HELOC Calculator
Model both phases of a home equity line of credit — the low interest-only payment now, and the payment shock when the draw period ends.
On a $50,000 HELOC at 8%, your interest-only payment is about $333/mo during the draw period — but when the draw ends, the balance amortizes and the payment jumps to about $418/mo, a +25.5% shock. This calculator models both phases from your own numbers: the low interest-only payment now, the higher fully-amortizing payment later, and the total interest over the life of the line.
These results are estimates for informational purposes only and are not financial, tax, or legal advice. Your actual figures from a lender or the IRS may differ. Consult a qualified professional before making decisions.
Payment shock when the draw ends
$333.33/mo → $418.22/mo
Your payment jumps $84.89 (+25.47%) when interest-only ends and the balance begins to amortize.
Month-by-month schedule
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About the HELOC Calculator
A HELOC is not a fixed loan — it is a revolving line with two very different phases, and the gap between them is what surprises people. During the draw period you typically pay interest only, so the payment is low: on a $50,000 balance at 8% that is about $333 a month, and none of it touches the principal. When the draw period ends, the outstanding balance is amortized over the repayment term, so the payment now includes principal too and jumps — to about $418 a month here, a 25% increase overnight. That is the HELOC payment shock, and it is the single most important number to plan for, because your balance does not shrink on its own during the draw. This tool computes both phases the way a lender's ledger actually does: interest is posted and rounded to the cent every month, and the totals are the sum of those posted amounts, so the numbers match your statement rather than a textbook approximation. One more thing to keep in mind: a HELOC is a variable-rate line. The rate you enter is an assumption you are stress-testing, not a forecast — if rates rise before your draw ends, the shock is larger.
Frequently asked questions
What is the HELOC payment shock?+
It is the jump in your monthly payment when the interest-only draw period ends and the balance begins to amortize. During the draw you pay interest only; after it, you pay principal plus interest over the repayment term, so the payment rises — often 20–35%. Planning for that jump is the whole point of running the numbers early.
How is the interest-only payment calculated?+
Interest only = your balance times the monthly rate (APR ÷ 12), posted and rounded to the cent each month. On $50,000 at 8%, that is 50,000 × 0.08 ÷ 12 ≈ $333.33 per month. Because none of it reduces principal, your balance is the same at the end of the draw as at the start (unless you pay extra).
Is my HELOC rate fixed?+
Usually no — most HELOCs are variable-rate, tied to an index like the prime rate. The rate in this calculator is an assumption you are testing, not a prediction. If you want to see the risk, run it again at a higher rate: the interest-only payment and the payment shock both grow.
How much total interest will I pay?+
It depends on your balance, rate, and both terms, but it is usually larger than people expect because the draw period adds years of interest with no principal reduction. This tool sums the interest actually posted in every month of both phases so the total matches a real amortization, down to the cent.
By D.J. Gelner
D.J. Gelner is a former attorney and product builder who writes and verifies the math behind ManyTools' calculators. This HELOC model uses real lender-ledger conventions — interest posted and rounded to the cent every month, exactly as a line of credit accrues — and every result is checked against hand-verified test cases. It is an educational estimate, not financial advice; confirm the numbers with your lender.
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