Safe Withdrawal (4% Rule) Calculator Guide

The Safe Withdrawal Calculator shows how much income a retirement pot can provide and how long the money lasts. It is built around the famous "4% rule": withdraw 4% of your pot in the first year of retirement, then increase that amount with inflation each year so your spending power stays level.

For example, a $1,000,000 pot at a 4% rate gives $40,000 of income in year one, roughly $3,333 a month. If inflation runs at 2.5%, the second year's withdrawal rises to $41,000, the third to $42,025, and so on. Meanwhile the remaining balance keeps growing at your expected investment return. The calculator projects this year by year and tells you whether the pot survives your whole retirement.

How to Use

Pot at Retirement: Enter the total value of your retirement savings on day one of retirement. Use the currency toggle to switch between dollars and pounds.
Withdrawal Rate: The percentage of the starting pot you take in year one. The classic rule uses 4%, but you can test anything from 1% to 10%.
Expected Growth: The average annual return you expect on the remaining invested pot.
Inflation: The annual rate at which your withdrawals grow to keep pace with rising prices.
Retirement Length: How many years the money needs to last. The classic studies used 30 years.

Understanding the Results

Annual income (headline): your first-year withdrawal, the pot multiplied by the withdrawal rate.
Monthly income: the first-year withdrawal divided by twelve.
Final-year withdrawal: what the withdrawal has grown to by the last year after inflation increases. Over 30 years at 2.5% inflation, a $40,000 withdrawal grows to over $81,000.
End balance: what is left in the pot at the end of the retirement horizon. With favourable assumptions this can be larger than the starting pot.
Money lasts: whether the pot survives the full horizon or runs out early. If it runs out, the calculator flags the year it happens.

The Chart tab plots the projected balance at the end of each year so you can see whether the pot is climbing, coasting, or heading for zero.

Where the 4% Rule Comes From

The rule originates with financial planner William Bengen's 1994 study. Bengen tested historical US stock and bond returns for every retirement start year since 1926 and asked: what is the highest first-year withdrawal rate that never exhausted a portfolio within 30 years, even for retirees who hit the worst market sequences? The answer was close to 4%, and the figure stuck.

The finding was reinforced by the Trinity study, published in 1998 by three professors at Trinity University. It examined "portfolio success rates" across a range of withdrawal rates, time horizons, and stock/bond mixes, and found that a 4% inflation-adjusted withdrawal from a majority-stock portfolio succeeded in the overwhelming majority of historical 30-year periods. Together, Bengen's work and the Trinity study turned 4% into the default starting point for retirement income planning.

Assumptions and Criticisms

The 4% rule is a useful rule of thumb, not a guarantee. Keep its limits in mind:

Constant returns vs. real markets: This calculator (like the rule itself when quoted casually) assumes a steady growth rate. Real returns arrive unevenly, and the order matters.
Sequence-of-returns risk: Poor market years early in retirement do disproportionate damage, because you are selling assets at depressed prices to fund withdrawals and the pot never recovers. Two retirees with the same average return can have wildly different outcomes depending on when the bad years land.
The 30-year horizon: Bengen and the Trinity study tested 30-year retirements. If you retire early or live longer, the safe rate is likely lower; for shorter retirements it can be higher.
US-centric data: The original studies used US market history, which delivered unusually strong returns through the twentieth century. Studies using international data often find that a "safe" rate below 4% would have been needed in many other countries.
No fees or taxes: The classic studies ignore investment fees and taxes, both of which reduce the rate you can sustainably withdraw.
Rigid spending: The rule assumes you mechanically raise spending with inflation every year. In practice, flexible spending (cutting back in bad years) materially improves survival odds.

The Withdrawal Formula

Withdrawals are taken at the start of each year, and the remaining balance grows at the expected rate:

W_{1} W_{t} B_{t} = P \times r = W_{1} \times (1 + i)^{t - 1} = (B_{t - 1} - W_{t}) \times (1 + g)

Where P is the pot at retirement, r is the withdrawal rate, i is inflation, g is the expected growth rate, W_t is the withdrawal in year t, and B_t is the balance at the end of year t (with B_0 = P). The pot is depleted in the first year t where B_t reaches zero.

References & Further Reading

"Determining Withdrawal Rates Using Historical Data" (William Bengen, Journal of Financial Planning, 1994): the original study behind the 4% rule.
Trinity study (Wikipedia): overview of the 1998 portfolio success-rate research.
William Bengen (Wikipedia): background on Bengen and later refinements to the rule.