Monster Loan Calculator
Most loan calculators only solve for the payment. This one solves for whichever number you are missing — payment, amount, term or rate — and shows what extra payments do to your payoff date.
How it works
The loan payment formula is M = P × r(1+r)n / ((1+r)n − 1) with P the amount, r the monthly rate and n the number of payments. Solving for the amount or term rearranges the same formula; solving for the rate is done by testing rates until the payment matches yours exactly. Extra payments are applied to principal every month, which shortens the loan and cuts total interest — the table shows both versions side by side.
Formula & worked example
Formula: M = P × r(1+r)n ÷ ((1+r)n − 1). The same formula rearranges to solve for the amount, the term, or (by testing rates) the interest rate.
Worked example: Borrow $20,000 at 7% over 5 years and the payment is $396.02/month, with $3,761 of total interest over the life of the loan.
When to use this
- Any loan offer — bank, dealer, online lender — that you want reduced to payment, total interest and payoff date.
- You know the payment you can afford and need the amount or rate that fits (the solver works backwards).
- You are weighing a longer term's smaller payment against its larger lifetime interest.
- You want to see what an extra monthly payment does before you commit to it.
How to use it, step by step
- Pick what to solve for — amount, rate, term or payment.
- Fill the three values you know from the offer.
- Add an extra monthly payment if you plan to pay ahead.
- Read the payment, total interest and payoff timeline together — never the payment alone.
- Re-run at the next term up or down; the interest gap is the real price of the smaller payment.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Loan amount | Principal borrowed, before interest | Any — $1,000 to $1,000,000+ |
| Interest rate | Annual rate (APR-style) | Cards 20%+, auto 5–12%, personal 8–20% |
| Term | Repayment length | 1–7 years typical, 30 for mortgages |
| Extra payment | Optional amount added every month | $25–$200 changes payoff dates fast |
Another worked example
A $5,000 personal loan at 12% over 24 months costs $235.37 a month and $648.82 of interest in total — about 13 cents of interest per dollar borrowed. Stretch the same loan to 48 months and the payment drops, but you would pay roughly double the interest; the term, not the rate, is usually the expensive dial.
Frequently asked questions
How do I find the interest rate on my loan?
Choose "Interest rate" under Solve for, enter the loan amount, term and the monthly payment from your statement, and the calculator backs out the rate that produces that payment.
How much do extra payments really save?
A lot, because every extra dollar goes straight to principal. On a $30,000, 6%, 5-year loan, an extra $100 a month pays the loan off about 10 months early and saves roughly $500 of interest — try it above.
What is the difference between APR and interest rate?
The interest rate drives the monthly payment. APR folds in most lender fees as well, so APR is the better number for comparing two loan offers. This calculator uses the interest rate.
Can this calculator handle a 0% loan?
Yes. With a 0% rate the payment is simply the amount divided by the number of months, and total interest is $0.
🗓️ Want the month-by-month picture?
The Amortization Calculator lays out every payment — principal, interest and balance — for the whole loan.
What does 'solve for' mean on this calculator?
Most calculators only find the payment. This one rearranges the loan equation so any one unknown — amount, rate, term or payment — can be the answer, using the other three as givens. It is the same formula banks use, run in whichever direction your question points.