How much you can borrow mainly depends on how much your home is worth and how much you still owe on your mortgage. Most lenders use a formula called the “combined loan-to-value ratio” to set your limit. Generally, lenders will let you borrow 80% to 85% of your home's total value, including your current mortgage.
For example, if your home is worth $400,000 and your lender has an 85% limit, they’ll allow your total debt to reach $340,000. So if you still owe $250,000 on your mortgage, you could qualify for a line of credit up to $90,000.
Just keep in mind that your credit score and monthly income also play a role. If your debt-to-income ratio is too high — usually over 43% — a lender might offer you a smaller amount to make sure you can comfortably handle the monthly payments.