Max Loan Amount From DSCR: The Back-Solve Every Borrower Should Know
Most borrowers meet the debt service coverage ratio as a test their deal must pass. Sophisticated borrowers run it the other direction: given the lender's minimum coverage, the property's income, and the loan terms, the maximum loan is a two-step calculation you can perform before any application — and the calculation's guts, a quantity called the mortgage constant, explain exactly which lever moves your proceeds and by how much.
This explainer derives the back-solve, unpacks the constant properly, and runs one deal through every variant — rate moves, amortization moves, interest-only — with verified numbers throughout. All figures are illustrative examples.
The Derivation, in Three Lines
Start from the coverage test and invert it:
- DSCR = NOI ÷ Debt Service — the lender's requirement,
DSCR ≥ minimum - Therefore Max Debt Service = NOI ÷ Min DSCR — the largest annual payment the income supports at the required cushion
- Therefore Max Loan = Max Debt Service ÷ Mortgage Constant — the loan whose payment equals that maximum
Or in one line: Max Loan = NOI ÷ (Min DSCR × Constant). Three inputs, one output, and everything interesting hides in the third input.
The Mortgage Constant, Properly
The mortgage constant is the annual debt service per dollar borrowed — the full year's principal-and-interest payment on a $1 loan at a given rate and amortization. It is what converts "how much payment can I afford" into "how big a loan is that."
Two properties define its behavior:
It always exceeds the interest rate on an amortizing loan, because the payment carries principal as well as interest. At an illustrative 6.75% on a 30-year schedule, the constant is about 7.78% — the extra 103 basis points are the amortization.
It moves with both rate and schedule — and borrowers systematically underweight the second. A table makes the point (illustrative, principal-and-interest constants):
| 30-year am | 25-year am | |
|---|---|---|
| 6.75% | 7.78% | 8.29% |
| 7.25% | 8.19% | 8.67% |
Read down a column and across a row: five years of amortization moves the constant about as much as fifty basis points of rate. Since the constant sits in the back-solve's denominator, every move in this table is a move in your proceeds — inversely, and immediately.
The Worked Back-Solve, Every Variant
The deal: $240,000 of underwritten NOI, a lender requiring 1.25x minimum coverage.
Base case — 6.75%, 30-year amortization:
- Max debt service = $240,000 ÷ 1.25 = $192,000
- Max loan = $192,000 ÷ 0.0778 ≈ $2,467,000
Rate variant — 7.25%, same schedule: constant ≈ 8.19%; max loan ≈ $2,345,000. Fifty basis points cost about $121,000 of proceeds — equity the borrower must produce, on the same property, with the same income.
Amortization variant — 6.75%, 25-year schedule: constant ≈ 8.29%; max loan ≈ $2,316,000. The shorter schedule cost about $151,000 — more than the half-point of rate did. This is the negotiation insight buried in the table: when a lender's rate is firm, the amortization period is often the more valuable term to push, and it is the one borrowers argue about least.
Interest-only variant: with no principal in the payment, the constant is the rate, and the back-solve becomes NOI ÷ (DSCR × rate) = $240,000 ÷ (1.25 × 0.0675) ≈ $2,844,000 — some $378,000 more than the amortizing base case. Which is exactly why the sizing convention matters so much: a lender who sizes on the amortizing payment even during an IO period is refusing to lend against payment relief that expires. Ask which payment sizes the loan; the two answers are a third of a million dollars apart on this deal.
The income variant: raise NOI 5% and the maximum loan rises exactly 5% — to about $2,590,000. The back-solve is linear in NOI, which converts the lender's underwriting haircuts into precise dollar terms: at this deal's box, every dollar of NOI the lender disallows (a vacancy floor, a management floor, a reserve deduction) removes about $10.28 of proceeds. When you contest an underwriting adjustment, that is the exchange rate you are contesting at.
The Constant's Second Job: Diagnosing Leverage
Once you can compute the constant, it earns a second use beyond sizing: compared against the property's cap rate, it tells you in one subtraction whether debt is helping or hurting your current income.
