After 20 years it becomes $40,387.00 — $30,387.00 of that is interest.
$10,000 at 7% for 20 years — how much?
After 20 years it becomes $40,387.00 — $30,387.00 of that is interest.
An example with the starting values. Change the inputs above for your own numbers.
How compounding feeds on time
Compounding adds interest to the principal so it earns interest in turn. The final amount is principal times (1 + rate ÷ periods)^(periods × years). Regular contributions grow the same way for the time each one is invested.
The gap against simple interest widens sharply with time. Ten thousand at 7% reaches $17,000 simple but $19,672 compounded after ten years; after thirty it is $31,000 versus $76,123, a 2.5-fold difference.
Doubling time is exactly log2 ÷ log(1+rate) — 10.24 years at 7%. The familiar rule of 72 gives 10.29, close enough for mental arithmetic.
Where long-range plans go wrong
An optimistic return assumption invalidates the whole plan. Projecting twenty years at 10% versus 6% differs by more than double. Use a conservative figure and let a better outcome be upside.
Ignoring inflation overstates future purchasing power. A 5% return against 3% inflation is a real return of 1.94%, not 2% — precisely (1.05 ÷ 1.03) − 1.
Compounding frequency matters less than people expect: daily versus annual over twenty years at 7% differs by roughly 5%. Rate and time dominate.
Dividing 72 by the rate approximates the doubling time — about 10.3 years at 7%. This calculator solves it exactly with logarithms and gets 10.24 years. The rule is fine for mental math.
Does compounding frequency matter much?
Less than people expect. Over 20 years at 7%, daily versus annual compounding differs by roughly 10%. Rate and time dominate.
Why is tax not deducted?
Taxation differs entirely by product. This shows pre-tax growth; after-tax figures for deposits live in the savings calculator.
This is a reference calculation using common finance assumptions and is not a substitute for professional advice. We accept no liability for decisions made from it — see the terms of use for details.