Most HELOC calculators answer one question: how much can I borrow? That is the easy half. The half that costs people their homes is what happens in year eleven, when the interest-only payment they have been making for a decade turns into a payment with principal in it.

This calculator shows both, side by side, plus the gap between them. Work through the explanation below before you act on any of the numbers.

What a HELOC actually is

A home equity line of credit is a revolving credit line secured against your house. It works more like a credit card than a mortgage: you are approved for a limit, you draw what you need when you need it, and you pay interest only on what you have actually drawn.

That last point matters and is routinely misunderstood. If you are approved for $100,000 and draw $20,000, you owe interest on $20,000. The other $80,000 sits there costing you nothing until you touch it.

The difference from a credit card is the collateral. A credit card is unsecured — default and the consequences are financial. A HELOC is secured by your home. Default and the lender can foreclose. That single fact should sit behind every decision you make about one.

The two phases nobody explains properly

A HELOC has two distinct periods, and they behave so differently they are almost two different products.

Phase one: the draw period

Usually ten years. During this time you can borrow, repay and borrow again, up to your limit. Most lenders require only an interest payment each month. On a $60,000 balance at 8.5%, that is $425 a month.

Some lenders require a small principal payment, typically 1% to 2% of the balance. Most do not. The ones that do not are not being generous — they are setting up phase two.

Phase two: the repayment period

Usually twenty years. The line closes. You cannot draw any more. The balance you are carrying now has to be fully repaid, with principal, over the remaining term.

That same $60,000 at 8.5% over twenty years becomes $520.69 a month.

If you had drawn more, the gap widens sharply. At $150,000 the interest-only payment is $1,062.50 and the repayment payment is $1,301.73. On a shorter repayment term it is worse still: fifteen years instead of twenty pushes that figure to $1,477.

And a minority of HELOCs do not amortise at all. They end the draw period with a balloon payment, meaning the full balance is due. If your agreement says balloon, the calculator above does not describe your loan and you need to read the paperwork carefully.

How much you can actually borrow

Lenders work from combined loan-to-value, usually written CLTV. It counts everything secured against the house, not just the new line.

Maximum total debt  =  home value  ×  maximum CLTV
Available credit    =  maximum total debt  −  mortgage balance

A worked example. Your home is worth $450,000. You owe $260,000. Your lender allows 85% CLTV.

450,000 × 0.85 = 382,500
382,500 − 260,000 = 122,500 available

You have $190,000 of equity but can access $122,500 of it. The difference is the lender’s buffer — they will not lend against the last slice of your equity, because if prices fall they want to still be covered.

Typical maximum CLTV sits between 80% and 85%. Some lenders go to 90% for strong applicants, and a few advertise 95%, almost always at a materially higher rate. Treat anything above 85% as expensive rather than generous.

The payment shock, in plain numbers

This is the part worth sitting with.

For ten years you pay $425 a month and the balance does not move. You may even have drawn more during that time, so it has gone up. Then one month, without a warning letter or a phone call, the payment becomes $520.69 and stays there for twenty years.

A 23% increase sounds survivable. Two things make it worse in practice.

You probably drew more than you planned. A revolving line available for a decade tends to get used. People open one for a kitchen, then use it for a car, then for a medical bill. The payment shock is calculated on the balance you end up with, not the one you started with.

The rate probably moved. Almost all HELOCs are variable. If your rate rose from 8.5% to 10.5% during the draw period, that $60,000 repayment payment is $599 rather than $521 — and the interest-only payment you had grown used to was already $525.

Run the stress test in the calculator at +2%. If the resulting repayment figure is uncomfortable, borrow less now rather than discovering it in year eleven.

How the variable rate actually works

HELOC rates are almost never fixed. They are expressed as the prime rate plus a margin:

Your rate  =  prime rate  +  lender's margin

Prime moves with the Federal Reserve’s decisions. Your margin is set at approval, based on your credit profile, and does not change. So a borrower at “prime + 0.5%” and one at “prime + 3%” both move up and down by the same amount — one just started from a much better place.

Three things to check in the agreement:

• The lifetime cap. Federal law requires HELOCs to have one. It is often far above anything you would want to pay — 18% is common — but you should know the number.
• Whether there is a periodic cap. Some agreements limit how much the rate can move in a single adjustment. Many do not.
• Whether an introductory rate applies. A low teaser rate for six or twelve months is a marketing device. Work out your payment at the go-to rate, not the teaser.

Some lenders offer a fixed-rate conversion option, letting you lock part of the balance. If you plan to carry a balance long term, that feature is worth more than a slightly lower starting rate.

HELOC, home equity loan, or cash-out refinance

These three get confused constantly and they suit genuinely different situations.

The decision usually turns on one question: do you know the amount?

A kitchen renovation with a signed quote is a home equity loan. A multi-year series of repairs, or a business with uneven cash needs, is a HELOC. If your existing mortgage rate is well above today’s market, a cash-out refinance can beat both — but if you are sitting on a mortgage below 4%, refinancing to access equity means giving that rate up, and that is almost always the wrong trade.

Costs that are not the interest rate

The advertised rate is not the whole price.

