Calculation Methodology
This page explains how SavingsSolver turns your input into a result and why the assumptions matter.
Fixed-rate amortizing loans
For a fixed payment loan, the periodic installment follows the standard formula Payment = P × r / (1 − (1 + r)^−n), where P is outstanding principal, r is periodic interest and n is remaining installments. When r is zero, the payment is P ÷ n.
In an amortization schedule, monthly interest is calculated on the previous balance. After applying each payment, the remaining balance declines. Monthly scheduled interest and payment amounts are rounded to cents in the exported breakdown. The last payment is capped to what is owed.
Savings accumulation and compounding
Savings projections use future-value mathematics for an opening balance combined with recurring deposits. A nominal annual rate is converted to a periodic rate consistent with the deposit schedule. Where a stated APY is used, it is converted to a corresponding monthly effective rate.
Calculator labels distinguish total contributions from hypothetical interest or investment growth. The timing of deposits matters: beginning-of-month deposits compound one month longer than end-of-month deposits.
Interest rates, inflation and return estimates
Annual rates are entered as percentages and converted to decimal values inside the math library. Effective annual yield, simple interest and annual investment compounding are distinct models, each identified on the relevant tool page.
Inflation and investing tools do not import live rates, asset prices or CPI datasets. These values are supplied by the visitor, and the result illustrates what would happen if the chosen rate persisted.
Debt payoff simulations
Debt payoff models accrue monthly interest, apply scheduled minimum payments and allocate additional budget by the snowball or avalanche priority. A paid-off debt no longer consumes its minimum, allowing the remaining budget to roll into other debts.
If a payment cannot cover the associated monthly interest, the calculator reports that the input does not produce a valid payoff projection rather than fabricating a debt-free date.
Precision, limitations and verification
The financial formulas use JavaScript numerical arithmetic with explicit rounding to cents in installment schedules and formatted financial outputs. This is appropriate for simplified consumer planning, but a bank’s day-count convention, taxes, rounding rules or fees may produce a different figure.
The test suite includes reference cases and boundary scenarios. The models are open to inspection in the downloaded source code. Visitors can report any suspected discrepancy through the contact page.
Questions about these standards? Contact us.