Retirement Compound Interest Calculator
See how compound interest supercharges retirement savings with our free calculator. Time is the most powerful factor in building wealth—starting early and letting compound interest work over decades can turn modest contributions into substantial nest eggs.
The Power of Time: Starting Age Comparison
| Starting Age | Monthly Contribution | At Age 65 (7% return) | Total Contributed | Interest Earned |
|---|---|---|---|---|
| 25 | $500 | $1,199,175 | $240,000 | $959,175 |
| 30 | $500 | $829,421 | $210,000 | $619,421 |
| 35 | $500 | $566,765 | $180,000 | $386,765 |
| 40 | $500 | $379,494 | $150,000 | $229,494 |
| 45 | $500 | $246,197 | $120,000 | $126,197 |
Retirement Account Tax Advantages
Account type significantly impacts compound growth:
| Account | Tax Benefit | 2024 Contribution Limit | Best For |
|---|---|---|---|
| 401(k) | Pre-tax contributions, tax-deferred growth | $23,000 (+$7,500 catch-up) | Employer match |
| Traditional IRA | Tax-deductible, tax-deferred growth | $7,000 (+$1,000 catch-up) | Tax deduction now |
| Roth IRA | After-tax, tax-free growth | $7,000 (+$1,000 catch-up) | Tax-free in retirement |
Retirement Growth Calculator
``javascript
function calculateRetirementGrowth(currentAge, retirementAge, monthlyContribution, annualReturn, currentBalance = 0) {
const yearsToRetirement = retirementAge - currentAge;
const months = yearsToRetirement * 12;
const monthlyRate = annualReturn / 100 / 12;
let balance = currentBalance;
for (let m = 0; m < months; m++) {
balance = (balance + monthlyContribution) * (1 + monthlyRate);
}
const totalContributions = currentBalance + (monthlyContribution * months);
const compoundGrowth = balance - totalContributions;
const percentFromCompounding = (compoundGrowth / balance * 100);
return {
projectedBalance: balance.toFixed(2),
totalContributions: totalContributions.toFixed(2),
interestEarned: compoundGrowth.toFixed(2),
percentFromCompounding: percentFromCompounding.toFixed(1) + '%'
};
}
`
The 4% Rule for Retirement Income
Financial planners often use the 4% rule: withdraw 4% of your portfolio annually in retirement. With $1 million saved, that's $40,000/year. To replace $80,000 in annual income, target $2 million in retirement savings.
The Projection Behind This Page
Starting from $10,000, adding $200 a month at 7%:
| Year | Deposited | Balance | Growth | Growth on deposits |
|---|---|---|---|---|
| 1 | $12,400 | $13,201 | $801 | 6% |
| 5 | $22,000 | $28,495 | $6,495 | 30% |
| 10 | $34,000 | $54,714 | $20,714 | 61% |
| 20 | $58,000 | $144,573 | $86,573 | 149% |
After 20 years, 60% of the balance is growth rather than money you
put in. That crossover — the point where returns exceed contributions — is the whole reason
compounding is worth waiting for, and it arrives later than most people expect.The Formula
`
A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) ÷ (r/n)]
└── initial principal ──┘ └────── regular contributions ──────┘
`
The second term usually dominates. On these numbers the $200 monthly contribution
accounts for the larger share of the final balance, which is the practical lesson: how much
you add matters more than the rate, right up until the balance gets large.
How Long Until It Doubles
The Rule of 72 divides 72 by the rate to estimate doubling time. At 7%:
`
72 ÷ 7 = 10.3 years
`
The exact answer is 10.2 years — the rule is accurate to within a few months
for rates between 6% and 10%, and drifts at the extremes. It works because ln(2) ≈ 0.693 and
72 has convenient divisors.
| Rate | Rule of 72 | Exact |
|---|---|---|
| 2% | 36.0 yr | 35.0 yr |
| 5% | 14.4 yr | 14.2 yr |
| 7% | 10.3 yr | 10.2 yr |
| 10% | 7.2 yr | 7.3 yr |
| 15% | 4.8 yr | 5.0 yr |
Compounding Frequency at This Rate
A nominal 7% turns into a different effective yield depending on how often it
compounds — this page uses monthly:
| Compounded | Effective annual yield |
|---|---|
| Annually | 7.000% |
| Quarterly | 7.186% |
| Monthly | 7.229% |
| Daily | 7.250% |
| Continuously | 7.251% |
The gap between annual and monthly is worth having. The gap between monthly and daily is
0.021 percentage points — rounding. Compare accounts on
APY, which already folds the frequency in, rather than on the nominal rate.Inflation Is the Number That Matters
A 7% nominal return against 3% inflation is a 4.0% real return.
Real return is what buys anything:
`
real ≈ nominal − inflation
``
Over 10 years, 3% inflation cuts purchasing power by about 26%. A projection quoted in nominal dollars therefore overstates what the money will actually be worth, which is why retirement targets are usually stated in today's dollars.