Compound Interest & SIP Calculator
Project how regular investing grows over time.
Added each contribution period.
'Start of period' (SIP) earns one extra period of growth.
Future value
- Total contributions
- $130,000.00
- Total growth
- $170,850.72
Projection only. Assumes a constant rate and reinvested returns; real markets vary.
How the compound interest calculator works
This tool projects what a starting amount plus a stream of regular contributions will be worth after a set number of years. It assumes a constant rate and contributions of equal size at a steady cadence.
The core formula for end-of-period (ordinary) contributions is FV = P*(1+r)^n + PMT*((1+r)^n - 1)/r, where P is the starting principal, PMT is each contribution, r is the rate per contribution period, and n is the total number of periods. The first term grows the lump sum; the second term grows the contribution stream. For start-of-period (SIP) timing, the contribution term is multiplied by (1+r) so each deposit gets one extra period of growth.
Because compounding frequency and contribution frequency can differ, the calculator reconciles them through the effective annual rate. It computes EAR = (1 + i/m)^m - 1 from the nominal annual rate i and compounds-per-year m, then derives the per-contribution-period rate as r = (1 + EAR)^(1/contributions_per_year) - 1. The period count is n = years * contributions_per_year.
Worked example
Starting amount of 10,000, monthly contribution of 500, 7% nominal annual rate, monthly compounding, monthly contributions, over 10 years, end-of-period timing.
EAR = (1 + 0.07/12)^12 - 1 = 0.072290(about 7.229%)r = (1.072290)^(1/12) - 1 = 0.0058333per monthn = 10 * 12 = 120periods(1+r)^n = 2.009661
| Result | Value |
|---|---|
| Future value (ordinary) | 106,639.02 |
| Total paid in | 70,000.00 |
| Total growth | 36,639.02 |
With start-of-period (SIP) timing, the same inputs return a future value of 107,143.85, about 505 more, because every monthly deposit compounds for one extra month.
How to use it
- Enter your starting amount as the principal, even if it is zero for a pure SIP.
- Set the contribution to one period’s deposit, not an annual total.
- Match compounding to how the account states it (savings often daily or monthly, funds typically annual) and set contribution frequency to how often you actually invest.
- Choose start-of-period timing for a standard SIP or recurring deposit, and end-of-period for a contribution that posts after the period closes.
- Use the total paid in and total growth figures to see how much of the result is your own money versus compounding.
Limitations
This is an estimate, not financial advice. It assumes a fixed rate and unbroken contributions, while real returns vary year to year and few people contribute without interruption. It excludes taxes, fees, and inflation, all of which lower the amount you keep. Treat the output as a planning guide for comparing scenarios, not a guaranteed outcome, and consult a qualified advisor for decisions that matter.
Frequently asked questions
What is the difference between ordinary and start-of-period (SIP) timing?
Ordinary timing assumes each contribution lands at the end of its period, so it earns one period less of growth. Start-of-period timing (an annuity-due, how most SIPs work) deposits at the start, giving every contribution one extra period to compound. The calculator handles this by multiplying the contribution term by (1+r), which is why the SIP result is always slightly higher for the same inputs.
Can the compounding frequency differ from how often I contribute?
Yes. You can set, for example, daily compounding with monthly deposits. The calculator first converts the nominal annual rate into an effective annual rate using the compounding frequency, then derives a per-contribution-period rate from that. This keeps the two cadences consistent so the result is not distorted by mismatched periods.
Does this account for taxes, inflation, or fees?
No. The projection is a pre-tax, pre-fee, nominal figure. Taxes on interest or capital gains, fund expense ratios, and inflation all reduce real returns. To approximate purchasing power, you can enter a rate that is your expected return minus expected inflation.
What happens if I enter a 0% rate?
With a zero rate there is no growth, so the future value is simply your starting amount plus every contribution you make (principal + contribution × number of periods). The calculator detects this case directly to avoid dividing by a zero rate.
Can I model a negative return?
You can enter a negative rate to model a declining balance, and total growth will show as a negative number. The calculator rejects rates so negative that the per-period factor would fall to zero or below, because the compounding math becomes undefined there.