Federal and private principal channels with interest sediment and extra-payment diversionLive current inputs
Finance & Money
Student Loan Payment Calculator
Calculate federal and private student-loan payments, blended interest cost, payoff timing, and the effect of targeted extra payments.
Combined required monthly payment—
Planned payment with extra—
Projected payoff month—
Projected interest paid—
Interest saved by extra payment—
Months saved—
First-month interest—
First payment principal share—
Repayment river
Federal and private principal channels with interest sediment and extra-payment diversion
Two balance channels flow through the term at their own rates. Interest sediment narrows as principal falls, while the extra-payment diversion targets the higher-rate balance.
Live decision table
Student-loan amortization checkpoints
Track federal balance, private balance, cumulative interest, and principal share through repayment.
| Month | Federal balance | Private balance | Interest paid | Principal paid | Active extra target |
|---|
Cash-flow schedule
Annual and monthly cash-flow schedule
How to calculate
Keep loan groups separate by rate and terms
- Use current principal plus capitalizing interest.
- Enter fixed nominal APRs.
- Use the contractual repayment term.
- Set only an extra payment you can sustain.
Amortization method
One blended APR can distort payment allocation
Required payments are calculated for each group independently. The highest-rate-first extra payment reduces financing cost while normal payments continue on both groups.
Capitalization increases principal before repayment and therefore raises future interest.
Calculation method
Amortize each loan group independently and target extra cash to the higher rate
Each loan group has its own required amortized payment. Capitalized interest is allocated proportionally to starting balances. Monthly interest accrues on each active balance; extra payment targets the highest-rate loan, then rolls to the other group.
Detailed calculation process and general formulas
P_i = B_i r_i / [1-(1+r_i)^(-n)]I_im = B_i,m-1 × r_iPrincipal_im = P_im - I_imB_i,m = max(B_i,m-1 - Principal_im,0)Extra_m → active loan with highest APRSymbols, meanings, and units
- B_i
- opening balance of loan group icurrency
- r_i
- monthly interest rate of loan group idecimal/month
- n
- contract repayment monthsmonths
- P_i
- required payment for loan group icurrency/month
- I_im
- interest charged to group i in month mcurrency
- Principal_im
- principal reduction in month mcurrency
The live worked example substitutes current inputs in formula order and reconciles the headline result with the visual and decision table.
Loan protections
Do not refinance away valuable federal options casually
- Review income-driven repayment.
- Check forgiveness eligibility.
- Compare deferment protections.
- Confirm private-loan prepayment terms.
Payment execution
Tell the servicer how extra money should be applied
Extra payments should reduce principal rather than advance the due date when the goal is faster payoff. Verify allocation on the next statement.
Maintain emergency liquidity before committing an aggressive extra amount.
Payment anatomy
How the first payment becomes a faster payoff
The river links required payment, rate targeting, and interest avoided.
Required payment
—Sum of independently amortized group payments.
First-month interest
—Interest charged before principal reduction.
Payoff acceleration
—Months removed by the extra payment.
Interest avoided
—Difference from required-payment-only amortization.
Decision takeaway: Keep normal payments on every loan and direct sustainable extra cash to the highest rate.
Practical applications
Decisions this calculator is designed to support
Federal and private loan mix
A graduate has lower-rate federal loans and a smaller high-rate private loan.
What the result clarifies: The extra-payment diversion targets the private balance without blending away federal protections.
Capitalized grace-period interest
Unpaid interest is added before standard repayment begins.
What the result clarifies: The model shows its effect on payment and lifetime interest.
Worked example
Current-input substitution and reconciliation
Important note
The model assumes fixed rates, monthly compounding, and standard amortization. Income-driven payments, subsidies, forgiveness, fees, daily interest, and tax treatment require program-specific analysis.
Student Loan Payment Calculator FAQ
Why calculate each group separately?
Different rates produce different required payments and interest, even with the same term.
Where is capitalized interest assigned?
It is allocated proportionally across entered starting balances.
Does extra payment always target the private loan?
It targets whichever active group has the higher entered APR.
Are federal forgiveness benefits valued?
No. Review those benefits before refinancing or accelerating eligible debt.