🏖️ Retirement & 401(k) Calculator
A retirement calculator that projects your nest egg with the 4% rule — plus what a single-average projection hides: sequence-of-returns risk.
What Retirement & 401(k) Calculator Does
Retirement projections work by compounding contributions at an assumed rate until a target age, then drawing them down. The arithmetic is simple. The problem is that the output looks like a prediction when it is a single scenario drawn from an enormous range of possible ones.
The most consequential thing a projection hides is that the *order* of returns matters, not just the average. Two retirees who earn exactly the same set of annual returns — same arithmetic mean — can end up with balances that differ by a factor of two, purely because one hit the bad years first while drawing an income. That is sequence-of-returns risk, and no single-number calculator can show it.
This calculator gives you the deterministic projection because it is a useful starting point. The sections below explain where the standard assumptions come from, how far they can be wrong, and how to sanity-check a number before you rely on it.
How to Use Retirement & 401(k) Calculator
- Enter your current age and planned retirement age
- Input your current retirement account balance (401k, IRA, savings)
- Enter your planned monthly contribution amount
- Set an estimated annual return rate (typically 7%–10% for stock index funds)
- Review your total projected nest egg and safe monthly withdrawal income
Formula Used by Retirement & 401(k) Calculator
Future value of savings with regular contributions
FV = P(1 + r)^n + PMT × [ ((1 + r)^n − 1) ÷ r ]
- P
- Current retirement savings
- PMT
- Contribution per period, including any employer match
- r
- Assumed return per period
- n
- Number of periods until retirement
Worked example
$50,000 saved, $1,000 a month added, 7% annual return, 25 years to go.
- Monthly rate: 0.07 ÷ 12 = 0.00583333, periods: 300
- Growth factor: (1.00583333)^300 = 5.7254
- Existing savings: 50,000 × 5.7254 = 286,271
- Contributions: 1,000 × ((5.7254 − 1) ÷ 0.00583333) = 810,072
Result: About $1,096,000 — of which $300,000 is money you paid in. Before inflation, which is the next section.
The 4% rule, inverted
nest_egg = annual_spending ÷ withdrawal_rate
- withdrawal_rate
- 0.04 for the classic 4% rule; lower rates are more conservative
Worked example
You expect to need $80,000 a year from the portfolio, after Social Security.
- 80,000 ÷ 0.04 = 2,000,000
Result: $2.0 million, which is 25× annual spending. At a more conservative 3.5% the target rises to $2.29 million (28.6×).
Converting a future balance to today's money
real_value = nominal ÷ (1 + inflation)^years
- inflation
- Long-run US CPI has averaged roughly 2.5–3% a year
Worked example
The $1,096,343 projected above, 25 years out, at 3% inflation.
- (1.03)^25 = 2.0938
- 1,096,343 ÷ 2.0938 = 523,619
Result: About $524,000 in today's purchasing power — less than half the headline figure. This is the single biggest reason projections feel wrong later.
Sequence-of-Returns Risk — Identical Returns, Different Order
A $1,000,000 portfolio drawing $40,000 a year for 15 years. Both columns use exactly the same fifteen annual returns and therefore exactly the same 8.80% arithmetic average. Only the order differs.
| Scenario | Average return | Balance after 15 years |
|---|---|---|
| Poor returns arrive first | 8.80% | $1,228,248 |
| Strong returns arrive first | 8.80% | $2,396,763 |
| Difference | none | 1.95× — from ordering alone |
Withdrawal Rate and the Multiple You Need
How much you must accumulate, expressed as a multiple of the annual spending the portfolio has to cover. Small changes in the assumed safe rate move the target a long way.
| Withdrawal rate | Multiple of spending | For $60k/yr | For $100k/yr |
|---|---|---|---|
| 3.0% | 33.3× | $2,000,000 | $3,333,000 |
| 3.5% | 28.6× | $1,714,000 | $2,857,000 |
| 4.0% | 25.0× | $1,500,000 | $2,500,000 |
| 4.5% | 22.2× | $1,333,000 | $2,222,000 |
| 5.0% | 20.0× | $1,200,000 | $2,000,000 |
US Social Security Full Retirement Age
The age at which unreduced benefits begin. Congress raised it in 1983; it now tops out at 67. Claiming earlier permanently reduces the benefit, and delaying past FRA increases it until age 70.
