THE NUMORIX GUIDE
How to use the Student Loan Calculator
Last reviewed September 14, 2026
What this calculator does
The route has three calculations.
Formula and method
The route has three calculations. Simple mode solves a monthly payment, term, or implied rate for a balance under monthly amortization. Repayment mode compares the original schedule with a simulation that applies extra monthly, yearly, and first-month one-time payments. Projection mode adds yearly borrowing during school, capitalizes interest when school interest is not paid, applies grace-period interest, and then solves the post-grace monthly payment.
Variables and inputs
Simple inputs are loan balance, remaining term in years, rate, monthly payment, and solve-for choice. Repayment inputs add extra payments. Projection inputs are years to graduate, loan amount per year, current balance, repayment term, grace months, rate, and whether interest is paid during school.
Worked example
In Simple mode, a $30,000 balance at 5% over 10 years has monthly rate 0.05 / 12 = 0.0041667 and payment 30000 x r x (1+r)^120 / ((1+r)^120 - 1) = about $318.20. Total payments are about 120 x 318.20 = $38,184, so modeled interest is about $8,184.
How to interpret the result
Simple mode is a regular repayment estimate; Repayment mode shows the effect of extra principal; Projection mode illustrates how borrowing and capitalized school or grace-period interest can change the balance before repayment starts. Federal program rules may produce a different payment or forgiveness path.
Common mistakes to avoid
Do not treat loan amount per year as the total degree cost. In projection mode, specify whether school interest is paid because unpaid interest is capitalized by this engine. In repayment mode, keep extra yearly and one-time amounts separate from the regular monthly payment.
Assumptions and limitations
The route does not model federal income-driven formulas, capitalization exceptions, subsidies, forgiveness, servicer rules, deferments, fees, or changing interest rates. The simple rate solver only returns a result object built around the entered payment and does not surface the solved rate in its output.