Calculator Methodology
Updated September 12, 2026
This page documents the models used by all 16 SumStride calculators. Percentages are divided by 100 before use in formulas. Dollar displays use USD. Inputs, example rates and outputs do not establish eligibility, current prices or professional advice.
Precision and interpretation
Dollar results are in USD and generally rounded to the nearest whole dollar; salary equivalents and percentage outputs use two decimal places. Calculations retain JavaScript floating-point precision between steps rather than rounding every payment to cents. Displayed components may not sum exactly to displayed totals. Payoff times use whole monthly payment periods. Defaults are examples, not current rates or recommendations.
Input limits keep calculations finite and responsive: most dollar inputs run from zero to one trillion, projection terms up to 100 years, and rates generally up to 100%. Growth models permit negative annual assumptions down to −99%. These are software bounds, not recommended financial ranges. Invalid or incomplete inputs show a message instead of a new estimate.
Mortgage Calculator
P is the loan amount, r is the annual interest percentage ÷ 100 ÷ 12, and n is the term in years × 12. Monthly housing cost adds annual property tax ÷ 12, annual insurance ÷ 12, monthly HOA and monthly PMI to M. Interest is M × n − P. Total paid includes these entered housing costs for the entire term, assumes they stay constant, and does not stop PMI automatically. Closing costs, down payment, maintenance, rate changes and lender escrow adjustments are excluded. The table previews up to the first 12 payments; it is not a full amortization schedule.
Loan Calculator
P is the amount borrowed, r is the annual interest percentage ÷ 100 ÷ 12, and n is years × 12. Total repayment is M × n; interest is total repayment − P. Assumes fixed rates and equal month-end payments, with no origination fees, insurance, penalties or extra payments. Use the contractual interest rate; an APR that includes fees is not necessarily the rate used to accrue interest. The table previews up to the first 12 payments.
Auto Loan Calculator
Financed principal P = max(0, price − down payment − trade-in + max(0, price − trade-in) × sales-tax rate + fees). Here r is the entered annual rate ÷ 100 ÷ 12 and n is months. The APR input is used as an interest rate; fee-inclusive lender APRs can produce different payments. The model assumes a trade-in sales-tax credit, which is not available everywhere. Trade-in is treated as value applied to the purchase; an outstanding trade-in loan is not modeled. Total value + payments includes down payment and trade-in value plus all loan payments, so it is not solely cash paid. Excludes ongoing ownership costs and any unentered fees.
Compound Interest Calculator
a is the annual percentage ÷ 100, f is compounds per year, y is years, P is starting balance and C is the monthly contribution. At a = 0, FV = P + 12 × y × C. In this calculator the selected compounding frequency applies only to the starting balance; monthly contributions always use monthly compounding and arrive at month-end. Thus frequencies other than 12 use a mixed convention, not a single account compounding schedule. Total contributed is P + 12 × y × C. Taxes, fees, inflation and variable returns are excluded. Fractional years use fractional exponents; charts show completed years.
Savings Calculator
P is the starting balance, C is the contribution at each month-end, r is the entered annual percentage divided by 100 and then 12, and n is years × 12. The rate is treated as a nominal annual rate, not APY or an effective annual return. Returns are constant in this illustration; no taxes, fees, inflation, withdrawals or market volatility are modeled. Fractional years use fractional exponents; the chart shows completed years only. Growth is FV minus the starting balance and deposits. Bank daily accrual, tiered balances and changing rates may differ.
Investment Calculator
P is the starting balance, C is the contribution at each month-end, r is the entered annual percentage divided by 100 and then 12, and n is years × 12. The rate is treated as a nominal annual rate, not APY or an effective annual return. Returns are constant in this illustration; no taxes, fees, inflation, withdrawals or market volatility are modeled. Fractional years use fractional exponents; the chart shows completed years only. Total return shown is (FV − total contributed) ÷ total contributed × 100, or not applicable if nothing was contributed. It is a cumulative gain relative to contributions, not annualized, time-weighted or money-weighted performance. Investment losses are possible.
Retirement Calculator
P is the starting balance, C is the contribution at each month-end, r is the entered annual percentage divided by 100 and then 12, and n is years × 12. The rate is treated as a nominal annual rate, not APY or an effective annual return. Returns are constant in this illustration; no taxes, fees, inflation, withdrawals or market volatility are modeled. Fractional years use fractional exponents; the chart shows completed years only. Years is retirement age minus current age. This projects accumulation only: it does not calculate retirement income needs, withdrawal sustainability, Social Security, pensions, eligibility or contribution limits. It cannot determine whether you have enough to retire.
