Compound interest calculator

This calculator takes a starting balance, a monthly addition, a period and a nominal annual rate. It applies one month's modeled growth first, adds the monthly contribution at month-end, and repeats for a rounded number of months. The ending balance is a hypothetical result, not a bank quote or an investment forecast.

Use the tool for a narrow question: how does one changed assumption affect this same monthly model? Keep the other inputs fixed when making that comparison. A higher output does not tell you which account to open, which investment to buy or how much you can afford to contribute.

Follow one modeled month

The annual percentage input is divided by 1,200 to turn a percentage into a monthly decimal rate. Each step is next balance = previous balance × (1 + monthly rate) + monthly addition. The addition comes after that month's growth. It does not receive growth during the month in which it enters.

For an illustrative one-month comparison, a $1,000 starting balance, $100 monthly addition and 12% nominal annual rate give $1,110: $1,000 grows by 1%, then $100 is added. A beginning-of-month addition would instead produce $1,111 under that same assumed monthly rate. That second figure is a separate timing example, not this tool's output.

The Investor.gov compound interest calculator asks for an initial investment, monthly contribution, time, interest-rate assumptions and compounding frequency. It is a useful primary reference for the inputs involved. Valora's served model has a fixed monthly frequency, so results should not be compared without aligning the assumptions.

Check the modeled period, not just the years field

The served calculation uses Math.round(years × 12) monthly steps. A fractional-year entry is therefore rounded to whole months before the loop runs. It is not a daily accrual model. Two different year entries can create the same number of monthly steps and the same result.

At a zero nominal rate, no growth is added. The result is the starting balance plus the monthly addition multiplied by the modeled month count. With $1,000, $100 a month and 12 monthly steps, that is $2,200. At a zero-month period, the result is the starting balance; there is no contribution step.

The visible result rounds the modeled ending balance to a whole dollar. A small change may not change that display even when the unrounded balances differ. Save the inputs, month count and display together rather than treating the displayed dollar amount as an account statement.

Keep the nominal rate separate from APY

This tool expects a nominal annual percentage rate that it divides into monthly periods. It does not accept a bank's annual percentage yield as an interchangeable rate label. The CFPB definitions of interest rate and annual percentage yield distinguish a rate that does not reflect compounding from APY, which reflects interest and compounding over the stated period.

For this model alone, a 12% nominal rate means 1% each month. Twelve repeated 1% steps produce about 12.68% growth over a year on a starting balance with no additions. That arithmetic does not establish a real product's yield. Check actual disclosures, compounding and deposit timing when discussing a bank account.

Do not relabel an assumed investment return as a quoted interest rate. The tool repeats the same rate every month. It does not reproduce variable market returns, cash distributions or the timing of actual gains and losses. Keep the real product or investment question separate from this calculation.

Use a private comparison worksheet

Keep six fields privately: scenario label; starting balance; monthly addition; entered years and rounded month count; nominal annual rate and its source or hypothetical status; and the displayed result with the unresolved decision. Add a date so another person can understand which inputs you used.

Write a second scenario that changes just one input. Describe which change created the difference, rather than calling the larger result the better choice. If the inputs came from an assumption rather than a document, say so in both rows. The blank CSV included with this guide is a record aid, not an account recommendation.

Separate money put into the model from modeled growth. With month-end additions, total input money is starting balance + monthly addition × modeled months. The ending balance minus that amount is the modeled gain or loss under the constant-rate assumption. It is not realized earnings or an after-tax return.

Respect the input and model limits

The served tool requires nonnegative starting balances and monthly additions, years from 0 through 100, and a nominal annual rate above -100% and at most 100%. It rejects empty fields and nonfinite values; balances and additions must not exceed JavaScript's maximum safe integer. Those are input checks, not an endorsement of any rate as reasonable.

This model does not include fees, taxes, inflation, withdrawals, changing rates or a different contribution calendar. A negative assumed rate can produce modeled losses, but repeating one negative rate is still not a market path. An output is not a probability, guarantee or test of account eligibility.

The retirement savings growth guide is the neighboring accumulation tool. This companion's distinct question is how monthly frequency, month-end additions, rounded months and rate labels change what you are comparing. For an actual household payment change, use the new-job guide for document questions rather than treating a modeled contribution as affordable or permitted.

Bring the question, not the forecast

If you seek professional help, state the inputs you used, the timing assumption you want checked and the decision the tool cannot answer. Ask what records and written comparison are included, what is excluded and what the work costs. Keep private account details out of a public inquiry. Valora is not arranging advisor matches or introductions at this time.