Every loan calculator asks one question. This one asks the other.

Type “loan calculator” into Google and you’ll get a hundred tools that all do the same thing: tell the borrower what their monthly payment will be. None of them answer the question that actually matters to a cosigner — what happens to your money, your credit, and your next mortgage if the person you’re vouching for stops paying. That gap is the whole reason this tool exists.

And it’s not a rare situation. A 2017 survey of parents who had cosigned private student loans, run by the lending-research firm LendEDU, found that 65.8% had already made at least one payment on the borrower’s behalf, 56.8% had watched their own credit score drop as a result, and 35% said outright that they regretted signing. Separately, the Consumer Financial Protection Bureau reviewed co-signer release applications across the industry and found that roughly 90% were rejected — meaning most cosigners who try to get their name off the loan can’t, even years later. Most lenders do offer a release option in principle, usually after a set number of consecutive on-time payments, but given that rejection rate, refinancing the loan into the borrower’s name alone tends to be the more realistic path off it than waiting on a release application to be approved.

Why your “yes” is legally identical to theirs

When you cosign, the lender doesn’t see a backup plan. It sees a second full borrower. This is called joint-and-several liability, and it means the lender can come after you for 100% of the balance — not half, not “your share” — without exhausting collection efforts against the primary borrower first. Lenders aren’t required to warn you before reporting a missed payment either; the first you might hear about a problem could be a drop in your own credit score.

A worked example, with real numbers

Say you cosign a $20,000 auto loan at 8.5% APR over 5 years. The monthly payment comes to $410.33. A year in, the loan still has a remaining balance of $16,647 — that’s the number that matters if things go wrong, not the original $20,000.

That’s a swing from a “minor inconvenience” moderate case to a five-figure worst case — on a loan that was only ever $20,000. The collection and legal fees alone can add more than the original car was worth.

The hit you don’t see coming: your own debt-to-income ratio

Here’s the part most people miss entirely. Even if the borrower pays every installment on time, the loan still counts as your debt the moment you sign. In the example above, a cosigner earning $5,500 a month with $900 in existing debt has a debt-to-income ratio of 16.4% before cosigning. Add the $410 payment and that ratio jumps to 23.8% — instantly, with no missed payment required. Mortgage lenders generally treat 43% as the line where qualifying for a Qualified Mortgage gets difficult, so this loan alone won’t sink most people, but it eats into the room you have for anything else: a car, a renovation loan, or the mortgage you’re planning next year.

The score drop is bigger if your credit is better

This sounds backwards, but FICO’s own published simulation data confirms it: a borrower with an excellent credit profile loses more points for the same missed payment than someone whose file already shows risk. For a cosigner starting at a 725 score, a 90-day late payment on the cosigned account could realistically drop them into the 623–643 range. The reasoning is simple — a clean file has more “room to fall,” because the scoring model had nothing else suggesting risk to offset against.

This calculator provides estimates based on published industry data and standard lending formulas. It is not legal or financial advice. Collection costs, court fees, and credit score impacts vary by state, lender, and individual credit file — talk to a financial advisor or attorney before making a cosigning decision.