• Closing costs. Often 2% to 5% of the credit line, though many lenders waive them. Where they are waived, check for an early closure fee.
• Early closure fee. Commonly charged if you close the line within the first three years, sometimes clawing back the waived closing costs.
• Annual fee. Typically $50 to $100, charged whether or not you draw anything.
• Inactivity fee. Some lenders charge for not using the line.
• Minimum draw. Many require an initial draw at closing — sometimes $10,000 or more — whether you need it that day or not.
• Appraisal. Required by most lenders, between $300 and $600 depending on the property and whether an automated valuation is accepted.

A line with no closing costs, a $75 annual fee and a three-year early closure clause can easily cost more than one with $1,500 in closing costs and no ongoing fees, depending on how long you keep it. Work out the total over the period you actually expect to hold it.

What lenders look at

A HELOC application is underwritten more like a mortgage than a credit card.

Credit score. Most lenders want 680 or better. The best rates generally go to 740 and above. Below 620 your options narrow sharply and the margin widens.

Debt-to-income ratio. Your total monthly debt payments, including the proposed HELOC, divided by gross monthly income. Most lenders cap this at 43% to 50%. Important detail: they often calculate the HELOC payment as though the line is fully drawn, not as though you will draw nothing.

Equity. Covered above. Most lenders also require you to keep at least 15% to 20% of the home’s value untouched.

Income stability. Two years of consistent income is the usual expectation. Self-employed applicants should expect to supply two years of returns.

The appraisal. The lender’s valuation governs, not your estimate and not the figure a property site shows you. If it comes in below expectation, your available credit falls accordingly, and the calculator result changes with it.

Good reasons and bad reasons

Nobody can decide this for you, but the pattern is clear enough to describe.

Reasonable uses

• Home improvements that add value, where costs arrive in stages
• Consolidating genuinely high-interest debt, with a plan to not re-accumulate it
• A funding gap between buying and selling a property
• A business or medical cost where the alternative is much more expensive credit

Uses that end badly

• Holidays, weddings, cars, anything that does not outlast the debt
• Investing the money in markets, hoping to beat the interest rate
• Covering an ongoing income shortfall, which postpones the problem and adds interest
• Paying off unsecured debt without changing the behaviour that created it — this converts debt you could have negotiated into debt secured against your house

That last point deserves emphasis. Moving credit card balances onto a HELOC lowers the rate but changes the risk category entirely. Unsecured debt in a crisis can be restructured. Secured debt in a crisis can take your home.

Tax treatment

Interest on home equity borrowing has been deductible only when the money is used to buy, build or substantially improve the home that secures the loan, subject to overall mortgage debt limits. Using a HELOC to consolidate credit cards or buy a car has not qualified.

This area of US tax law was scheduled to change at the end of 2025 and the treatment may differ for the current tax year. Confirm the current rule with a tax professional before counting on any deduction. Do not let a deduction you have not verified change a borrowing decision.

The risk most people never consider

Lenders can reduce or freeze a HELOC that you have already been approved for.

If your home value falls, if your credit score drops significantly, or if the lender’s own circumstances change, they can cut your available credit or suspend draws entirely. This happened at scale during the 2008 housing downturn, and the agreements still permit it.

The practical consequence: a HELOC is not an emergency fund. If you are keeping one open as a safety net, be aware that the net can be withdrawn in exactly the conditions where you would need it.

How to use the calculator above

1. Enter your home’s current value. Use a realistic figure. An appraisal will govern, and optimism here produces a result you cannot borrow against.
2. Enter what you still owe. Mortgage balance, plus any existing second lien.
3. Pick a combined LTV. 85% is a reasonable default. Use 80% if you want a conservative figure.
4. Enter what you plan to draw — not the maximum, unless you genuinely intend to use it all. The payments are calculated on what you draw.
5. Enter the rate you have actually been quoted. If you have no quote yet, use the current average for your credit band rather than the best advertised rate.
6. Set the stress test to +2%. This is the number to plan around, not the one at today’s rate.

Then read the repayment payment, not the draw payment. The draw payment is what the next ten years feel like. The repayment payment is what the loan costs.

Questions

Is a HELOC the same as a second mortgage? It is a type of second mortgage. The term covers any loan secured against a property that already has a mortgage on it, which includes both HELOCs and home equity loans.

Can I pay a HELOC off early? Yes, and during the draw period there is normally no penalty on repaying principal. Check for an early closure fee if you intend to close the line entirely within the first few years.

What happens if I sell the house? The HELOC is repaid from the sale proceeds at closing, like your mortgage. You cannot transfer it to a new property.

Does opening one hurt my credit score? The hard inquiry causes a small temporary dip. After that, the effect depends on usage. A large balance relative to the limit is read similarly to high credit card utilisation.

How long does approval take? Typically two to six weeks, longer if a full appraisal is required. It is slower than unsecured credit because the property has to be valued and the title checked.

Can I get one on a rental or second home? Sometimes, but fewer lenders offer it, the maximum CLTV is lower, and the rate is higher. Expect to need stronger credit than on a primary residence.

What does the calculator not account for? Closing costs, annual fees, minimum draw requirements, and rate changes during the draw period. It also assumes a standard amortising repayment phase rather than a balloon. Treat it as a planning estimate and confirm every figure with the lender.

This calculator and article are for general information. They are not financial advice, not an offer of credit, and not a substitute for speaking to a qualified lender or adviser. Your home is at risk if you do not keep up repayments on a loan secured against it.