HELOC Calculator: The Payment Step Nobody Budgets For

Introduction

A HELOC is not one loan. It is two loans glued together, and the seam between them is where the trouble is.

TL;DR: A HELOC pays interest only during the draw period, then amortises the whole balance over a much shorter term than a mortgage. The payment does not drift up at the changeover — it steps, typically by 40% or more, on a date fixed at closing. And because the rate is variable, the payment you are underwriting is a range rather than a number.

The two halves

The draw period, usually ten years. You may borrow up to your limit, repay, and borrow again. The required payment is interest only. Nothing is repaid unless you choose to repay it, which is exactly why most borrowers reach the end of the draw period owing precisely what they drew.

The repayment period, usually fifteen or twenty years. The line closes. The full balance now amortises over that term alone.

The step comes from the second half having to repay in fifteen years what the first half never touched. The rate has not changed. Nothing about the loan has gone wrong. Principal simply became due.

A worked example

A $500,000 property with a $300,000 first mortgage, an 85% combined LTV limit, and $100,000 drawn at 8.5%.

Line	Amount
Credit available	$125,000
Left after this draw	$25,000
Payment during the draw period	$708
Payment once repayment starts	$985
Payment step at changeover	$276
Combined LTV after drawing	80%
Total interest over both periods	$162,253

The payment rises 39% overnight, from $708 to $985, on a balance that never moved and a rate that never changed.

That is the benign version. Push the balance to the full $125,000 and the step is proportionally the same but larger in dollars. Add a rate rise — and the rate is variable — and the two effects compound.

Why the variable rate matters more here than on a mortgage

Most HELOCs are priced at prime plus a margin and adjust monthly. There is no fixed period. A borrower who takes a HELOC at 8.5% is not underwriting an 8.5% loan; they are underwriting whatever prime does over the next twenty-five years.

The sensitivity chart above prices the repayment payment across a range for exactly this reason. If the deal only works at today's rate, it is not a deal — it is a bet on the rate, and the lender has written the terms so the bet is yours.

What investors actually use a HELOC for

The interest-only draw period makes a HELOC well suited to short holds and badly suited to long ones:

• A BRRRR down payment or rehab budget, drawn at purchase and repaid from the cash-out refinance. The line is repaid inside the draw period, so the repayment step never arrives. This is the strongest use, and the BRRRR calculator models the exit that repays it.
• A bridge between an acquisition and a permanent loan, where the HELOC is measured in months.
• Reserves that stay undrawn. An open line costs nothing until you use it, and it is cheaper insurance than idle cash.

The weak use is funding a long-term buy-and-hold and leaving the balance outstanding, because that is the case that walks straight into the repayment step with the balance intact.

Where this calculator is deliberately simple

It assumes a single draw held at a constant balance, rather than the revolving draw-and-repay pattern a HELOC allows. If you repay during the draw period the repayment payment is smaller than shown, and if you draw further it is larger.

It holds the rate constant within each period, and prices rate movement separately in the chart rather than modelling a path. It does not model annual or lifetime rate caps, draw minimums, inactivity fees, or the lender's right to freeze or reduce a line — which is not theoretical, and happened widely when values fell in 2008.

For a fixed-rate alternative on an investment property, compare against the DSCR calculator; to test whether a cash-out refinance beats a line of credit, use the refinance break-even calculator.

FAQ

How much can I borrow on a HELOC?

Your property's value at the lender's maximum combined LTV, minus the balance on your first mortgage. At 85% CLTV on a $500,000 property with a $300,000 first, that is $125,000. Investment properties are usually held to a lower CLTV than primary residences, often 70% to 80%, and some lenders will not write a HELOC on a rental at all.

What happens when the draw period ends?

The line closes to new borrowing and the outstanding balance begins amortising over the repayment term. The payment steps up immediately — commonly by 40% or more, and by considerably more if the rate has risen since you drew. There is no grace period; the date is set at closing.

Is a HELOC interest-only forever?

No. Interest-only applies to the draw period only, typically the first ten years. Some lenders offer a fixed-rate conversion option on part of the balance, which trades the variable rate for a fixed one but does not remove the amortisation.

Can I get a HELOC on a rental property?

Some lenders write them, at lower CLTV limits and higher margins than on a primary residence, and the pool of lenders is much smaller. Many investors use a cash-out refinance or a dedicated investment-property line instead.

Is HELOC interest tax deductible?

Since 2018, interest on a HELOC secured by a primary residence is only deductible if the proceeds are used to buy, build or substantially improve that home. Using a HELOC on your house to fund an investment property changes the analysis — interest tracing rules may allow the deduction against the investment instead. This is a question for your CPA, not a calculator.

Should I pay down the balance during the draw period?

If you will still hold the balance when the draw period ends, yes — every dollar repaid early is a dollar that does not have to amortise over the shorter repayment term. If your plan repays the line entirely from a refinance or sale inside the draw period, the interest-only payment is doing exactly what you took the line for.

Conclusion

Size the draw against what the repayment payment will be, not against what the interest-only payment is. Then run the rate up two points and check that the answer still holds. A HELOC used inside its draw period is one of the cheapest tools an investor has; a HELOC still outstanding when the draw period ends is a repricing you scheduled years earlier and forgot about.