When the cap rate exceeds the constant, each borrowed dollar earns more (as property yield) than it costs (as debt service) — leverage is positive, and adding debt raises the cash-on-cash return on your equity. When the constant exceeds the cap rate, the relationship inverts: the debt consumes more income than the leveraged dollars produce, cash-on-cash falls below the unlevered yield, and the deal is — whatever the pitch says — an appreciation bet by construction, because current income is being traded away for it.
On the worked deal's terms, a 6.75%/30-year loan carries a 7.78% constant; a property bought at a 6.5% cap sits in negative-leverage territory against it, while one bought at 8.5% enjoys a full point of positive spread. Neither situation is forbidden — negative leverage with a genuine growth story can be rational — but the constant-versus-cap comparison names which game you are playing before the financing papers do.
What the Back-Solve Is For
Three practical uses, in ascending order of leverage:
1. Sizing your own term sheet. Before any lender conversation, you know the coverage-constrained proceeds at any rate — and can bracket the equity check across the rate range between now and closing. No surprises at commitment.
2. Pricing a purchase from the debt side. Combine the back-solve with your target leverage and the equity you have, and an asking price converts into a feasibility test in minutes: does the income support the loan this price requires? Run alongside the value and debt-yield tests — the coverage answer is one of three constraints, and the loan sizing calculator shows which of the three actually governs a given deal.
3. Planning a refinance. For BRRRR-style and value-add plans whose harvest is a refinance, the back-solve is the plan: stabilized NOI, the program's minimum DSCR, and the forward rate produce the proceeds — and stress-testing that trio before you buy is the difference between a capital-recycling machine and capital buried in a rental.
One caution spans all three: use underwritten NOI, not your pro forma. The lender's vacancy and management floors and reserve deductions come off the income before their coverage test runs, and — per the exchange rate above — every haircut dollar is a proceeds multiple. The construction of the underwritten-NOI block, in Excel, is covered in the DSCR calculator build guide; the constant's own arithmetic gets a dedicated treatment in the mortgage constant calculator guide.
Frequently Asked Questions
What is the formula for maximum loan amount from DSCR?
Max loan = NOI ÷ (minimum DSCR × annual mortgage constant), where the constant is the annual principal-and-interest payment per dollar borrowed at the quoted rate and amortization. In Excel: =-PV(rate/12, amort_years*12, (NOI/MinDSCR)/12).
What is a mortgage constant? The annual debt service on a $1 loan — the rate plus the amortization component. At 6.75% on a 30-year schedule it is about 7.78%; interest-only makes it equal the rate.
Why did my loan proceeds drop when rates rose, even though my NOI didn't change? Because the constant sits in the denominator: a higher rate means each borrowed dollar costs more to service, so the fixed payment your income supports amortizes a smaller loan. On the worked deal, 50 basis points removed about $121,000.
Does a longer amortization always mean more proceeds? Under a DSCR constraint, yes — a longer schedule lowers the constant and raises the coverage-constrained maximum. Whether the lender offers the longer schedule, and whether other constraints (LTV, debt yield) then bind instead, are separate questions.
Is the max loan from DSCR the loan I will actually get? Only if coverage is the binding constraint. Lenders size to the most restrictive of DSCR, LTV, and often debt yield — the coverage back-solve is one leg of a triple test.
Run the Back-Solve With the Full Machinery
The DSCR & Debt Underwriting Calculator implements the back-solve inside the complete sizing framework: the lender-convention NOI adjustments as inputs, all three constraints computed with the binding one flagged, the constant exposed rather than buried, full amortization with by-year coverage, and rate stress across the range that matters — fully unlocked, formula-transparent, versioned, with a documented methodology PDF.
This article is for educational purposes only and does not constitute investment, legal, or tax advice. All figures are illustrative examples, not rate quotes or lender terms. Consult qualified professionals before making investment decisions.
Get the next breakdown in your inbox
New CRE modeling walkthroughs 3× per week. No spam, unsubscribe anytime.
