How this compound interest calculator works
Most compound interest calculators ask for one rate, one contribution, and one time horizon — as if your financial life were a single flat line. Real money doesn't work that way. You contribute $300 a month in your twenties and $1,500 in your forties. You earn equity-like returns while accumulating and shift conservative near retirement. You inherit money once, buy a house once, and eventually stop contributing entirely and start withdrawing.
This calculator models that reality as phases: a sequence of periods, each with its own duration, monthly contribution (or withdrawal), and expected annual return. Within each phase, the balance compounds monthly — each month the balance grows by one-twelfth of the annual rate, then the month's contribution is added. One-time lump sums (an inheritance, a bonus, a home down payment leaving the portfolio) can be dropped anywhere on the timeline. The chart stitches the phases together so you can see the whole arc, from first paycheck to final withdrawal.
The formula, and why contributions change everything
The textbook compound interest formula covers a lump sum left alone:
A = P(1 + r/n)nt
where P is the starting principal, r the annual rate, n the number of compounding periods per year, and t the years. $10,000 at 7% compounded monthly for 30 years becomes about $81,200 — an eightfold increase without adding a cent.
But almost nobody invests as a single lump sum. The moment you add regular contributions, each deposit becomes its own little principal compounding from its own start date, and the math (a future-value-of-annuity series) stops being something you want to do by hand. That's the real job of this calculator: it runs the month-by-month arithmetic across every phase, contribution, and lump sum, so the only thing you need to bring is honest assumptions.
What the phases teach that one flat rate can't
Early money dominates. Run a scenario with $300/month for the first ten years and nothing after, against nothing for ten years and $600/month for the next ten. The early saver usually wins despite contributing half as much per month — those first dollars compound for decades. Seeing that on your own numbers is more convincing than any lecture about starting early.
Drawdown is a phase, not an afterthought. Model retirement as a phase with a negative monthly contribution — say −$2,500/month at a 4% return. The calculator shows whether the balance outlives the plan or hits zero, and warns you when it depletes. One honest limitation: this is a deterministic model, meaning it assumes the same return every single year. Real markets deliver returns unevenly, and a bad stretch early in retirement (sequence-of-returns risk) can sink a plan that looks fine on average. If your drawdown phase barely survives, treat that as a plan that needs more margin, not one that works.
Rate changes compound too. Dropping from 7% to 4% at retirement sounds small; over a 20-year drawdown it's often the difference between ending with a surplus and running dry. Phases make that visible.
Choosing honest assumptions
For long-run U.S. stock returns, roughly 10% annually before inflation and about 7% after inflation are the commonly cited historical averages — useful defaults for accumulation phases, with the reminder that history is not a guarantee. Bonds and cash run lower, so blended portfolios and retirement phases are often modeled at 4–5%. Results here are in nominal, pre-tax dollars: to think in today's purchasing power, subtract expected inflation from your rate; to think after taxes, remember that a 401(k) or IRA compounds untaxed while a brokerage account loses a slice to taxes each year. For newer account types with their own rules, see the HELOC payment calculator for borrowing-side math or the Trump Account calculator for the new children's accounts.
Frequently asked questions
What interest rate should I use?
Match it to the investment. Long-run U.S. stocks: ~7% real / ~10% nominal historically. Blended or conservative portfolios: 4–6%. Model accumulation and drawdown phases at different rates — that's what the phases are for.
How is compound interest different from simple interest?
Simple interest pays only on the original principal; compound interest pays on principal plus all previously earned interest, so growth accelerates. At 7%, $10,000 earns $700 in year one either way — but by year ten, the compounded version earns over $1,300 a year.
Does monthly vs. annual compounding matter?
Modestly. $10,000 at 7% for 30 years: about $76,100 compounded annually vs. $81,200 monthly. This calculator compounds monthly, which also matches how people actually contribute.
Does it account for inflation or taxes?
No — results are nominal and pre-tax. Subtract expected inflation from your rate for real-dollar results, and remember tax treatment depends on the account (401(k)/IRA vs. taxable brokerage).
Can it model running out of money?
Yes — give a phase a negative monthly contribution and the calculator flags the year the balance hits zero. Because it assumes steady returns, treat a barely-surviving plan as one that needs more cushion.