Monthly Interest Calculator
Calculate how monthly compounding grows your savings and investments with our free interest calculator. Monthly compounding is the most common frequency for CDs, investment accounts, and many loans, striking a balance between frequent compounding benefits and practical accounting.
Monthly Compounding in Action
| Starting Balance | Interest Rate | Years | Final Balance | Interest Earned |
|---|---|---|---|---|
| $10,000 | 4% APR | 5 | $12,209.97 | $2,209.97 |
| $10,000 | 5% APR | 5 | $12,833.59 | $2,833.59 |
| $10,000 | 6% APR | 5 | $13,488.50 | $3,488.50 |
| $10,000 | 7% APR | 5 | $14,176.25 | $4,176.25 |
Monthly vs Annual Compounding
Monthly compounding earns slightly more than annual compounding because interest starts earning interest sooner. The formula: A = P(1 + r/12)^(12t)
For a $10,000 investment at 5% over 10 years:
- Monthly compounding: $16,470.09
- Annual compounding: $16,288.95
- Advantage: $181.14 extra
Monthly Compound Interest Calculator
``javascript
function calculateMonthlyCompound(principal, annualRate, years, monthlyContribution = 0) {
const monthlyRate = annualRate / 100 / 12;
const months = years * 12;
let balance = principal;
for (let i = 0; i < months; i++) {
balance = (balance + monthlyContribution) * (1 + monthlyRate);
}
const totalContributions = principal + (monthlyContribution * months);
const interestEarned = balance - totalContributions;
return {
finalBalance: balance.toFixed(2),
totalContributions: totalContributions.toFixed(2),
interestEarned: interestEarned.toFixed(2),
effectiveAPY: ((Math.pow(1 + monthlyRate, 12) - 1) * 100).toFixed(3) + '%'
};
}
`
Where Monthly Compounding Is Common
Most CDs compound monthly (though they may pay interest quarterly or at maturity). Investment accounts, bond funds, and 401(k) accounts typically compound monthly. Credit cards also compound monthly—but that works against you, making it important to pay balances in full.
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.