Extra Payment Calculator: What +$100 a Month Really Saves
An extra payment calculator answers a simple question: if you pay a little more than the required amount every month, how much sooner is the loan gone and how much interest do you skip? Enter your balance, APR and current payment below, then try different extra amounts. The sample is a $25,000 car loan at 7.5% APR with a $500 payment and $100 extra.
How extra payments hit principal
Every regular payment is split in two. First the lender takes the interest that built up since the last payment; whatever is left reduces the principal, the amount you actually owe. Interest for the next month is then charged on the smaller principal.
Here's the split in the first month for three loans:
| Loan | Payment | To interest | To principal |
|---|---|---|---|
| $300,000 mortgage at 6.5%, 30 years | $1,896.20 | $1,625.00 | $271.20 |
| $25,000 car loan at 7.5% | $500.00 | $156.25 | $343.75 |
| $6,000 credit card at 24% | $180.00 | $120.00 | $60.00 |
Look at the credit card: two thirds of the payment goes to interest. The mortgage is similar in its early years, which is why a 30-year loan barely moves at first.
An extra $100 is different. The interest for the month is already covered by the regular payment, so every dollar of the extra goes straight to principal. Less principal means less interest next month, which means more of next month's regular payment goes to principal too. That compounding is why a small extra amount shortens a loan by months or years.
There's one catch: the extra has to actually be applied to principal. With installment loans such as mortgages, car loans and student loans, some servicers treat an extra amount as an early payment toward next month's bill, or hold it until it adds up to a full payment. Look for an option like "apply to principal" or "additional principal" when you pay online, or call and ask how extra payments are handled. Then check your next statement to confirm the principal went down by the extra amount.
Mortgage vs car vs card
The same $100 behaves very differently depending on the rate and the size of the loan.
- Credit card. High APR and a balance you can realistically clear, so the extra has the biggest effect relative to the balance. Cards also have no prepayment penalties, and the extra goes to the balance automatically. For how long a card takes at different payments, see how long it takes to pay off a credit card.
- Car loan. A mid-range rate and a short term. The savings are real but modest, and the main payoff is freeing up the monthly payment sooner.
- Mortgage. A lower rate, but a huge balance and a very long term. $100 a month is a small share of the payment, yet over decades it saves the most in dollars. The trade-off is time: the money is locked in your home until you sell or refinance.
Example table
Here's what +$100 a month does for each loan. The mortgage payment covers principal and interest only; taxes and insurance paid through escrow aren't included.
| Loan | Without extra | With +$100 a month | Interest saved | Paid off sooner |
|---|---|---|---|---|
| Mortgage: $300,000, 6.5%, $1,896.20 | 360 months, $382,633.47 interest | 312 months, $321,638.68 | $60,994.79 | 48 months |
| Car loan: $25,000, 7.5%, $500 | 61 months, $5,069.24 interest | 49 months, $4,051.02 | $1,018.22 | 12 months |
| Credit card: $6,000, 24%, $180 | 56 months, $3,986.50 interest | 29 months, $1,913.24 | $2,073.26 | 27 months |
Three things stand out:
- The card nearly halves its payoff time. At $180 a month, $120 of the first payment is interest. Adding $100 lifts the principal portion from $60 to $160, and the payoff drops from 56 months to 29.
- The mortgage saves the most in dollars, but over a long horizon. You'd add $31,200 in extra payments over 312 months and avoid $60,994.79 in interest, four years earlier.
- The car loan is the smallest win. $4,900 in extra payments over 49 months saves $1,018.22.
A quick way to compare: each extra dollar earns a guaranteed return equal to the loan's APR. Paying down a 24% card works like a 24% return; paying down a 6.5% mortgage works like 6.5%. If you have several debts, the extra should usually go to the highest rate first, and our debt payoff plan in 5 steps shows how to order them.
Pay off car loan early — worth it?
Often yes, but it depends on three things.
How your loan charges interest. Most car loans are simple interest: interest is figured on the balance you owe, so paying it down early directly cuts what you pay. Some loans, typically older or subprime ones, use precomputed interest, where the interest is set up front and paying early saves less. Your contract or your lender can tell you which one you have.
Prepayment penalties. Many car loans have none, but some contracts do. Check your contract before you start sending extra.
Your rate compared with your other options. At 7.5% APR, $100 extra saves $1,018.22 over the life of our sample loan. With a 2.9% promotional rate and a $450 payment, the same $100 a month saves only $363.10 and brings the payoff forward by 11 months. At a rate that low, the money may do more elsewhere:
- Higher-rate debt first. A credit card at 24% is a much better target than a car loan at 7.5%.
- An emergency fund. A car you've paid down early won't cover a surprise bill. Many people keep a cash cushion before sending extra to a low-rate loan.
- An employer 401(k) match. If your employer matches contributions, getting the full match usually beats extra payments on a low-rate loan.
The same logic applies to a mortgage. Before adding principal payments, make sure your high-rate debt is gone, your emergency fund is in place and you're getting any 401(k) match. We weigh this in more detail in pay off debt or save. One more mortgage detail: if you pay private mortgage insurance, extra principal can get you to the equity level where you can ask your lender to cancel it sooner. Ask your servicer about their rules.
Calculator
The calculator at the top of the page works for any single fixed-payment loan: a car loan, personal loan, student loan or a credit card where you pay the same amount every month.
- Balance: what you owe today, from your latest statement.
- APR: the annual rate on the loan.
- Monthly payment: your current regular payment. For a mortgage, use the principal and interest part only.
- Extra per month: the amount you plan to add every month.
- One-time extra: a lump sum paid now, like a tax refund or bonus. It can be combined with the monthly extra.
The results show the payoff time and interest without the extra, with it, and the difference. The calculator charges APR divided by 12 on the balance each month; lenders accrue interest daily, so your statements may differ by a few dollars.
If you have more than one debt, the full DelDebt planner takes it further: it puts all your debts in one plan, shows where an extra payment saves the most, and previews a lump sum before you send it, with the choice to shorten the term or lower the payment. It's free, and you can use it without an account.