Last reviewed: October 15, 2026
The most common surprise for first-time HELOC shoppers is that the amount a lender will let them borrow is a lot smaller than the equity they think they have. If your home is worth $450,000 and you owe $220,000, you have $230,000 of equity on paper, yet a typical lender will not offer a $230,000 line. This guide walks through the arithmetic behind that gap with five worked examples, so you can predict roughly where your own number will land before you ever talk to a lender.
Lenders cap how much total debt can sit against your home. That cap is called the combined loan-to-value ratio, or CLTV, and it is a percentage of your home's value. The maximum new credit line is what is left after you subtract your existing home-secured debt from that cap:
Maximum HELOC = (home value × maximum CLTV) − existing home-secured debt
The calculator on our home page uses exactly this formula with your mortgage balance as the existing debt, and it shows $0 rather than a negative number when the mortgage already exceeds the cap. The CLTV percentage is not set by law for HELOCs in general; each lender chooses its own limit, and it can change with your credit profile and the property type. Many lenders publish limits in the neighborhood of 80% to 85%, which is why we use 80% as a default starting assumption, but you should check each lender's actual figure.
Home value $450,000, mortgage balance $220,000, lender limit 80% CLTV.
Total equity is $230,000, but the estimated HELOC is $140,000, about 61% of it. The remaining $90,000 is the cushion the lender insists you keep in the home.
Keep the same $450,000 home and $220,000 mortgage, and change only the lender's limit:
| Lender CLTV limit | Max combined debt | Estimated HELOC |
|---|---|---|
| 70% | $315,000 | $95,000 |
| 80% | $360,000 | $140,000 |
| 85% | $382,500 | $162,500 |
A swing of fifteen percentage points in the limit moves the estimate by $67,500. This is why we suggest running the calculator at a conservative number and a generous number instead of trusting a single result. Whichever lender you eventually talk to will tell you its own limit.
Home value $300,000, mortgage balance $250,000, limit 80%.
The owner has $50,000 of equity on paper but no room for a HELOC at an 80% limit, because the existing mortgage already uses more than the lender will allow in total. This is common for recent buyers who made a small down payment, or owners whose home value has fallen.
Home value $600,000, mortgage balance $380,000, limit 85%.
Even with a generous 85% limit, the line is $130,000 against $220,000 of total equity.
This is the case our calculator does not model directly, and it matters. Suppose your home is worth $500,000, your first mortgage is $300,000, and you already have a home equity line with a $40,000 limit of which you have drawn only $10,000. If a lender uses an 80% limit:
If you entered only the $300,000 first mortgage, the calculator would show $100,000, overstating the room by $40,000. Fannie Mae's Selling Guide, for example, says the full amount of a HELOC counts in the combined ratio whether or not funds have been drawn. Other lenders may handle it differently, so treat the existing credit limit as debt when you use the calculator. Simply add it to the mortgage balance field.
The estimate tells you how much you might be able to access, not how much you should borrow. Every dollar drawn must be repaid with interest, usually at a variable rate, and the loan is secured by your home. Before choosing an amount, read how HELOC payments change after the draw period and think about the smallest amount that solves your problem.