Compound interest calculator

Compound interest calculator projects a Starting balance plus equal Monthly contributions over a Term in years. The entered Annual return percentage is divided by 100 and then by 12. Years are multiplied by 12 to create the number of monthly periods.

The starting balance grows for every period. Contributions use the future-value formula for an ordinary annuity, which treats each contribution as arriving at the end of its month. A contribution at the start of each month would receive one additional period of growth and produce a different balance.

The rate stays constant, contributions never change, and no withdrawals, taxes, fees, inflation, volatility, or missed months are included. This is a deterministic scenario, not a forecast or promise of investment performance.

For a nonzero monthly rate r and n months, the starting balance grows as principal times (1 + r) raised to n. Contributions grow as monthly contribution times (((1 + r) raised to n minus 1) divided by r). The page adds those two amounts.

Your contributions is starting balance plus monthly contribution times months. Estimated growth is projected balance minus that contribution total. The label growth includes only the modeled return effect under the formula; it does not separate income, price movement, fees, or tax.

At a zero annual rate, the page uses starting balance plus monthly contribution times months. This branch avoids dividing by zero and gives Estimated growth of zero. Negative rates are accepted mathematically but are not validated as a suitable scenario.

Enter 5000 as Starting balance, 7 as Annual return, 10 as Term, and 200 as Monthly contribution. The calculator uses 120 months and a monthly rate of 0.07 divided by 12. It treats each $200 contribution as arriving at month end.

The computed projected balance is about $44,665.26837 and displays as $44,665.27. Contributions equal $5,000 plus 120 times $200, or $29,000. Estimated growth is about $15,665.27.

These values assume the same monthly rate every month. A real account can rise and fall, charge fees, receive contributions on different days, and hold cash temporarily. Use the example to understand formula behavior, not to predict a particular investment.

End-of-month timing is embedded in the ordinary-annuity term. If money is deposited at the beginning of each month, multiply the annuity portion by one plus the monthly rate for the corresponding annuity-due model. The workbench has no timing selector and always uses the end-of-month version.

A single annual deposit is not equivalent to twelve equal monthly deposits at the same nominal total because money enters at different times. Irregular bonuses and skipped contributions also need a cash-flow schedule. Do not average them into one monthly figure when timing materially affects the decision.

The first starting balance is present before month one and receives all periods of growth. A contribution entered in Monthly contribution is separate from that principal. Place money already invested in Starting balance rather than counting it again as the first monthly deposit.

The annual percentage is treated as a nominal rate divided by 12, not an effective annual return converted to a monthly equivalent. Seven percent entered here produces a monthly rate of 7% divided by 12. The resulting effective yearly compounding is slightly above 7% when the rate is positive.

Investment returns are not credited as a stable monthly rate in most market assets. Historical averages do not arrive smoothly and do not guarantee future outcomes. Sequence, fees, tax, asset allocation, and withdrawals can change results materially.

For savings products, confirm compounding frequency and whether the quoted rate is nominal or effective. For investments, test several clearly labeled assumptions and review risk separately. The calculator does not recommend a rate or assess suitability.

The formula uses JavaScript floating-point values and formats final lines as USD with at most two decimals. Intermediate monthly balances are not rounded to cents because the calculation uses a closed-form expression. An account that posts and rounds each month can differ slightly.

Currency is fixed to USD display and there is no conversion. The numerical formula can be applied to another currency only if every cash amount uses that same currency, but the dollar symbol would be misleading. Label any external use explicitly.

Do not subtract displayed rounded lines to diagnose a one-cent difference. Compare underlying assumptions and use a decimal cash-flow model when cent-level reconciliation is required. The page is designed for estimates rather than account statements.

The form checks numeric finiteness but does not require positive balances, contributions, rates, or years. Zero years gives zero months and leaves the starting balance. Negative periods or a monthly rate of negative 100% can create undefined or misleading mathematics. Use realistic inputs.

