Student loan debt follows you for years if you don't have a clear payoff plan. Calculating your timeline and total interest cost arms you with the information to choose the right repayment strategy.
The Basic Payoff Formula
For a standard fixed-payment loan:
Monthly Payment = P ร (r(1+r)^n) รท ((1+r)^n โ 1)
Where:
- P = principal balance
- r = monthly interest rate (annual rate รท 12)
- n = number of monthly payments
Example: $30,000 loan at 6.5% APR, 10-year standard plan:
- r = 0.065 รท 12 = 0.005417
- n = 120
- Payment = $30,000 ร (0.005417 ร 1.005417^120) รท (1.005417^120 โ 1) = $340.48/month
- Total paid = $340.48 ร 120 = $40,857
- Total interest = $40,857 โ $30,000 = $10,857
Federal vs Private Loans
| Feature | Federal | Private |
|---|---|---|
| Income-driven repayment | Yes | No |
| Loan forgiveness programs | Yes | No |
| Deferment/forbearance | Yes | Limited |
| Interest rate | Fixed by Congress | Variable or fixed, credit-based |
| 2024โ25 rates | 6.53%โ9.08% | Varies widely |
Always exhaust federal loan repayment options before considering refinancing.
Repayment Plan Comparison ($30,000 at 6.5%)
| Plan | Monthly Payment | Payoff Period | Total Interest |
|---|---|---|---|
| Standard (10 years) | $340 | 10 years | $10,857 |
| Extended (25 years) | $202 | 25 years | $30,609 |
| Graduated (starts low) | $191โ$326 | 10 years | $12,671 |
| SAVE (income-driven) | Varies | 20โ25 years* | May be forgiven* |
*Remaining balance after 20โ25 years on SAVE may be forgiven (potentially taxable).
Income-Driven Repayment (IDR) Calculation
Under SAVE (Saving on a Valuable Education), the current primary IDR plan:
Monthly Payment = (Discretionary Income ร 5%) รท 12 (for undergrad loans)
Discretionary income = Adjusted Gross Income โ 225% of federal poverty line
Example: $45,000 AGI, single, 48 contiguous states (2025 poverty line โ $15,060):
- 225% of poverty line = $33,885
- Discretionary income = $45,000 โ $33,885 = $11,115
- Monthly payment = ($11,115 ร 5%) รท 12 = $46.31/month
Note: SAVE plan is currently subject to litigation โ check studentaid.gov for the latest status.
Calculating Extra Payment Impact
Every dollar of extra principal payment saves you interest compounding forward.
Interest saved = Extra Payment ร r ร Remaining Months
(Approximate โ actual savings are higher due to reduced future compounding)
More precise: Use an amortisation calculator. A $100/month extra payment on the $30,000 example:
- Original payoff: 120 months, $10,857 interest
- With $100 extra: ~81 months, ~$6,900 interest
- Savings: ~$4,000 in interest, 3+ years earlier
Refinancing: When It Makes Sense
Refinancing federal loans into a private loan at a lower rate makes sense only if:
- You will not qualify for PSLF or other forgiveness
- You have stable income and don't need IDR flexibility
- You can get a rate at least 1โ2% lower than current
Warning: Refinancing federal loans permanently removes access to income-driven plans, forgiveness programs, and federal forbearance.
Public Service Loan Forgiveness (PSLF)
Work full-time for a qualifying employer (government, 501(c)(3) nonprofit) and make 120 qualifying payments (10 years) on an IDR plan โ remaining balance is forgiven tax-free.
Best strategy for PSLF: Minimise payments by choosing the lowest-payment IDR plan, maximise the forgiven amount.
Payoff Priority Order
- Capture full employer 401(k) match (guaranteed 50โ100% return)
- Build a $1,000 emergency fund
- Pay minimums on all student loans
- For high-interest private loans (>7%): aggressively pay down
- For federal loans at <6%: consider investing the difference instead
Use our loan payment calculator to model different payment scenarios and find your optimal payoff timeline.