Savings Drawdown Calculator
See how long a pot of savings lasts when you withdraw a fixed amount each month while the balance keeps earning a return — and the withdrawal that would make it last forever.
How it works
Months = −ln(1 − r·B ÷ W) ÷ ln(1 + r), where r = annual return ÷ 12, B = balance, W = monthly withdrawal · If r = 0: months = B ÷ W · Perpetual when B·r ≥ W
Each month the balance earns a return, then you take a withdrawal. If the return on the whole balance is bigger than the withdrawal, the pot grows and never runs out. Below that point the balance shrinks and the formula gives the exact number of months until it hits zero. This is the core math behind retirement drawdown and the "safe withdrawal rate" — the sustainable figure shown is simply the balance times the monthly return.
Worked example
With $500,000 earning 5% a year (about 0.4167%/month) and $2,500 withdrawn monthly: the first month earns ~$2,083, less than the withdrawal, so the balance falls. The formula gives roughly 431 months — about 35 years and 10 months — after withdrawing around $1.08 million in total.
Frequently asked questions
What return should I assume?
A conservative real (after-inflation) return for a balanced retirement portfolio is often 3–5%. Using a lower figure builds in a safety margin against poor market years early in retirement.
What is the sustainable withdrawal?
It is the amount equal to one month’s return on the balance. Withdraw exactly that and the principal stays intact; withdraw less and it grows; withdraw more and it slowly depletes.
Does this account for inflation?
Not directly — it assumes a flat monthly withdrawal. To keep purchasing power steady you would raise the withdrawal each year, which shortens how long the money lasts. Use a real (after-inflation) return as a simple workaround.
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Educational estimate, not financial advice.