Retirement Savings Model

About you

Current age

Retirement age

Plan to age

Savings & income

Current savings $

Annual savings (today's $) $

Retirement spending

Annual spending (today's $) $

Social Security / pension (today's $/yr) $

Benefits start at age

Effective tax rate on withdrawals %

Grosses up portfolio withdrawals to cover income tax. Enter 0 for Roth accounts or if your spending inputs are already pre-tax.

Portfolio mix & returns

Stocks / Bonds % / 30%

Stock return (nominal) %

Bond return (nominal) %

Inflation %

Annual volatility (σ) %

Volatility is the standard deviation of annual portfolio returns. Typical values: 100% stocks ≈ 17%, 70/30 ≈ 12%, 100% bonds ≈ 6%.

Blended portfolio return

–

Real (after inflation): –

Portfolio at retirement

–

In today's $: –

Nest egg needed

–

Today's $ to fund through age 95

Surplus / shortfall

–

in today's $

Probability of success

–

Monte Carlo, 1,000 simulations

Earliest breakeven retirement age

–

Portfolio value over time

MC P5–P95 MC P25–P75 (core) Constant returns (today's $)

Year-by-year projection (deterministic)

Age	Year	Contribution	Investment returns	Spending	SS/Pension	Portfolio withdrawal	Net impact	Portfolio (nominal)	Portfolio (today's $)

How this model works

Each year the portfolio earns a blended return based on your stock/bond mix. During working years, contributions are added (and grown with inflation). Starting at retirement, you withdraw the amount needed to cover spending (also inflation-adjusted) minus any Social Security or pension income.

The blue line on the chart is the deterministic (constant returns) path — the same numbers shown in the year-by-year table. The green band shows the Monte Carlo interquartile range (P25–P75): 1,000 simulations where each year's return is drawn from a normal distribution centred on your blended return with the volatility (σ) you set. Because volatility creates variance drag, the deterministic line typically sits above the band's midpoint — this gap grows with time and equity allocation. Probability of success is the share of MC simulations where the portfolio survives to your plan-to age.

"Today's dollars" divides nominal balances by cumulative inflation. The "nest egg needed" is the present-value lump sum — in today's dollars — to fund all net withdrawals through your plan-to age. When surplus equals zero in the deterministic model, the portfolio hits exactly $0 at plan age.

Limitations: Returns are assumed normally distributed (no fat tails), no taxes, no healthcare shocks, no RMDs. The ~4% rule is a common rule of thumb for a 70/30 US portfolio.