Compound Interest Calculator
This free tool calculates how your regular contributions grow over time, and how much of that growth comes from compounding rather than what you put in. Most people overestimate how much they need to start, and underestimate how much they lose by waiting.
Compound Interest Calculator
Why does time matter more than the amount you invest?
Compounding rewards time more than it rewards the size of your contribution. The same weekly amount, the same return, can end up wildly different depending only on when you start.
$50 a week at 7% grows to around $107,000 after 20 years. Leave it running for 30 years instead, same $50 a week, and it grows to around $245,000. Ten extra years more than doubles it.
Common Questions
What is compound interest?
Compound interest is interest earned on both your original deposit and on the interest you've already earned. Each period, the amount you earn interest on grows, so the balance grows faster over time than it would with simple interest. Over long periods, this compounding effect does most of the work.
How does starting early affect compound interest?
Starting early gives compounding more time to work, and time matters more than the size of your contribution. The same weekly amount, invested for an extra ten years, can more than double the final balance. Waiting even a few years to start can cost more than adding a larger contribution later.
What's the difference between compound and simple interest?
Simple interest is calculated only on the original amount deposited, so it grows at a steady, flat rate. Compound interest is calculated on the original amount plus all interest earned so far, so the growth accelerates the longer the money is invested. Over short periods the difference is small; over decades it is large.
How often should interest compound for the best growth?
More frequent compounding, such as monthly instead of annually, produces a slightly higher return for the same rate, because interest starts earning interest sooner. The effect is real but modest compared to the impact of the interest rate itself and how long the money stays invested.
How much difference does a higher interest rate make?
A higher rate compounds faster, so even a small increase can produce a meaningfully larger balance over a long time horizon. The gap between two different rates widens the longer the money is left invested, which is why the rate matters most for long-term goals.