Calculating a HELOC involves three separate pieces of math: how much you can borrow, what you’ll pay each month during the draw period, and what your payments jump to during the repayment period. Each calculation is straightforward once you know the inputs.
How Much You Can Borrow
Lenders use a metric called the combined loan-to-value ratio (CLTV) to determine your maximum credit line. Most lenders cap this at 80% to 85% of your home’s appraised value, though some go as high as 90%. The formula works like this:
Maximum HELOC = (Home value × CLTV limit) − Remaining mortgage balance
Say your home appraises at $400,000, you owe $250,000 on your mortgage, and the lender allows up to 85% CLTV. Multiply $400,000 by 0.85 to get $340,000. Subtract your $250,000 mortgage balance, and your maximum HELOC credit line would be $90,000. If the lender only allows 80% CLTV, that drops to $70,000.
Your actual credit line may come in below the maximum. Lenders also factor in your credit score, debt-to-income ratio, and income stability. But the CLTV calculation sets the ceiling.
How Your Interest Rate Is Set
Most HELOCs carry a variable interest rate built from two components: an index and a margin. The index is almost always the prime rate, which moves with the Federal Reserve’s decisions. The margin is a fixed percentage the lender adds on top, typically ranging from 1% to 3% or more depending on your creditworthiness.
If the prime rate is 6.75% and your lender’s margin is 3%, your HELOC rate would be 9.75%. When the prime rate rises by 0.25%, your rate climbs to 10%. When it falls by 0.25%, your rate drops to 9.5%. The margin never changes, so your rate always moves in lockstep with the index. This matters for every payment calculation below, because the numbers will shift as rates move.
Monthly Payments During the Draw Period
A HELOC typically has a 10-year draw period followed by a 20-year repayment period, though these terms vary by lender. During the draw period, most lenders require only interest payments on whatever balance you’ve borrowed. You can draw funds, repay them, and draw again up to your credit limit.
The formula for your monthly interest-only payment is simple:
Monthly payment = Outstanding balance × Annual interest rate ÷ 12
If you’ve drawn $45,000 on a HELOC with a 9.75% rate, your monthly payment would be $45,000 × 0.0975 ÷ 12, which equals $365.63. If you only drew $20,000, the payment drops to $162.50. This recalculates each billing cycle based on your current balance and current rate, so the amount you owe can change month to month even if you don’t borrow more.
Some lenders require a small principal payment during the draw period as well, often 1% to 2% of the outstanding balance. Check your loan agreement to see which structure applies.
Monthly Payments During Repayment
Once the draw period ends, you can no longer borrow against the line. Your remaining balance converts to a standard loan that you repay with principal and interest over the remaining term, usually 20 years. This is where many borrowers get surprised, because payments can increase significantly.
The standard amortization formula for a fixed monthly payment is:
Monthly payment = P × [r(1+r)^n] ÷ [(1+r)^n − 1]
In this formula, P is the principal balance at the start of repayment, r is the monthly interest rate (annual rate divided by 12), and n is the total number of monthly payments.
Using the same $45,000 balance at 9.75% over 20 years: the monthly rate (r) is 0.0975 ÷ 12 = 0.008125, and the number of payments (n) is 240. Plugging those in gives a monthly payment of roughly $425. That’s a noticeable jump from the $365.63 interest-only payment, and the gap would be even wider if your balance were higher or rates had risen during the draw period.
Keep in mind that if your HELOC still carries a variable rate during repayment, this payment amount will shift as the prime rate changes. Some lenders offer the option to lock a fixed rate on all or part of your balance before repayment begins.
Costs Beyond Interest
Interest isn’t the only expense. HELOCs can come with upfront and ongoing fees that affect your total cost of borrowing.
- Appraisal fee: Lenders typically require a formal property valuation, which can run $300 to $600.
- Application fee: Some lenders charge this upfront, and it may not be refunded if you’re turned down.
- Closing costs: These can include title search fees, attorney fees, mortgage filing charges, and title insurance. Some lenders waive closing costs entirely, while others charge amounts similar to a traditional mortgage closing.
- Annual fee: Some lenders charge a yearly maintenance fee whether or not you’re actively using the credit line. This often ranges from $50 to $100.
- Transaction or inactivity fees: A few lenders charge per-draw fees or penalize you for not using the line at all.
- Early termination fee: If you close the HELOC within the first few years, some lenders charge a cancellation penalty.
When comparing HELOC offers, add these costs to your interest projections. A lender offering a slightly higher rate but no closing costs and no annual fee could cost you less over time than a lower-rate lender that charges several hundred dollars in fees each year.
Putting the Full Calculation Together
To estimate the total cost of a HELOC from start to finish, work through each phase. Start with the amount you plan to borrow and when. Calculate interest-only payments for the draw period, keeping in mind that your balance may change as you draw funds or make voluntary principal payments. Then calculate the amortized payments for the repayment period based on whatever balance remains. Finally, add in all fees.
For a practical example: suppose you open a $60,000 HELOC, draw the full amount immediately, and make only interest payments for 10 years at an average rate of 8.5%. Your monthly interest payment would average $425. Over 10 years, that’s $51,000 in interest alone. Then you’d repay the $60,000 principal plus interest over 20 years, with monthly payments around $521 at the same rate, totaling roughly $124,920. Add $500 in closing costs and $75 per year in annual fees over 30 years ($2,250), and your all-in cost approaches $178,670 on a $60,000 credit line.
That total drops dramatically if you pay down principal during the draw period or repay the balance early. Even adding $100 per month toward principal during the draw phase can save tens of thousands in long-term interest.