Retirement Runway: How Long Will Your Savings Last?

Runway is an aviation term, and it fits retirement better than "savings." It is not how much fuel you carry. It is how far you can fly at the rate you burn. Your savings last exactly as long as the gap between what you spend and what arrives without you working.

The example is small and honest. Spend $6,000 a month, receive $2,500 a month from Social Security and a pension, and the gap is $3,500 a month — $42,000 a year. A $1.2 million portfolio covers that gap at a 3.5% withdrawal rate, a sustainable reference on current assumptions. The retirement income runway calculator computes your own number in minutes.

Runway is a gap, not a pile

Guaranteed income is the part you do not manage: Social Security, a pension, an annuity. Expenses are the part you do. The gap between them is what the portfolio funds.

This framing changes the planning question. The goal is a smaller gap — lower fixed expenses, higher guaranteed income, or both — because every dollar of gap reduction extends the runway without any change to the portfolio.

Guaranteed income does not have to be permanent to count. A fixed annuity, a rental stream, or a spouse's pension each reduce the gap in their own years. The honest version of the table lists income by the year it arrives, because the gap is different at 60 and at 70.

The worked example

At 3.5%, the portfolio covers the gap exactly as a reference. The withdrawal rate is the dial: at 4%, the same portfolio funds a $48,000 gap, but with less margin. The runway calculator tests both rates against your own numbers.

The sensitivity runs both ways. Every $500 of monthly expenses changes the gap by $6,000 a year — half a point of withdrawal rate on a $1.2 million portfolio. Cutting $1,000 a month of fixed costs moves the needed rate from 3.5% to 2.5%, and the same portfolio suddenly covers a much longer runway.

Why 4% is a reference, not a law

The 4% rule comes from historical research on diversified portfolios and long horizons. It is a starting point for stress testing, not a guarantee and not a legal limit. It assumes flexible spending, low fees, and a balanced portfolio — assumptions that fail quietly in concentrated holdings.

Use it as the upper bound of a test range, then build the plan around the gap and the risks below.

The bridge years

The runway is rarely flat. In the years before Social Security and Medicare start, the gap is at its widest: no guaranteed income, full healthcare cost, and the portfolio paying for all of it. Most plans stress-test this window separately, because it is the one where sequence risk and healthcare cost arrive together.

The standard bridge is 1-2 years of cash for the bridge years, with the rest of the portfolio left alone. That cash converts a forced sale at a bad moment into a scheduled withdrawal from a stable account.

Four risks that shorten the runway

Sequence of returns

A market drop in the first years of retirement hurts more than the same drop later, because withdrawals lock in losses. Early retirement years are the fragile ones. A cash buffer of 1-2 years of gap spending smooths exactly this risk. The buffer is not an investment opinion; it is a calendar. Withdrawing from cash for the first two years lets the portfolio ride out the early market noise.

Inflation

At 3% inflation, costs double in about 24 years. A fixed pension loses a third of its purchasing power in that window. The 3.5% reference assumes the portfolio can grow enough to keep pace. Social Security adjusts for inflation; private pensions mostly do not. The fixed part of the guaranteed line is the part that erodes.

Taxes

Withdrawals from tax-deferred accounts are ordinary income, and brokerage sales create capital gains. The capital gains guide shows the cost, and the payroll and tax tools cover the rest of the withdrawal stack. A tax-aware drawdown order extends the runway without spending less. Roth dollars are the exception: tax-free in, tax-free out. The order in which you spend taxable, tax-deferred, and Roth accounts changes how much of the runway the tax bill eats.

Healthcare before Medicare

The years between retirement and Medicare are the most expensive healthcare years of the plan, and premiums plus deductibles can run into five figures. Medicare starts at 65; the gap years before it are the expensive ones, and a serious condition can double the line. The health plan comparison prices that risk, and the income protection guide covers the disability side for anyone retiring early.

How to extend the runway

• Delay Social Security. Every year of delay raises the monthly benefit for life. Two years of delay can add 15% or more to the guaranteed line. The math is simple: bigger guaranteed income shrinks the gap for life, and the break-even point lands in the late 70s for most couples.
• Shrink the fixed gap. A paid-off house removes the largest fixed expense. The extra payment calculator shows the cost of getting there.
• Work part-time early. A few years of partial income at the start of retirement is the cheapest runway insurance available, because it protects the sequence-of-returns years. Even $20,000 a year of part-time income cuts the withdrawal rate by nearly half a point on a $1.2 million portfolio.
• Build the buffer years. The savings goal timeline and compound growth calculators show how long the accumulation side takes before retirement starts.

Bottom line

Retirement planning is gap math. Know the expenses, count the guaranteed income, and measure the withdrawal rate the gap requires. The retirement income runway calculator gives you the number — the risks above tell you how much margin to add.