Auto Loan Cosigner Calculator
A car loses value faster than the loan shrinks. See the months you'd be underwater — and what you'd owe after a repossession sale.
What you'd still owe after the car is sold
In month 18 the loan balance is $23,213.44 while the car is worth about $25,500.61. A repossession sale at 80% of that brings in $20,400.49 — leaving $2,812.96 unpaid. The lender can pursue you for that shortfall, plus repossession and collection costs, and you never had the car.
Monthly payment
$529.97
You're liable for every one of them.
Months underwater
0
The loan never exceeds the car's value.
Above water from
Month 1
When the car is finally worth more than the loan.
| Month | Loan balance | Car value | Gap |
|---|---|---|---|
| 0 | $29,000 | $32,000 | $3,000 |
| 6 | $27,162 | $29,328 | $2,167 |
| 12 | $25,234 | $26,880 | $1,646 |
| 18 | $23,213 | $25,501 | $2,287 |
| 24 | $21,095 | $24,192 | $3,097 |
| 30 | $18,873 | $22,951 | $4,077 |
| 36 | $16,544 | $21,773 | $5,228 |
| 42 | $14,103 | $20,655 | $6,553 |
| 48 | $11,542 | $19,596 | $8,053 |
| 54 | $8,858 | $18,590 | $9,732 |
| 60 | $6,044 | $17,636 | $11,592 |
| 66 | $3,094 | $16,731 | $13,637 |
| 72 | $0 | $15,872 | $15,872 |
Every sixth month shown. The highlighted row is the default month you picked.
The shortfall, worked
- Loan balance at month 18
- $23,213.44
- Car's market value
- $25,500.61
- Sale at 80% of value
- $20,400.49
= $23,213.44 − $20,400.49
Excludes repossession, storage and collection fees, which are added on top
How to use
- Enter the car, the deposit, and the loan terms you've been asked to cosign.
- Set the depreciation assumptions — they vary hugely by vehicle, so they're yours to choose, not ours to assert.
- Slide the default month. The headline is what the lender would come to you for after selling the car — the number that makes this different from cosigning anything else.
How it works
Cosigning a car loan carries a risk that cosigning anything else doesn't, and almost nobody explains it at the dealership: the car loses value faster than the loan gets paid down.
For a stretch of the loan — longer with a small deposit and a long term — the balance is bigger than the car is worth. If the borrower stops paying inside that window, the lender repossesses the car, sells it for less than is owed, and comes after you for the difference. You pay for a car you never drove and cannot now touch.
This calculator runs the loan balance B(m) = P(1+r)^m − M((1+r)^m−1)/r against the car's declining value month by month, so you can see both the window and the number at the end of it. For the debt-to-income side of cosigning, use our general cosigner risk calculator as well.
Sources
- CFPB — Should I agree to co-sign someone else's car loan?
- CFPB — My car was repossessed and I was told it will be sold. What can I do?
- CARFAX — Car depreciation: how much value does a car lose per year?
- Experian — How much do cars depreciate per year?
- Investopedia — Amortization (payment and balance formulas)
Frequently asked questions
What happens to a cosigner if the car is repossessed?
The car is sold, usually at auction and usually for less than the balance owed. The difference — the deficiency — is still owed, and the lender can pursue the cosigner for it directly, along with repossession and collection costs. You end up paying for a car you never drove and no longer have any claim to. That deficiency is the number this calculator exists to show you.
What does it mean to be underwater on a car loan?
It means the loan balance is bigger than the car is worth — also called negative equity. New cars typically lose a large share of their value in the first year while the loan has barely moved, so most borrowers spend an early stretch of the loan underwater. Long terms and small deposits both make that stretch longer and deeper, which is exactly when a default hurts a cosigner most.
How long am I on the hook?
Until the loan is paid off, refinanced in the borrower's name alone, or the lender releases you — and release is rarely automatic. Assume you are liable for the entire term unless something specific changes that, and read the agreement for whether a release option exists at all before you sign.
Can I get off a car loan I already cosigned?
Usually through one of three routes: the borrower refinances in their own name (the cleanest and most reliable), the car is sold and the loan cleared, or the lender has a cosigner-release programme and approves you for it. If the borrower's credit has improved since the original loan, refinancing is the exit worth pursuing first.
Does cosigning a car loan affect my credit?
Yes. The loan appears on your credit report as your own obligation, its payment counts against your debt-to-income ratio when you apply for other credit, and any missed payment lands on your report exactly as if you'd missed it yourself.
Is there a safer way to help than cosigning?
Often. Gifting toward a larger deposit reduces the loan they need and can turn a decline into an approval, with no ongoing liability for you. A cheaper car financed alone is usually a smaller problem than an expensive one financed together. And a few months of credit-building can move someone from needing a cosigner to qualifying on their own.
Related tools
- Cosigner Release CalculatorTrack your progress toward release — and weigh it against refinancing, given that a 2015 CFPB review found ~90% of release applications on private student loans were rejected.
- Cosigner Loan Risk CalculatorSee the monthly payment you'd be liable for, your before-and-after debt-to-income ratio, and the mortgage buying power you'd give up — before you co-sign.
- Medical Bill Payment Plan CalculatorCompare a 0% hospital plan, a deferred-interest medical credit card, and a personal loan — including the retroactive interest charge if you miss the promo.
- Airbnb Profit CalculatorMonth-by-month seasonal cash flow for short-term rentals — peak profits, off-season losses, and the cash buffer you need, not a flat vacancy rate.
This tool is an educational estimate, not financial or legal advice. Depreciation and repossession-sale figures are assumptions you set, not values we assert; deficiency balances, fees and your rights after a repossession vary by lender and by state. Read the loan agreement before you sign it.