Guide · Financial Calculator

Student loan calculator — monthly payment & repayment guide

Student loans are a classic Time Value of Money problem: you borrow a lump sum today and repay it with interest over a fixed period. Understanding the numbers behind your loan — not just the monthly payment, but the total interest you'll pay — puts you in a much stronger position to manage your debt or pay it down faster.

This guide walks through how to calculate your monthly payment, compare repayment terms, and model accelerated payoff using Pebble's free financial calculator. The same TVM approach works for federal and private student loans alike.

What each TVM field means for student loans

Switch Pebble to Financial mode (tap the mode button and select 💰). Here's how to map your loan details to the five fields:

• N — Repayment term in months (e.g. 10 years = 120, 20 years = 240)
• I/Y — Annual interest rate (e.g. 6.5 for 6.5%)
• PV — Total loan balance you're repaying
• PMT — Monthly payment (what you want to find)
• FV — Enter 0 (the loan is fully paid off at the end)

Enter four of those values and tap CPT followed by the one you're solving for. The calculator handles all the amortisation math automatically.

Example: 10-year standard repayment

You graduate with $35,000 in federal student loans at a 6.5% annual interest rate. You enrol in the standard 10-year repayment plan. What will your monthly payment be, and how much interest will you pay in total?

Over 10 years you'll pay roughly $47,760 in total — about $12,760 in interest on top of the $35,000 principal. That's a manageable amount compared to longer repayment terms.

Example: 20-year extended repayment

Same $35,000 loan at 6.5%, but stretched over 20 years to lower the monthly burden. How much does extending the term actually cost you?

The monthly payment drops by $137, but the total interest rises to around $27,640 — more than double the 10-year figure. The extra decade of repayments costs you an additional $14,880 in interest alone. Lower monthly payments come at a real long-term cost.

Modelling accelerated repayment

How long to pay off with extra payments?

You can use the financial calculator in reverse to model accelerated repayment. Say you can afford $550 per month instead of $398 — enter that as PMT (negative), keep PV at $35,000 and I/Y at 6.5, set FV to 0, then tap CPT → N. The result tells you exactly how many months — and how many years of interest — you'll shave off your loan.

What if I refinance at a lower rate?

If you can refinance to a lower interest rate, enter your current balance as PV, the new rate as I/Y, your remaining term (in months) as N, and 0 as FV. Solve CPT → PMT to see your new monthly payment, then compare to what you're currently paying to quantify the monthly savings.

Note: refinancing federal loans into a private loan means giving up federal protections like income-driven repayment and potential forgiveness programmes. The interest savings need to outweigh those trade-offs for your specific situation.

Federal vs private loans: what changes in the calculation?

The TVM math is identical for federal and private student loans — the difference is in the available terms and rates. Federal loans have fixed rates set annually by Congress; private loans offer variable or fixed rates based on your credit profile. Whatever your rate and term, enter the numbers the same way. The calculator doesn't know or care whether the loan is federal or private.

If you have multiple loans at different rates, you can run the calculation separately for each and add the monthly payments — or model a consolidated loan at a blended average rate for a simplified overview.

See also: mortgage calculator guide · compound interest calculator · financial independence guide

Frequently asked questions