To calculate a HELOC payment, you use one of two formulas depending on where you are in the loan. During the draw period, the minimum payment is interest only: multiply the outstanding balance by the annual interest rate and divide by 12. During the repayment period, the payment includes principal and is set by the standard amortization formula, which sizes each payment so the balance reaches zero by the maturity date. Because most HELOCs carry a variable rate, both calculations have to be rerun whenever the rate or balance changes.
Numbers You Need Before You Start
Every input lives in your loan agreement, your most recent statement, or your online banking portal. Pull these before you open a calculator:
- Outstanding balance: what you’ve drawn and not yet repaid.
- Index rate: the benchmark your lender uses. Most HELOCs track the U.S. prime rate, which sat at 6.75% as of March 2026. Some older lines reference SOFR or a Treasury index.1Board of Governors of the Federal Reserve System. H.15 – Selected Interest Rates (Daily)
- Margin: a fixed percentage the lender adds to the index, commonly 1% to 2%. It doesn’t change over the life of the line.2eCFR. 12 CFR Part 226 – Truth in Lending (Regulation Z)
- Current phase: draw period (typically 5 to 10 years) or repayment period (typically 10 to 20 years after that).
- Remaining term: months left before the balance must be paid in full.
- Lifetime rate cap and floor: the highest and lowest rates the lender can charge. The maximum must be disclosed in your agreement; many lenders also set a floor.2eCFR. 12 CFR Part 226 – Truth in Lending (Regulation Z)
Your interest rate at any given moment is the index plus the margin. If prime is 6.75% and your margin is 1.5%, you’re paying 8.25%. The margin and the cap are locked in the original agreement. Everything else moves.
Calculating a Draw-Period Payment
During the draw period, the minimum payment covers only the cost of borrowing. The formula:
Monthly payment = (Balance × Annual interest rate) ÷ 12
Say you’ve drawn $50,000 and your current rate is 8.25%. Multiply $50,000 by 0.0825 to get $4,125 in annual interest. Divide by 12. Your monthly interest-only payment is $343.75.
That payment keeps the account current but doesn’t touch principal. You’ll owe the same $50,000 next month unless you voluntarily pay more. Rerun the math whenever the balance changes because you drew more or paid extra, or whenever the rate adjusts. A $10,000 additional draw at the same rate raises the payment to $412.50 immediately.
Missing an interest-only payment can trigger late fees, and if the delinquency persists the lender can freeze further draws or declare the loan in default. The payment feels small compared to what’s coming in the repayment period, which is exactly why paying more than the minimum now pays off later.
Calculating a Repayment-Period Payment
When the draw period ends, you can no longer borrow against the line, and the payment jumps because it now includes principal. The lender resets the payment so the entire balance amortizes to zero by the maturity date. The standard amortization formula:
M = P × [ r (1 + r)n ] ÷ [ (1 + r)n – 1 ]
- M = monthly payment
- P = outstanding principal balance
- r = monthly interest rate (annual rate ÷ 12)
- n = total number of remaining monthly payments
Using the same $50,000 balance at 8.25% with a 20-year repayment period (240 months): the monthly rate is 0.0825 ÷ 12 = 0.006875. Plug that in and the monthly payment comes to roughly $429. Compare that to the $343.75 you were paying during the draw period. If the repayment term is only 10 years (120 months), the payment jumps to about $614.
The exponents make this tedious by hand, and an online amortization calculator is the practical tool. But knowing the formula lets you spot errors on your statement. Plug in your balance, rate, and remaining months, then compare against what your lender is charging. If the numbers don’t line up, call your servicer before the discrepancy compounds.
Federal regulations require your lender to notify you at least 15 days before the minimum payment increases.2eCFR. 12 CFR Part 226 – Truth in Lending (Regulation Z) That is not much runway. Borrowers who handle the transition well tend to run the amortization formula a year or two before the draw period ends and start adjusting their budget early.
Rerunning the Math After a Rate Change
Most HELOCs carry variable rates, so the payment can shift every time the index moves. After a Federal Reserve rate change, most borrowers see the impact within one or two billing cycles. The recalculation process is the same in either phase: take the new rate, apply the correct formula, and you have your updated payment.
Draw Period
If prime rises from 6.75% to 7.00%, your total rate goes from 8.25% to 8.50%. On a $50,000 balance, the interest-only payment goes from $343.75 to $354.17. That quarter point adds about $10 a month. Scale to a $150,000 balance and the same move costs an extra $31 every month.
Repayment Period
The same rate change has a bigger effect here because the amortization formula is more sensitive to the rate input. On a $50,000 balance with 15 years remaining, a jump from 8.25% to 8.50% pushes the monthly payment up by roughly $7 to $8. Rates rarely move just once, though. Three quarter-point increases in a year can add $20 to $25 per month on a relatively small balance.
The way to stay ahead of these shifts is to watch the index your HELOC is tied to. If the Federal Reserve signals upcoming moves, you have a window to recalculate and prepare before the new payment posts.
Testing the Worst Case Against the Rate Cap
Every HELOC agreement discloses a lifetime rate cap, the absolute highest rate the lender can charge. Many lenders set this at 18%, though yours may differ.3U.S. Bank. Home Equity Rate and Payment Calculator Some agreements also set a floor, often 3% to 4%. Both figures appear in your original loan documents.
Running the worst case through both formulas tells you whether the HELOC stays affordable under extreme conditions. Take your current balance, plug the lifetime cap into the amortization formula, and calculate. On a $50,000 balance with a 10-year repayment term at 18%, the monthly payment would be roughly $901. If that number would break your budget, it is worth paying down principal now or asking your lender about a fixed-rate conversion.
The floor rate matters less for budgeting, but it explains why your payment may not drop as far as expected when the index falls. If your floor is 4% and the index plus margin produces 3.75%, you will still be charged 4%.
How Extra Payments Change the Math
Because draw-period payments cover only interest, every extra dollar goes straight to principal. That shrinks the balance, which immediately lowers next month’s interest-only payment. More importantly, it reduces the balance that gets amortized when the repayment period starts, meaning a smaller payment shock down the road.
Extra payments during the repayment period also compress the remaining principal and cut total interest. One practical approach: if rates drop and your payment decreases, keep paying the old higher amount. The difference chips away at principal without changing your budget. When rates climb again, you’re working from a smaller balance and the increase hurts less.
Most HELOCs have no prepayment penalty, but verify this in your agreement before making a large lump-sum payment. For borrowers still in the draw period, extra principal is the single easiest way to control what the repayment period will cost.