How to Run DTI Math Before Using a HELOC to Buy a Rental

The Quick Read: Divide all monthly debt payments by gross monthly income, add the estimated payment on the new line, then test the purchase loan separately. A home equity line faces a personal debt-to-income test, while a DSCR purchase loan is reviewed primarily on property-level rental income covering the payment, subject to lender guidelines.

• The HELOC gets its own personal DTI test, even when the rental purchase loan is a DSCR loan.
• On this network, DTI tops out at 50%, and a ratio above 45% needs a 680 minimum credit score.
• The line is qualified on the interest-only payment calculated on the maximum draw, not on what you plan to use.
• An investment-property line is capped at 70% CLTV and $500,000.
• Clearing 1.00 coverage on the purchase is not positive cash flow. The line payment still comes out of your pocket.

This is the lending ratio, not the imaging technique that shares the abbreviation.

Why Are There Two Tests?

There are two separate approvals on this deal. The line is a personal-credit product, so it is tested on your income and debts. The rental purchase loan may be tested differently. Most files that stall do so because the investor ran one test and assumed it covered both.

Sequence matters too. The line has to be approved and funded before an offer goes under contract, because the purchase lender will want to trace where the down payment came from. Treat it as line approval first, purchase underwriting second.

Lendmire’s complete DSCR loans guide covers the purchase side in full. This piece covers the arithmetic that comes before it.

Key Terms Defined

Debt-to-income ratio (DTI): Your total monthly debt payments divided by your gross monthly income.

Gross monthly income: Income before taxes and other deductions.

Qualifying payment: The payment a lender assigns to a new line for the ratio, which may differ from what you actually pay.

CLTV: Combined loan-to-value, meaning all liens on the property divided by its value.

DSCR: Monthly rent divided by the property’s full monthly payment (principal, interest, taxes, insurance, and any HOA dues).

Draw period: The stretch when you can borrow against the line, typically interest-only.

Repayment period: The later phase when principal and interest are both due.

What Is the DTI Formula?

The formula is simple. The CFPB defines DTI as all monthly debt payments divided by gross monthly income, and it adds that different loan products and lenders set different limits. No single federal number governs a HELOC.

Older benchmarks still float around. Cornell’s Legal Information Institute notes the traditional 28/36 benchmark for housing costs and total debt. That is background. The ceiling that matters is the one on the program you are applying to.

On this network, the ceiling for the line is 50%. Credit profiles from 600 to 679 are held to 45%, and anything above 45% needs a 680 minimum. Those figures are subject to lender guidelines and full file review.

How Do You Run the Math, Step by Step?

Run it in this order.

1. List every recurring debt. Mortgages, auto loans, student loans, card minimums, and support payments. Utilities, groceries, and insurance premiums generally stay out.

2. Find gross monthly income. Use pre-tax, documentable income. Rental income may count, but the haircut varies by lender, so ask before you assume.

3. Add the qualifying payment on the new line. On this network, the line is qualified on the interest-only payment calculated on the maximum draw. A line sized at the full amount looks heavier than the draw you intend to take.

4. Divide and compare. Total monthly debts divided by gross monthly income, then compare to the ceiling for your credit tier.

5. Rerun it for the purchase loan. This is where the structure of the purchase loan decides the answer.

6. Stress-test the repayment phase. More on that below.

A modeled illustration, with no dollar figures: say an investor’s existing debts already use about 30% of gross income. A new line payment that adds 12 points of ratio lands the file near 42%. That clears the 45% mark for lower credit tiers. Add a second line or a new personal-DTI mortgage and the number moves past 45%, which now requires a 680 score. The cushion disappears quickly in practice, so run the combined case before you apply, not after. Exact terms depend on lender and investor guidelines, credit profile, reserves, and property review.

Which Purchase Loan Changes the Math?

The type of purchase loan decides what goes in your ratio.

The second column trips people up. DSCR does not mean DTI disappears. The line itself still faces the personal test, and it will show on your credit report when the purchase lender looks at the file.

Coverage on purchase is a separate question. Most DSCR purchases land at 75% to 80% LTV, and select programs reach 85% LTV with roughly a 700+ score. Select programs start at 1.00 coverage. A separate path through select lenders in the network takes coverage below 1.00, with leverage and terms adjusted. These specifics are subject to lender guidelines and a full review of property, leverage, and credit.

A larger down payment lowers the monthly payment and can lift the ratio, but it never erases leverage caps, credit floors, or reserve rules. The strongest files clear both tests: enough equity and enough rental coverage.

Where Does the Rule Break?

These are the edge cases that decide files.

