What it means
A bullet bond repays everything at maturity, so its life is obvious. Amortising instruments are different: mortgages, auto loans and asset-backed securities drip principal back month by month, and the question of when the money comes home has no single answer, so average life supplies the best single one.
The measure weights each principal payment by the time until it arrives and divides by total principal. A loan returning half its principal in year one and half in year five has an average life of three years, whatever its legal final maturity says.
Securities filings formalise the concept, since United States Securities and Exchange Commission structured data for funds holding mortgage-backed securities includes maturity breakdowns by estimated weighted average life, showing that regulators and issuers treat it as a standard descriptor of these portfolios. Prepayments make the estimate a moving target.
Mortgage borrowers refinance when rates fall and stay put when rates rise, so the average life of a mortgage pool shortens and lengthens with the rate cycle, which is why analysts quote average life under stated prepayment assumptions. Extension risk is the mirror image of prepayment risk, because when rates rise prepayments slow, the average life stretches, and investors are stuck longer in paper yielding less than the new market.
Average life drives risk and return decisions for amortising paper. Reinvestment risk concentrates around it, since that is when the cash actually returns to be redeployed, and the yield quoted for such securities is meaningful only against the assumed life, so a bond priced to a four-year life that prepays in two delivers a different realised return.
An asset-backed security's promised yield also assumes principal returns on schedule, which means comparing two deals on yield without comparing average lives is comparing apples of different ripeness. The concept differs from duration, though they rhyme.
Duration measures price sensitivity to rate changes using all cash flows including interest, while average life tracks only principal return, and for amortising bonds both fall as prepayments accelerate but they answer different questions. Pricing models quote average life under several rate scenarios precisely to expose both directions of risk.
For treasurers buying asset-backed paper, the discipline is to compare average life across deals under identical prepayment scenarios and to stress the assumption, since the figure is a model output wearing a fact's clothing. Servicers' monthly remittance reports show how much principal actually arrived against projections, and serious investors track the error and chart realised against projected lives, because a pool that persistently prepays faster than modelled has rate-sensitive borrowers.
Credit enhancement sits below the average-life maths, so principal timing and loss protection must be read together, and some structures include cleanup calls that cap the tail by retiring the deal once the remaining pool shrinks below a set share.
In practice
Real-world examples.
Example
An investor compares two mortgage-backed securities by average life rather than final maturity. One has a 30-year legal maturity but an average life of seven years, while the other has a 15-year legal maturity and an average life of six. She learns that the two commit her money for similar periods in practice.
Example
An analyst restates a pool's average life after a refinancing wave speeds up principal return. The estimate drops from 5.5 years to 4.2 years under the new prepayment assumption. The restated figure changes the security's expected yield and its fit with the fund's mandate.
Example
A portfolio manager matches an asset-backed holding's average life to a known three-year liability. She tests the holding under fast and slow prepayment scenarios to see whether the match survives a change in rates. Only the version that stays near three years in both scenarios is kept.
Formula
Calculation
Average life = sum of (principal repaid at time t x t) / total principal. A pool returning 50% of principal in year 1 and 50% in year 5 has an average life of (0.5 x 1) + (0.5 x 5) = 3 years.
Worked example. A $1,000,000 loan pool repays $200,000 of principal in year 1, $300,000 in year 2 and $500,000 in year 4. The weighted sum is ($200,000 x 1) + ($300,000 x 2) + ($500,000 x 4) = $200,000 + $600,000 + $2,000,000 = $2,800,000. Dividing by total principal of $1,000,000 gives an average life of 2.8 years.
Now suppose refinancing accelerates and the same pool repays $500,000 in year 1, $300,000 in year 2 and $200,000 in year 4. The weighted sum becomes $500,000 + $600,000 + $800,000 = $1,900,000, so the average life shortens to 1.9 years. The legal final maturity has not changed, but the investor's cash returns almost a year sooner and must be redeployed at whatever rates then prevail.Case study
Seen in the real world.
This is a fictional example. Lakeside Credit Union, an invented institution, buys a $5,000,000 auto-loan security quoted with a 2.1-year average life under base assumptions. When rates fall and prepayments accelerate, the realised life shortens to 1.6 years, so principal comes back about half a year earlier than planned and must be reinvested at lower yields. The treasurer reviews the purchase and realises the quote came with a single prepayment assumption. For later purchases she asks dealers for average life under fast, base and slow scenarios, for example 1.6, 2.1 and 2.9 years, and rejects any deal whose range is wider than her liability schedule can absorb.
Watch out
Common mistakes.
- Using final maturity instead of average life for amortising paper. Most principal returns far earlier, so maturity overstates commitment.
- Quoting average life without the prepayment assumption. The number is scenario-dependent and moves with interest rates.
- Confusing average life with duration. Average life tracks principal return timing; duration measures rate sensitivity using all cash flows.
Questions
People also ask.
How is average life calculated?
Weight each principal payment by its timing and divide by total principal, using scheduled payments plus assumed prepayments.
Why does average life change over time?
Prepayment rates shift with interest rates and borrower behaviour, shortening or lengthening the actual principal return.
Is average life the same as weighted average life?
Yes; the full name is weighted average life, and both terms describe the same principal-weighted average.
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