Definition
The Debt Constant (also called the mortgage constant) is the ratio of a loan’s annual debt service to the total loan amount.
It shows the percentage of the loan balance that must be paid each year to cover both principal and interest.
Formula
Debt Constant = Annual Debt Service ÷ Loan Amount
Example:
- Loan Amount: $5,000,000
- Annual Debt Service: $375,000
Debt Constant = $375,000 ÷ $5,000,000 = 7.50%
Why It Matters
The debt constant allows investors to compare financing cost to property yield. It is especially useful when analyzing leverage relative to the cap rate.
Debt Constant vs Interest Rate
- Interest Rate: Cost of borrowing on outstanding principal.
- Debt Constant: Effective annual payment rate including amortization.
Because amortizing loans repay principal over time, the debt constant is always higher than the interest rate on fully amortizing loans.
Relationship to Cap Rate
Comparing the cap rate to the debt constant helps determine whether leverage is accretive or dilutive.
- If Cap Rate > Debt Constant → Positive leverage.
- If Cap Rate < Debt Constant → Negative leverage.
Connection to DSCR
The Debt Service Coverage Ratio (DSCR) can be expressed using the debt constant:
DSCR = NOI ÷ (Loan Amount × Debt Constant)
This relationship is frequently used in lender underwriting.
How It’s Used in Underwriting
- Quickly estimate maximum loan size based on NOI.
- Compare financing structures across lenders.
- Evaluate impact of amortization changes.
- Test refinance scenarios.
Common Pitfalls
- Confusing interest-only loans (where debt constant equals interest rate).
- Ignoring amortization term differences.
- Failing to adjust for floating-rate risk.
Investor Checklist
- Verify amortization schedule.
- Understand if loan is interest-only.
- Compare debt constant to going-in cap rate.
- Stress test refinance rates.