401(k) Calculator
P is the starting balance, C is the contribution at each month-end, r is the entered annual percentage divided by 100 and then 12, and n is years × 12. The rate is treated as a nominal annual rate, not APY or an effective annual return. Returns are constant in this illustration; no taxes, fees, inflation, withdrawals or market volatility are modeled. Fractional years use fractional exponents; the chart shows completed years only. Years is retirement age minus current age. Monthly contribution C = annual salary × (employee percentage + employer percentage) ÷ 100 ÷ 12. Salary and percentages remain constant. Employer percentage means the actual contribution as a percentage of salary, not the percentage of employee contributions matched. Enter the effective employer amount yourself. The model does not enforce tax-year contribution limits, match eligibility, vesting, catch-up rules, salary growth or tax treatment; verify these with your plan.
Simple Interest Calculator
P is principal, a is the annual percentage ÷ 100 and y is time in years. No interest earns interest. The model assumes unchanged principal with no installments, fees or taxes. Fractional years are allowed, but calendar dates and 360/365-day accrual conventions are not modeled.
Credit Card Payoff Calculator
Each month adds interest at the annual percentage ÷ 100 ÷ 12, then subtracts the fixed payment. The final payment is reduced to the remaining balance plus interest. No new borrowing, fees, daily accrual or changing rates are included. Payments that do not cover interest cannot repay the balance. Estimates beyond 1,200 months are flagged rather than presented as a completed payoff; a residual of one cent or less ends the simulation. Credit cards often accrue interest daily and use changing minimum payments; compare your issuer’s statement.
Debt Payoff Calculator
Each month adds interest at the annual percentage ÷ 100 ÷ 12, then subtracts the fixed payment. The final payment is reduced to the remaining balance plus interest. No new borrowing, fees, daily accrual or changing rates are included. Payments that do not cover interest cannot repay the balance. Estimates beyond 1,200 months are flagged rather than presented as a completed payoff; a residual of one cent or less ends the simulation. This is a single-debt model and does not optimize repayment across multiple debts.
Home Affordability Calculator
The calculator searches home prices from $0 to $3,000,000, using 70 bisection steps. At each price, principal is max(0, price − down payment); fixed monthly loan payment uses the amortization formula above. Annual property tax and insurance percentages of price are divided by 12 and added to that payment. The highest modeled price within the housing budget is shown. A result at $3,000,000 reaches the search ceiling, not a proven maximum. If existing debts already exceed target DTI, there is no housing budget under that target. Excludes PMI, HOA, closing costs, utilities, maintenance, reserves, living expenses, credit checks and lender-specific rules. This is neither a loan approval nor a recommendation about a comfortable budget.
Mortgage Payoff Calculator
P is the current balance, r is the annual percentage ÷ 100 ÷ 12, and n is remaining years × 12. The scheduled payment is recalculated from these inputs; it may differ from your actual contractual payment. Two monthly simulations compare total interest and payoff months, reducing the last payment to the amount due. Extra money is assumed to reduce principal immediately. No escrow, fees, prepayment penalties, recasts or changing rates are included. Simulations stop at a residual of one cent or less or 1,200 months; an unfinished payoff is flagged.
ROI Calculator
Initial investment must be positive and final value nonnegative. A final value of zero represents a 100% loss. Annualized return requires a positive holding period; otherwise it is not applicable. Assumes one initial outflow and one final value, with no intermediate deposits, withdrawals or distributions. Include relevant costs in your own inputs; taxes and fees are not added automatically. This is not IRR and does not measure risk or predict future returns.
Salary to Hourly Calculator
Results are gross pay equivalents, before taxes, deductions and benefits. Hours and weeks must be positive; daily pay always assumes five working days per week. Monthly pay spreads salary across 12 months even if fewer weeks are worked. Unpaid leave, overtime premiums, holiday calendars and wage-law eligibility are not modeled.
Percentage Calculator
A zero denominator makes the corresponding ratio undefined, shown as not applicable. The A% of B result is a number, not a currency amount. Negative values are allowed; percentage changes from a negative baseline can be counterintuitive and should be interpreted alongside the original values. Results are rounded to two decimal places.
Content and calculation checks: September 12, 2026. Educational estimates; no independent professional certification is claimed.
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