Equity is your car's market value minus what you still owe on it. Worth $18,000 with a $14,000 payoff, you have $4,000 of positive equity: money you own. Worth $18,000 owing $21,000, you have $3,000 of negative equity: you are 'upside down,' and that shortfall follows you into your next deal if you trade now.
Positive equity is a down payment you already own: it shrinks your next loan, can improve your rate band, and in trade-in credit states it also cuts your sales tax. Negative equity does the reverse: rolled into a new loan it raises your amount financed and loan-to-value, which can raise the rate and even push a deal outside lender parameters.
It also matters if the car is totaled: insurance pays market value, not your payoff. Negative equity is exactly the gap that GAP coverage exists for.
Yes, it happens every day: the shortfall is either paid in cash or added to the new loan. Rolling it works when the new deal has room, but it raises your payment and starts the next loan deeper underwater, so treat it as a last resort, not a habit.
Request a payoff quote from your current lender, then get a market value for your car. Market value minus payoff is your equity, positive or negative.
In trade-in credit states, the full trade-in allowance reduces your taxable price, which stacks with the equity lowering your loan. See our state-by-state trade-in tax guide.