Payment basis. A line qualified on the interest-only payment at the maximum draw looks heavier than one qualified on the drawn balance. The CFPB’s HELOC booklet notes that lenders often size the limit as a percentage of appraised value minus the existing mortgage balance. Know which number your lender uses.

The step-up. Investment lines on this network run a 5-year interest-only draw followed by a 25-year fully amortizing repayment. DTI at origination does not show the jump when principal starts. Pricing also floats through both phases and never converts to fixed. Stress-test the repayment payment yourself.

Drawn at closing. At least 75% of the line is drawn at closing. The idea of borrowing only a sliver of the line does not fit this structure.

Title. LLCs, corporations, partnerships, and land or irrevocable trusts cannot hold title on the line. Only the individual borrower or a revocable living trust can. A rental already deeded to an LLC needs a vesting change or a DSCR cash-out instead.

Exposure. A borrower is limited to three lines, and an investor with more than 15 financed properties is not eligible.

Property type. Single-family, 2-4 units, PUDs, townhomes, and condos are eligible. Manufactured homes, co-ops, condotels, log homes, commercial, and mixed-use are not.

Availability. Lines are offered in Lendmire’s 16 full-service states, which is narrower than the DSCR footprint.

What Does the Line Itself Allow on a Rental?

A HELOC against a rental is a thinner product than one against a primary home. NerdWallet reports a smaller lender pool and lower borrowing limits on investment properties, and it describes a typical interest-only draw followed by principal-plus-interest repayment.

On this network, an investment line is capped at 70% CLTV and $500,000, with a 700 minimum credit score. A 720 score buys no extra leverage on an investment line, only eligibility. Because the line stays at or below $500,000, it usually runs on an automated valuation with no traditional appraisal, though a higher CLTV may call for a secondary valuation.

Higher ceilings exist for primary residences and second homes. A 90% CLTV ceiling applies only with a 720-or-better credit profile. That is not an investment-property figure and should not be used for one.

If equity sits in your primary home, the same DTI math applies, but the occupancy tier changes the ceiling.

What If the Number Fails?

Work the fix list in order.

1. Shrink the line. A smaller maximum draw lowers the qualifying payment directly.

2. Pay off a small debt. Removing a card or auto payment frees ratio room faster than raising income.

3. Check your credit tier. Crossing 680 lifts the ceiling from 45% to 50%.

4. Reverse the math. Start from the ceiling, subtract existing debts, and see what payment room remains. That sets your maximum line size.

5. Consider a property-based route. If personal DTI is the bottleneck, a DSCR cash-out refinance on an existing rental pulls equity based on the property, not on your personal ratio. Cash-out on the network tops out around 75% LTV, with about 6 months of seasoning expected. See the related piece Using HELOC To Buy Rental Property for the line-first route. Terms depend on the lender’s guidelines, the property type, the leverage, and a full review of the credit profile.

Where Do Investors Get Caught?

• Assuming the line only counts after a draw. Lenders commonly assign an estimated payment before any draw.
• Treating 43% as a legal cap. It was an old qualified-mortgage rule of thumb cited by CNBC Select, and it does not govern this product.
• Reading 1.00 coverage as cash flow. DSCR compares rent to the payment only. Repairs, vacancy, management, utilities, and capex sit outside it.
• Forgetting reserves. Down payment and reserves are separate pots, and borrowed funds often cannot satisfy reserves. Reserves commonly run around 6 months of PITIA, varying by lender, leverage, and loan size.
• Stacking lines. Each personal line raises DTI and shows on the next application.

Tax treatment can depend on how the funds are used and how the property is held; investors should keep clear records and speak with a qualified tax professional before relying on any deduction.

For the judgment call on whether the leverage is wise at all, Lendmire has a separate piece on using a HELOC to buy a rental and whether it is smart or over-leveraged that walks through it.

If you are considering a home equity line and want to see how the numbers work, Lendmire can help you compare HELOC options based on the property, the equity available, credit profile, combined leverage, and your goals.

About Lendmire

As a mortgage broker (NMLS# 2371349), Lendmire helps homeowners and real estate investors in its 16 full-service states open home equity lines of credit arranged through wholesale lenders. Lendmire is never the lender; line amounts, ceilings and terms are set by the lender under its guidelines. Scotsman Guide named Lendmire a Top Mortgage Workplace in both 2025 and 2026.

References

1. CFPB – What is a debt-to-income ratio

2. Cornell LII – Debt-to-income ratio

3. CFPB HELOC booklet

4. NerdWallet – Can you get a HELOC on an investment property

5. CNBC Select – How to calculate debt-to-income ratio