HELOC Formula Explained
The math behind the HELOC Calculator: the equation, the variables, and the assumptions it makes.
By Onias Derilus, Mortgage Capital · NMLS# 1859012 · Last Updated: June 2026
It estimates your available home equity line of credit and the interest-only or principal-and-interest payment during the draw and repayment periods.
The Formula
Available line = (value × max CLTV) − first mortgage balance
Combined loan-to-value counts your first mortgage plus the HELOC against the home's value, commonly capped around 80% to 90%. The available line is whatever room is left under that ceiling.
HELOC rates are usually variable and tied to an index, so payments can change over time. During the draw period many HELOCs allow interest-only payments; once repayment begins, the balance amortizes and the payment rises.
Turn Your HELOC Estimate Into a Real Pre-Approval
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Rates are illustrative only. APR and payments vary by credit score, loan amount, and market conditions. Subject to credit approval. Not a commitment to lend. NMLS# 1859012. Equal Housing Lender.