Current-balance-weighted average of the mortgage note rates in a pool.
Weighted average coupon (WAC) is the average mortgage note rate in a pool, weighted by each loan’s current principal balance.
WAC matters because an MBS pool can contain many loans with different note rates and balances. Weighting by current balance describes the rate profile of the principal still outstanding more accurately than a simple average by loan count.
For borrowers, WAC is a bridge between one contractual note rate and a pool-level statistic. It does not replace or average the rates borrowers owe on their individual notes.
Borrowers usually do not see WAC during application or closing. The term appears in secondary-market reporting, MBS pool descriptions, and investor analysis after many loans have been pooled.
It becomes practical when explaining why a Mortgage Pool can have a single summary rate even though it contains many mortgages. Because balances change through amortization and prepayment, current WAC can change over time even when no surviving loan’s note rate changes.
Here, B_i is loan i’s current principal balance and r_i is its note rate. Disclosure rules determine the balance date, included loans, and rounding.
| Term | What it describes |
|---|---|
| Note Rate | The rate on one borrower’s mortgage note |
| Weighted average coupon | The balance-weighted loan-rate profile of the pool |
| MBS Coupon | The security-level coupon label |
| Pass-Through Rate | The rate used for cash flow passed through to investors |
A pool contains a $300,000 balance at 6.00% and a $100,000 balance at 7.00%:
The simple average of 6.00% and 7.00% is 6.50%, but that would give the smaller loan too much influence. The balance-weighted result is 6.25%.
Weighted average coupon differs from Note Rate because note rate belongs to one loan, while WAC summarizes many loans in a pool.
It also differs from MBS Coupon. WAC describes the underlying loan-rate profile. MBS coupon is the security-level label used for investor cash flow.
It also differs from Pass-Through Rate. Pass-through rate reflects what is passed through to investors after the security and servicing structure, while WAC describes the underlying loan coupons.