What is weighted average life for an ABS or MBS, and how does it differ from maturity and duration?

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AcadiFi · Accepted Answer

Weighted average life (WAL) measures the average time until principal is repaid, weighted by the amount of principal received in each period. Unlike stated maturity (the legal final payment date) or Macaulay duration (the weighted-average time of all cash flows including interest), WAL focuses exclusively on principal cash flows.

**Why WAL Matters for Amortizing Securities**

For a bullet bond, WAL equals maturity — all principal is returned at the end. But ABS and MBS return principal gradually through scheduled amortization and prepayments, so the average dollar of principal comes back much sooner than the stated maturity.

**WAL Formula**

WAL = Σ (tᵢ × Principalᵢ) / Total Principal

Where tᵢ is the time (in years) of each principal payment and Principalᵢ is the amount received.

**Worked Example — Harborview Auto ABS (3-Year Sequential Tranche A):**

Original principal balance: $200M. Monthly amortization schedule at 0% CPR (no prepayments):

| Period (Months) | Time (Years) | Principal Received | tᵢ × Principalᵢ |
|---|---|---|---|
| 1-12 | 0.5 (avg) | $72M | $36.0M |
| 13-24 | 1.5 (avg) | $68M | $102.0M |
| 25-36 | 2.5 (avg) | $60M | $150.0M |
| **Total** | | **$200M** | **$288.0M** |

WAL = $288M / $200M = **1.44 years**

The stated maturity is 3 years, but the WAL is only 1.44 years because most principal is received in the first two years.

**Prepayment Sensitivity:**

If prepayment speed increases (higher CPR), principal returns faster and WAL shortens:

| Prepayment Speed | WAL | Change |
|---|---|---|
| 0% CPR | 1.44 years | Baseline |
| 5% CPR | 1.28 years | −0.16 |
| 10% CPR | 1.14 years | −0.30 |
| 20% CPR | 0.91 years | −0.53 |

**WAL vs. Maturity vs. Duration:**

| Measure | What It Captures | Includes Interest? | Includes Prepayment? |
|---|---|---|---|
| Stated maturity | Legal final payment date | No | No |
| WAL | Avg time to principal return | **No** | Yes (if modeled) |
| Macaulay duration | Avg time of all cash flows | **Yes** | Yes (if modeled) |
| Effective duration | Price sensitivity to rate change | Yes (implicitly) | Yes |

**Why Analysts Prefer WAL for ABS:**

1. **Reinvestment risk** — WAL tells you when your principal comes back and must be reinvested. A shorter WAL means earlier reinvestment at potentially lower rates.
2. **Tranche comparison** — In a sequential-pay CMO, the A tranche has a short WAL (receives principal first), while the Z tranche has a long WAL (receives principal last).
3. **Pricing** — ABS are often priced at a spread to the Treasury with a maturity matching the WAL, not the stated maturity.
4. **Cash flow planning** — Institutional investors match WAL to their liability outflows.

**Limitations:**

- WAL is an estimate — it depends on the assumed prepayment speed. Change the CPR assumption and WAL changes materially.
- WAL does not capture interest rate sensitivity like effective duration does.
- For credit-sensitive ABS, WAL should also incorporate expected defaults (some principal is lost, not just delayed).

Explore WAL and structured product analysis in our CFA Level II fixed income course.

What is weighted average life for an ABS or MBS, and how does it differ from maturity and duration?

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