Some homeowners run their income and expenses through a first-lien home equity line of credit instead of a checking account, so interest is only charged on a lower balance most of the time. See what that actually does to your payoff time and your total interest, using your own numbers.
Adjust anything below. Everything updates as you go.
Include everything that leaves the account in a typical month: bills, groceries, gas, subscriptions, dining, entertainment, and other discretionary spending. Leave out only the mortgage payment.
Total interest saved over the life of the loan, HELOC method vs. traditional mortgage
Over the life of this loan, running your income and expenses through a HELOC instead of a traditional mortgage could save $168,005 in interest and pay the balance off 19y 3mo sooner.
In addition to the interest savings, you'd save approximately $386,627 due to the ability to pay off your loan early, since you'd no longer be making mortgage payments during those years.
How each loan's balance declines, month by month.
You make one fixed payment a month. Interest is charged on the full remaining balance, and it only drops by whatever principal that month's payment includes. Any extra cash left over after bills sits in a checking account earning little or nothing.
Your paycheck lands in the HELOC and immediately lowers the balance interest is charged on, days before your bills go back out. There's no separate mortgage payment. Whatever's left over after expenses each month permanently reduces the balance, even though the rate is higher.
This is an illustration, not a loan estimate. It assumes a steady income and expense pattern every month, an average HELOC rate, since most first-lien HELOCs carry a monthly adjustable rate, no annual or draw fees, and the discipline to run every dollar through the line without spending the surplus elsewhere. Real first-lien HELOC products, credit limits, qualification, and rates vary by lender, and HELOC rates are usually variable, so payments can rise. A HELOC is secured by your home, so a gap in income is riskier here than with a fixed payment. Run your actual numbers with a loan officer before making a change like this.
Both a mortgage and a first-lien HELOC charge you interest. The difference is when they check your balance to figure out how much interest to charge. That one difference is the whole reason this strategy saves money.
Your lender looks at your balance one time each month and charges interest based on that number. It doesn't matter if you had extra cash sitting in the bank for two weeks before your bills were due. That money didn't lower your balance in the lender's eyes, so it didn't save you anything.
Balance only changes once a month, on payment day. Interest for the whole month is already locked in before that.
A HELOC adds up interest daily, based on whatever the balance actually is that day. When your paycheck lands, the balance drops right away, so interest for those days is calculated on a smaller number. When bills go out, the balance goes back up. It moves up and down all month, and interest follows it closely.
Balance drops on payday and climbs back up as bills go out. Every day spent lower means less interest that day.
Here's the second piece. Early in a 30-year loan, you owe the most money you will ever owe. Your payment is fixed, so most of it has to cover interest on that big balance, and only a small piece actually lowers what you owe. As years pass and the balance shrinks, more of each payment starts going toward principal instead. This is called amortization. It isn't a trick, it's just math: a fixed payment against a bigger balance is mostly interest, and against a smaller balance it's mostly principal. The chart below shows this using the numbers you entered above. Notice how much of each early payment is the darker, interest portion, and how that shrinks over time.
Put together, this is the whole idea: a HELOC charges interest on a balance that moves every day instead of a balance that's locked in once a month, and it lets extra money attack principal right away instead of waiting years for the amortization schedule to catch up.
Like any financial strategy, this one has real upsides and real trade-offs. Here's both sides, plainly.