What moves the numbers most
- Monthly contribution: because contributions compound for the rest of your working years, raising them early has an outsized effect on both the projected balance and the estimated FI age.
- Return assumption: a higher expected return shrinks the time needed to reach any given target, but real portfolios do not grow at a single constant rate every year the way this projection assumes.
- Desired spending: because the target nest egg scales directly with annual spending, trimming your planned retirement budget lowers the target by the same percentage as raising it would.
Assumptions and limits
This is a deterministic projection: it assumes a constant annual return, monthly compounding, and level monthly contributions, with no adjustment for inflation, taxes, fees, Social Security, or pensions. It does not model market volatility, early-withdrawal penalties on tax-advantaged accounts before age 59½, healthcare costs before Medicare eligibility, or sequence-of-returns risk in the years around retirement. Treat the result as a directional planning estimate, not a guarantee, and revisit it as your savings rate, expected return, or spending plan changes.