Rates & InterestLetter M

Mortgage Constant (Loan Constant)

The total annual debt service (principal plus interest) of an amortizing loan expressed as a percentage of the original loan principal, used by investors to test for positive or negative leverage.

Overview & Practical Application

The Mortgage Constant (also called the Loan Constant or Debt Constant) measures the total annual cash required to service an amortizing loan (both principal AND interest) as a percentage of the total initial loan balance.

While the nominal interest rate only reflects the cost of interest, the Loan Constant incorporates the amortization speed (e.g., 20-year vs. 30-year amortization).

Real estate investors compare the Loan Constant directly against a property’s Capitalization Rate (Cap Rate) to determine whether taking out a mortgage helps or hurts their cash yield: - Cap Rate > Loan Constant = Positive Leverage: Borrowing increases your Cash-on-Cash Return above the Cap Rate. - Cap Rate = Loan Constant = Neutral Leverage: Borrowing produces a Cash-on-Cash Return identical to the Cap Rate. - Cap Rate < Loan Constant = Negative Leverage: Borrowing reduces your Cash-on-Cash Return below the unleveraged Cap Rate.

Mathematical Formula & Calculation

Loan Constant (%) = (Annual Debt Service (P&I) / Original Loan Principal) * 100

Divides 12 months of scheduled Principal & Interest payments by the total loan amount.

Variable Definitions:

  • Annual Debt Service (P&I)=Monthly Principal & Interest payment multiplied by 12
  • Original Loan Principal=Initial borrowed mortgage balance

Real-World Underwriting Example

Scenario: An investor borrows $500,000 at a 6.50% interest rate amortized over 30 years ($3,160.34/month or $37,924.08/year) to buy a property with an 8.00% Cap Rate.

Calculation:
Loan Constant = ($37,924.08 / $500,000) * 100 = 7.58%.

Takeaway: Because the property’s Cap Rate (8.00%) is higher than the 7.58% Loan Constant, the investor achieves Positive Leverage.

Key Underwriting Takeaways

  • On an interest-only loan, the Loan Constant is equal to the nominal interest rate; on an amortizing loan, the Loan Constant is always higher than the interest rate.
  • Shortening the amortization period (e.g., from 30 years to 20 years) increases the Loan Constant even if the interest rate stays the same.
  • Comparing Cap Rate vs. Loan Constant is the fastest way to verify positive cash-flow leverage.
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Rates & Interest

Capitalization Rate (Cap Rate)

A real estate valuation metric that expresses a property’s unleveraged annual rate of return by dividing its Net Operating Income (NOI) by its current market value.

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Rates & InterestCoC

Cash-on-Cash Return (CoC)

A leveraged real estate return metric that measures the annual pre-tax cash flow generated by a property as a percentage of the actual out-of-pocket cash invested.

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Credit & UnderwritingDSCR

Debt Service Coverage Ratio (DSCR)

The primary investment-property underwriting ratio that measures whether a rental property’s cash flow is sufficient to cover its recurring mortgage debt obligations.

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Mortgages

Amortization

The mathematical scheduling of equal periodic loan payments that systematically reduce both interest and outstanding principal over time.

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