How is this different from APR?

This calculator solves for the stated interest rate (periodic rate annualized), which equals APR only when there are no fees. If the original loan had origination fees, the APR is higher than the rate this calculator shows. APR includes fees; the interest rate doesn't. Use the APR calculator if you need to account for fees in your comparison.

Why might my calculated rate differ from what I thought I had?

Common reasons: the loan had points or fees added to the balance upfront (which raises effective rate), a variable rate has since changed, an adjustable-rate loan reset, or the original disclosure was for a different loan product than what was funded. If there's a significant discrepancy (>0.5%), contact your servicer for the exact current rate from your loan statements.

Can I use this to find the rate on a credit card?

Partially — but credit cards compound daily and have variable balances, which makes precise rate calculation from payment and balance alone unreliable. For credit cards, the APR is always disclosed on your statement. Use this calculator for installment loans (auto, mortgage, personal loan) with fixed payments and declining balances.

How accurate is the reverse-calculation?

Highly accurate for standard installment loans with fixed payments and no changes since origination. Less accurate if: you've made extra payments, the rate changed (ARM or variable), fees were added or waived, or there are escrow components in a mortgage payment. For a mortgage, use only the principal+interest portion of the payment, not the full PITI payment.

🏦Borrowing

Interest Rate Calculator: What Rate Are You Actually Paying?

What interest rate are you paying?

What This Does

You know your monthly payment. You know your loan balance. But do you know your actual interest rate? Many borrowers don't — especially if the loan was originated years ago, restructured, or came with fees that effectively raise the rate. This calculator works backwards: you enter your payment amount, current balance, and remaining term, and it computes your actual interest rate. This reverse-calculation is more useful than it might sound. If you have an older loan and can't find the original paperwork, this tells you the rate you're being charged. If you suspect your rate is wrong, this lets you verify it. If you're considering refinancing, this gives you your current rate to compare against new offers. And if you're evaluating a seller-financed or private loan, this is how you find the implied rate buried in the payment terms. The calculator also works in forward mode: enter a desired rate and see what payment it implies for your balance and remaining term. This is useful for comparing refinancing scenarios — if a lender offers a new rate, you can see immediately how much your payment would change and what you'd save in total interest over the remaining life of the loan. Both directions of this calculation are immediately useful for anyone managing debt or evaluating borrowing decisions.

When Should You Use This?

→You have a loan but can't find the original paperwork with your interest rate
→Verifying that a lender is charging the correct rate on your loan
→Evaluating a seller-financed or private-party loan to find the implied rate
→Comparing your current rate to current market rates before refinancing
→Calculating the effective rate on a buy-here-pay-here auto loan or rent-to-own arrangement

Example Scenario

James bought a car from a dealership 2 years ago with a $485/month payment on a 60-month loan. His current balance is $18,200 with 36 months left. He suspects his rate is high but lost the paperwork. The calculator reverse-solves: his implied interest rate is 12.4% APR. Current market rate for his credit profile: 7.5%. He refinances immediately, dropping his payment to $400/month and saving $3,060 total. The 30-second calculation paid off substantially.

Interest Rate Calculator

Find Your Rate · Calculate Payments · Refinance Analysis

Two modes: reverse-engineer your rate from a payment, or calculate payments from a rate. Results update in real time.

Mode

Monthly Payment ($)

Current Balance ($)

Remaining Months

Market Rate Benchmark (%)

About This Calculator

This interest rate calculator operates in two modes: Reverse mode solves for your actual effective interest rate from a monthly payment, current balance, and remaining months using Newton-Raphson numerical iteration — revealing the true cost of any loan even if you never saw the original rate. Forward mode calculates monthly payment, total interest, and total cost from a loan amount, rate, and term. All results update in real time as you type.

The Overview tab provides a bar chart comparing total interest cost at your rate versus the market benchmark rate — making the dollar cost of an above-market rate immediately visible. The Sensitivity tab plots a line chart of total interest across a ±3% range of rates, with reference lines at your current rate and the market rate, plus an interactive table showing every 0.5% increment. The Amortization tab shows a stacked bar chart of principal vs interest by year, demonstrating how amortization front-loads interest.

A rate score (0–100) benchmarks your rate against the market: higher scores indicate a more favorable rate. Risk flags appear automatically for rates significantly above market, very high absolute rates, and very long terms. The market rate benchmark field is customisable — enter the prevailing rate for your specific loan type (auto, personal, mortgage) for an accurate comparison. Export to PDF generates a complete report with the full sensitivity table and key insights.

Results are estimates only and do not constitute financial, tax, or legal advice. Consult a qualified professional before making financial decisions.