Key takeaways
– The weighted average coupon (WAC) is the size-weighted average interest rate of the mortgages that back a mortgage‑backed security (MBS).
– WAC describes the coupon (interest) characteristic of a pool at a point in time; it can change as loans amortize, prepay, or default.
– Analysts use WAC to estimate interest income from an MBS and to compare pools; it should be used alongside other metrics (e.g., pass‑through rate, weighted average maturity, prepayment assumptions).
– Simple arithmetic (weights = outstanding principal balances) yields WAC; it’s easy to compute in a spreadsheet or with a short script.
What is the Weighted Average Coupon (WAC)?
The WAC is the principal-weighted average of the coupon rates on the individual mortgages in a mortgage pool or an MBS. Because each loan contributes interest in proportion to its outstanding principal, the WAC uses each mortgage’s remaining principal balance as its weight. WAC is typically expressed as an annual percentage and reflects the coupon at the time of measurement.
Why WAC matters
– Cash‑flow expectations: WAC indicates the gross interest rate the pool is earning and determines the pool’s gross coupon cash flow before servicing fees, credit losses, and prepayments.
– Comparison: WAC lets investors compare different pools or tranches on a basic yield characteristic.
– Modeling and pricing: WAC is an input for valuation models and scenario analysis (e.g., to estimate expected interest receipts under given prepayment speeds).
– Risk monitoring: Changes in WAC over time can signal that higher‑ or lower‑rate loans are prepaying or defaulting at different rates, which affects both yield and reinvestment risk.
How to calculate WAC — conceptual formula
WAC = (Σ (Coupon_i × OutstandingPrincipal_i)) / (Σ OutstandingPrincipal_i)
Steps to calculate WAC (manual / by hand)
1. For each loan or loan pool, record: coupon rate (as a decimal) and current outstanding principal balance.
2. Multiply each coupon rate by its loan’s outstanding balance to get interest‑weighted dollars for each loan.
3. Sum the interest‑weighted dollars across all loans.
4. Sum the outstanding principal balances across all loans (this is the total pool balance).
5. Divide the total interest‑weighted dollars by the total pool balance. The result is the WAC (convert to % if desired).
Worked example
Suppose three mortgage pools combine into one MBS with total outstanding principal $11,000,000:
– Pool A: $4,000,000 at 7.50%
– Pool B: $5,000,000 at 5.00%
– Pool C: $2,000,000 at 3.80%
Step 1–3: Multiply and sum interest‑weighted dollars:
– A: $4,000,000 × 0.075 = $300,000
– B: $5,000,000 × 0.05 = $250,000
– C: $2,000,000 × 0.038 = $76,000
Sum = $626,000
Step 4: Total outstanding principal = $11,000,000
Step 5: WAC = $626,000 / $11,000,000 = 0.0569 = 5.69%
Alternate (“weights-first”) approach
– Compute weights: wA = 4/11 = 36.36%, wB = 5/11 = 45.45%, wC = 2/11 = 18.18%
– WAC = wA×7.5% + wB×5.0% + wC×3.8% = 5.69%
Practical computing: Excel and Python examples
– Excel:
• Suppose coupon rates in column B (as decimals) and outstanding balances in column C for rows 2:n.
• Formula for WAC: =SUMPRODUCT(B2:Bn, C2:Cn) / SUM(C2:Cn)
– Python (pseudocode using lists):
• coupons = [0.075, 0.05, 0.038]
• balances = [4_000_000, 5_000_000, 2_000_000]
• wac = sum(c*p for c,p in zip(coupons, balances)) / sum(balances)
How and why WAC changes over time
– Amortization: As borrowers pay principal, the weights change; loans with faster paydown shrink relative weights.
– Prepayments: If higher‑coupon loans prepay faster (common when rates fall), the pool’s WAC tends to decline because the higher‑rate loans leave the pool earlier. The converse can happen when low‑coupon loans prepay more quickly.
– Defaults/foreclosures: Credit losses remove principal and can skew the remaining composition and thus the WAC.
– New originations are not added to a static pool, so WAC is dynamic and must be recalculated periodically.
WAC vs related metrics (brief)
– Pass‑through rate: The rate distributed to investors after deducting servicing, guarantee fees, and other fees from the gross interest collected. Pass‑through rate = WAC − fees (approximately).
– Weighted average maturity (WAM) / Weighted average life (WAL): These are timing measures (how long principal is expected to remain outstanding), not interest rate measures. Use WAC with WAM/WAL for cash‑flow modeling.
– WALA (weighted average loan age): useful to understand seasoning; it does not replace WAC.
When an MBS gets risky — how WAC fits in
– WAC alone doesn’t capture credit risk or prepayment risk. A high WAC may reflect higher coupons but could also indicate subprime or riskier underwriting.
– During the 2007–2008 crisis, many MBS were backed by subprime and poorly underwritten loans. Although some pools had attractive WACs, defaults and rapid deleveraging destroyed expected cash flows and valuations (see FDIC history and investor resources for background).
– Always combine WAC with credit quality metrics (FICO, LTV), documentation type, seasoning, and prepayment assumptions.
Practical steps for analysts and investors
1. Gather data: coupon rates, current outstanding principal per loan/pool, servicing/guarantee fees, loan attributes (FICO, LTV, documentation, origination date).
2. Compute WAC using the SUMPRODUCT / SUM approach above. Recalculate regularly to reflect paydowns and prepayments.
3. Calculate pass‑through rate: WAC minus servicing and guarantee fees (if those fees are available).
4. Model cash flows under multiple prepayment scenarios (e.g., PSA speeds) to see how WAC interacts with prepayment behavior and reinvestment risk.
5. Compare WAC to market yields and to comparable pools; a spread analysis helps identify pricing vs risk.
6. Monitor changes: if WAC shifts substantially over a short period, investigate whether prepayments are concentrated in certain coupon bands or whether credit losses are affecting specific cohorts.
7. Use WAC alongside credit and timing metrics (WAL/WAM, delinquency rates, loss severities) for a full risk assessment.
Limitations and common pitfalls
– WAC is a snapshot: it reflects current composition and does not predict future coupon composition without prepayment/default assumptions.
– Not a substitute for credit analysis: two pools with identical WACs can have very different credit profiles and risk.
– Fee structure matters: WAC is gross interest; net yield to investors will be lower after fees and losses.
– Prepayment behavior can materially alter realized returns even if WAC is unchanged initially.
Regulatory and historical context
MBS and related structured products were central to the 2007–2008 financial crisis. Many pools contained subprime or poorly underwritten loans; WAC or similar coupon figures did not reflect the true credit and prepayment risks embedded in those pools. For historical background and regulatory context, see FDIC historical summaries and investor education materials about MBS.
Selected sources and further reading
– Investopedia — “Weighted Average Coupon (WAC)” (source provided)
– U.S. Securities and Exchange Commission / Investor.gov — Mortgage‑Backed Securities explanation:
– FDIC — Crisis and Response: An FDIC History, 2008–2013 (historical background on MBS in the financial crisis) /
Editor’s note: The following topics are reserved for upcoming updates and will be expanded with detailed examples and datasets.