The Term field shows a one-year step, though typed decimals can create fractional payment counts after multiplication by 12. Use a term that maps to the intended whole months. Blank fields may convert to zero; fill the three core inputs and enter Monthly contribution explicitly when needed.

Very large rates or terms can overflow the exponent and display nonfinite currency. Such output is not an extreme forecast. It signals that the scenario exceeds practical numeric bounds or contains an entry error.

Loan calculator solves a fixed payment needed to amortize principal. Compound interest projects an asset balance from deposits. Using the growth result as a loan schedule ignores payment allocation and debt terms.

Percentage calculator applies one direct equation without time. A 7% one-time increase is not the same as monthly compounding from a 7% nominal annual rate. Discount and VAT calculations also apply rates to prices under different formulas.

Use this page for a clearly labeled constant-rate contribution scenario. Verify plans with provider terms, fees, taxes, and a qualified adviser when decisions are consequential. Preserve inputs and contribution timing with every copied result.

Hold starting balance, term, and contribution constant when comparing return rates. Then hold rate constant when comparing contribution amounts. Changing several inputs together makes it hard to explain why balances differ. Label each scenario as an assumption rather than expected performance.

A longer term amplifies both contributions and modeled compounding. Compare Your contributions as well as Projected balance so added deposits are not mistaken for investment growth. Estimated growth is simply the difference between those two lines under the model.

Inflation is absent. A future nominal balance does not show purchasing power. If an analysis needs real values, apply a consistent inflation model outside this calculator and state whether returns are nominal or inflation-adjusted.

Fees and taxes are also absent. Subtracting an annual fee from the return rate is only an approximation and may not match fee timing or tax treatment. Use account-specific projections for decisions.

Start with the zero-rate case. A $1,000 balance plus $50 for 24 months should produce $2,200, contributions of $2,200, and growth of zero. This checks month count and contribution addition without exponentiation.

For a positive-rate case, independently compute monthly rate, months, principal future value, and annuity future value. Confirm that their sum matches the unrounded projection within floating-point tolerance. Then verify contributions as principal plus monthly deposits.

If output is unexpectedly large, check whether 7% was entered as 7 rather than 0.07. The tool divides by 100. Also inspect term and contribution signs. Reset clears the scenario, so record values before comparing another run.

A provider statement may differ because cash arrives on specific dates and returns vary. Reconcile using transaction-level data rather than changing the constant rate until one final number happens to match.

Contribution frequency is fixed at monthly. Weekly or annual saving plans need conversion with explicit timing, not merely division or multiplication, because each deposit receives a different number of growth periods. Build a period-by-period schedule when cash flows vary.

The model assumes returns compound monthly at the nominal rate. If a product compounds daily, quarterly, or annually, use its stated frequency and rate definition. Matching the annual percentage while changing frequency can change the effective return.

Negative monthly contributions mathematically act like withdrawals, but the closed-form result does not stop when a balance reaches zero or model withdrawal constraints. Use a drawdown model for spending plans.

A negative starting balance is accepted but changes the interpretation from saving to debt without applying a loan payment formula. Do not use it as a shortcut for borrowing costs.

For scenario review, calculate zero contribution to isolate growth on principal, then zero rate to isolate deposits. These benchmark cases make sign, period, and timing errors easier to detect.

Save each scenario's starting balance, annual rate, years, monthly contribution, assumed timing, currency, and calculation date. Rounded output without assumptions cannot be reproduced reliably.

Market uncertainty is not represented by a single constant value. A range of assumptions can show formula sensitivity, but it still does not provide probabilities or downside paths.

Compare the projected balance with the contribution total, not only with the starting balance. Otherwise, deposits can be misreported as return. The output already separates these lines for that reason.

If monthly contribution is omitted, the code substitutes zero. Entering zero explicitly is clearer in a documented scenario. Other required blanks can also convert unexpectedly, so inspect every field before Calculate.

The Copy action transfers the three displayed currency lines but not the four inputs. Store the starting balance, rate, term, and monthly amount beside the projection so another reviewer can reproduce it.