How a HELOC credit line is sized
A home equity line of credit is a revolving loan secured by your home, second in priority behind the first mortgage. Lenders size it using the combined loan to value ratio, which is all debt secured by the property divided by the property value. The available line is the value times the maximum CLTV, minus what you already owe: maximum line = home value * max CLTV - first mortgage balance.
With the calculator defaults, a 500,000 home, a 280,000 mortgage balance, and an 85 percent maximum CLTV, the arithmetic is 500,000 * 0.85 = 425,000, minus 280,000, giving a maximum credit line of 145,000. If the draw amount you enter exceeds that, the calculator caps the amount it models at the maximum and flags the input, since no lender would advance more.
The maximum CLTV field exists because lenders differ. Eighty to eighty five percent is common, some go to 90 percent for strong credit profiles, and a few offer more at higher rates. The home value used by the lender comes from an appraisal or an automated valuation, not from your estimate, so a line sized on an optimistic value may not be approved.
Draw period and repayment period, and why the payment jumps
A HELOC has two phases. During the draw period, commonly ten years, you can borrow and repay freely and the required payment is usually interest only on the outstanding balance. During the repayment period that follows, commonly ten to twenty years, no further borrowing is allowed and the balance amortizes to zero.
This calculator models the interest only draw payment as balance times the monthly rate, and holds the balance constant for the whole draw period, which is what happens if you draw once and make only the minimum payment. The repayment payment uses the standard amortization formula on the same balance over the repayment months: payment = B * i * (1 + i)^n / ((1 + i)^n - 1).
The jump between the two is the feature most borrowers underestimate. It is sometimes called payment shock, and it arrives on a fixed schedule known at signing, so it is entirely predictable from the terms.
Worked example: drawing 50,000 at 9 percent
Using the defaults of a 9 percent rate, a 50,000 draw, a ten year draw period, and a twenty year repayment period, the monthly rate is 0.09 / 12 = 0.0075. The interest only payment during the draw period is 50,000 * 0.0075 = 375.00 a month. Over ten years of interest only payments, that is 375 * 120 = 45,000.00 in interest, with the balance still at 50,000 when the draw period ends.
Repayment then amortizes 50,000 over 240 months at 9 percent, giving a monthly payment of 449.86. Total paid during repayment is 107,967.11, of which 57,967.11 is interest. Adding the two phases, total interest is 102,967.11 and total cost is 152,967.11 on a 50,000 draw.
The comparison worth making is against paying down principal during the draw period. If instead you paid 449.86 a month from the start, the balance would amortize over thirty years rather than sitting flat for ten, and lifetime interest would be far lower. The interest only minimum is a minimum, not a schedule.
What this calculator assumes and what it omits
The largest assumption is a fixed rate. Most HELOCs carry a variable rate tied to the prime rate plus a margin, and prime moves with the federal funds rate. A line taken at 9 percent can be at 7 or at 11 within a couple of years. The calculator holds your entered rate constant for the entire draw and repayment period, so treat every figure as a snapshot at that rate rather than a forecast.
It also assumes a single draw at the start that is never repaid during the draw period and never added to. Real usage is revolving, with balances that move up and down, and the interest cost follows the actual balance day by day. If your intended usage is a single lump sum, such as a renovation, the model is a reasonable fit; if you plan to use it as a rolling facility, it is not.
Costs are excluded. HELOCs may carry application fees, appraisal fees, annual fees, inactivity fees, and early closure fees, and some lenders charge for converting a balance to a fixed rate. Closing costs are often waived on the condition that the line stays open for a minimum period. None of these appear in the total cost figure.
The mistake people make with a HELOC
The main mistake is treating the interest only payment as the cost of the loan. During the draw period, interest only payments retire no principal at all, so after ten years and 45,000 paid on the example above, the borrower still owes the full 50,000. The money was not wasted, it paid for the use of the funds, but nothing was repaid.
The second mistake is forgetting that the loan is secured by the home. Unlike a credit card, a HELOC that goes unpaid can lead to foreclosure. Consolidating unsecured debt into a HELOC lowers the interest rate and converts a debt the lender cannot seize your house over into one it can.
The third is assuming the line stays available. Lenders can freeze or reduce a line if the property value falls or your credit profile deteriorates, and many did so during the 2008 to 2010 period. A line that is part of an emergency plan may not be there when the emergency arrives.