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What Is a HELOC and How Does This Calculator Model It?

A Home Equity Line of Credit (HELOC) is a revolving loan secured by your home — similar to a credit card, but with your equity as collateral and significantly lower interest rates. Unlike a home equity loan (which delivers a fixed lump sum at a fixed rate) or a cash-out refinance (which replaces your entire mortgage), a HELOC gives you a flexible credit line you can draw from as needed during a set draw period, then repay during a separate repayment period.

What makes HELOCs uniquely complex — and what most generic mortgage calculators miss — is this two-phase structure. During the draw period (typically 10 years), you typically pay interest only on what you've borrowed, and your balance can fluctuate as you draw and repay. When the draw period ends, the outstanding balance converts to a fully amortizing loan with principal + interest payments spread over the repayment period (typically 10–20 years).

This calculator models both phases correctly. You'll see the interest-only draw payment, the P&I repayment payment, the total interest cost, and — critically — the payment jump between phases (often called "payment shock"). HELOC rates are variable in practice (typically Prime + a margin), but for projection purposes this calculator treats the input rate as fixed and discloses this clearly.

The maximum credit line is determined by your lender's Combined Loan-to-Value (CLTV) cap — typically 80–90% of the home's appraised value minus any existing mortgage balance. The calculator enforces this cap so you see a realistic borrowing limit, not an inflated theoretical maximum.

How the Math Works

The HELOC credit limit calculation starts with your Combined Loan-to-Value cap: Max HELOC = (Home Value × CLTV%) − Existing Mortgage. If the amount you plan to draw exceeds this cap, the calculator surfaces a warning and caps the calculation at the allowed maximum.

During the draw period, monthly interest-only payments equal Balance × (Annual Rate / 12). Because you're paying interest only, the principal doesn't shrink — the full drawn balance carries into the repayment phase.

During the repayment period, the outstanding balance amortizes using the standard P&I formula: PMT = P × r(1+r)ⁿ / ((1+r)ⁿ − 1), where P is the balance at repayment start, r is the monthly rate, and n is the number of repayment months. This gives a fixed monthly payment that fully retires the balance by the end of the repayment term.

Total interest = (draw-phase payments) + (repayment-phase payments − original principal). Because the draw-phase balance doesn't decrease, borrowers often underestimate how much interest a HELOC costs over its full life.

Worked Example

Scenario: Home worth $525,000, existing mortgage $220,000, lender CLTV cap 85%.

  • Max combined loan = $525,000 × 0.85 = $446,250
  • Maximum HELOC = $446,250 − $220,000 = $226,250
  • Borrower draws $50,000 at 8.5% rate
  • Draw period monthly payment (interest-only) = $50,000 × (0.085 / 12) = $354/month
  • At repayment start, balance is still $50,000. Amortized over 20 years at 8.5%: $434/month
  • Payment shock: $434 − $354 = $80/month increase
  • Total interest: 120 × $354 (draw) + (240 × $434 − $50,000) (repayment) = $42,480 + $54,160 = $96,640

Frequently Asked Questions

What's the difference between a HELOC and a home equity loan?

A HELOC is a revolving line of credit with a variable rate — you borrow what you need, when you need it, and pay interest only on what you draw during the draw period. A home equity loan gives you a lump sum at a fixed rate with fixed P&I payments from day one. If you need flexibility, a HELOC wins. If you want payment certainty, a home equity loan is simpler.

What happens when my HELOC enters repayment?

At the end of the draw period (typically 10 years), your HELOC converts to a fully-amortizing loan. You can no longer draw funds, and your payment jumps from interest-only to full P&I. This 'payment shock' is one of the most common HELOC surprises — this calculator shows you exactly how much it is before you sign.

Can my lender cut my credit line during the draw period?

Yes. Lenders can reduce or freeze a HELOC if your home value drops significantly, your credit score declines, or the lender experiences financial stress. This is a key risk vs. a home equity loan, where the funds are committed upfront. Keep your credit score strong and monitor your home equity throughout the draw period.

Is HELOC interest still tax deductible?

Under current tax law (TCJA through 2025, extended pending legislation), HELOC interest is deductible only if the funds are used to 'buy, build, or substantially improve' the home securing the loan. Interest used for debt consolidation, vacations, or other personal expenses is generally not deductible. Consult a tax advisor for your specific situation.

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