| Birth year | Full retirement age |
|---|---|
| 1943 – 1954 | 66 |
| 1955 | 66 and 2 months |
| 1956 | 66 and 4 months |
| 1957 | 66 and 6 months |
| 1958 | 66 and 8 months |
| 1959 | 66 and 10 months |
| 1960 and later | 67 |
Source: Social Security Administration — Retirement Age Calculator
The Common Rules of Thumb, and What They Assume
Each is a useful starting point and none is a plan.
| Rule | Says | Hidden assumption |
|---|---|---|
| 10–15% rule | Save 10–15% of pre-tax income throughout your career | That you start in your twenties and never stop |
| 80% rule | You need 70–80% of pre-retirement income | That your spending pattern stays similar |
| 4% rule | Withdraw 4% of the initial balance, adjusted for inflation | A 30-year horizon, a US stock/bond portfolio, and history repeating |
| 25× rule | Accumulate 25× annual spending | Identical to the 4% rule, stated the other way round |
How to Read Your Result
Where the 4% rule actually comes from
It originates in research on historical US market data, testing what initial withdrawal rate would have survived every rolling 30-year period, adjusted annually for inflation, from a stock-and-bond portfolio. Its conclusions are conditional on all of those things: a 30-year retirement, US returns, and a specific asset mix. A 40-year retirement, an internationally diversified portfolio, or high fees each change the answer. It is a well-tested rule of thumb, not a law.
Why a single number is over-confident
A deterministic calculator applies the same return every year. Real markets do not, and a projection built on the average silently assumes the ordering does not matter. As the table above shows, it matters by roughly 2×. This is why institutional planning tools run thousands of simulated paths and report a probability of success rather than a balance. Read any single figure — including the one this tool gives you — as the midpoint of a wide range, not a forecast.
The first decade of retirement carries most of the risk
Sequence risk is concentrated at the start of drawdown, because that is when the balance is largest and a withdrawal taken during a fall permanently removes shares that would otherwise have recovered. The common mitigations are holding one to three years of spending in cash or short bonds, and being willing to reduce withdrawals in a bad year rather than selling into it.
Employer match first, always
A 50% or 100% employer match on 401(k) contributions is an immediate, guaranteed return that no market assumption competes with. Whatever the projection says, contributing at least enough to capture the full match is the one part of retirement planning that does not depend on any forecast being right.
Limitations & Accuracy Notes
- This is a deterministic projection using a single constant return. It cannot represent volatility, sequence-of-returns risk, or the probability that a plan succeeds. Treat the output as one scenario among many, not an expected outcome.
- Taxes are not modeled. Traditional 401(k) and IRA withdrawals are taxed as ordinary income; Roth withdrawals generally are not; taxable accounts incur capital gains along the way. The same nominal balance funds very different lifestyles depending on which bucket it sits in.
- Fees are excluded. A 1% annual fee compounds against you exactly as returns compound for you, and over a full career it commonly removes a fifth or more of the final balance.
- Social Security, pensions and annuity income are handled as flat figures if entered at all, and no benefit estimate here should substitute for the Social Security Administration's own statement of your record.
- Life expectancy is an input, not a prediction. Planning to a life expectancy leaves roughly half of outcomes underfunded; most planners work to a longer horizon than the average deliberately.
- Healthcare and long-term care costs are the largest uncertainty in most retirements and are not modeled at all.
- This is arithmetic and general information, not financial, tax or investment advice. It does not account for your circumstances. For decisions of this size, speak to a licensed professional who is obliged to act in your interest.
Frequently Asked Questions
What is the 4% rule in retirement planning?
How much money do I need to retire?
How does compound interest help retirement savings?
What is the 4% rule and can I rely on it?
Should I use nominal or real returns?
Why does starting a few years earlier make such a large difference?
Does this account for tax?
What is sequence of returns risk?
Is this financial advice?
References & Further Reading
- Social Security Administration — Retirement Age Calculator — Official full retirement age by birth year, and why it changed
- US SEC (Investor.gov) — Compound Interest Calculator — Regulator reference implementation of the accumulation formula
- IRS — 401(k) contribution limits — Current annual deferral and catch-up limits
- US Bureau of Labor Statistics — Consumer Price Index — The official inflation series behind any real-terms adjustment
- Federal Reserve — Survey of Consumer Finances — Population data on actual US retirement account balances by age