Weighted average maturity, or WAM, is the value-weighted average of how long until the bonds in a portfolio reach their final repayment dates, with larger holdings counting more toward the average. A fund holding mostly 2-year Treasury notes will have a WAM near 2 years; a fund concentrated in 20-year corporate bonds will have one near 20. The number gives you a fast read on how much interest rate risk a fund carries and whether its time horizon matches yours.
What the Number Actually Captures
WAM applies mainly to pooled vehicles: bond mutual funds, exchange-traded funds, and structured products such as mortgage-backed securities. It reduces a basket of bonds maturing at different times to a single number by weighting each bond’s time to maturity by its market value in the portfolio. A $50 million position pulls the average more than a $5 million position does.
Two mechanical details shape what the figure means. The maturity used is each bond’s final stated maturity date, not any interim coupon or principal payment along the way. A 10-year bond counts as 10 years regardless of its coupon schedule. And the weighting runs on current market value, not par value or original purchase price, so a bond whose price has risen exerts proportionally more influence on the average.
The result is reported in years for most bond funds and in days for money market funds and other ultra-short vehicles. Regulatory filings require the figure for certain fund types, which makes it one of the easiest risk metrics to find when you are comparing funds.
How to Calculate Weighted Average Maturity
The formula is simple. Multiply each bond’s market value by its remaining time to maturity, add those products together, and divide by the portfolio’s total market value. Here is a worked example with three bonds:
- Bond A: $20 million market value, 1.5 years to maturity
- Bond B: $50 million market value, 3.0 years to maturity
- Bond C: $30 million market value, 5.0 years to maturity
Calculate each bond’s weighted contribution. Bond A contributes $20 million × 1.5 = $30 million. Bond B contributes $50 million × 3.0 = $150 million. Bond C contributes $30 million × 5.0 = $150 million. The sum is $330 million.
Add the total market value of the portfolio: $20 million + $50 million + $30 million = $100 million. Divide $330 million by $100 million, and the WAM is 3.3 years. The portfolio’s holdings, on a value-weighted basis, reach their final repayment dates in a little over three years on average.
Bond B and Bond C together account for 80% of the portfolio’s market value and have maturities of 3 and 5 years, which pulls the WAM well above Bond A’s 1.5 years. If Bond A were the largest position instead, the WAM would drop considerably. That is the point of value-weighting: the number reflects where the money actually sits, not how many different securities the fund owns.
Reading WAM as a Measure of Interest Rate Risk
The practical use of WAM is as a rough gauge of how sensitive a fund is to interest rate changes. When rates rise, bond prices fall, and the decline hits longer-maturity bonds harder than shorter-maturity ones. A fund with a WAM of 12 years will lose more value on a rate hike than a fund with a WAM of 2 years, all else equal.
The relationship works in reverse too. If rates drop, the longer-WAM fund gains more. WAM acts as a dial: turning it up increases both the potential reward and the potential pain from rate movements. Investors expecting rates to fall might favor a higher WAM; those bracing for rate increases might want a lower one. A portfolio concentrated in short-maturity instruments faces a different problem, reinvestment risk, because proceeds from maturing bonds may have to be redeployed at lower yields if rates fall.
For context, bond funds are grouped by maturity. Short-term funds typically hold securities with average maturities under about four years, intermediate-term funds fall roughly in the four-to-ten-year range, and long-term funds extend beyond ten years. These bands help you benchmark a fund’s reported WAM against its category peers. A fund marketed as intermediate-term but reporting a WAM of 14 years is taking a more aggressive maturity stance than its label suggests.
The 60-Day Limit for Money Market Funds
WAM has special regulatory weight for money market funds. Under SEC Rule 2a-7, a money market fund’s dollar-weighted average portfolio maturity cannot exceed 60 calendar days.1eCFR. 17 CFR 270.2a-7 – Money Market Funds The cap keeps these funds anchored to short-term rates so they can maintain a stable net asset value near $1.00 per share. Before 2010 the limit was 90 days; regulators tightened it after the financial crisis exposed how quickly longer-maturity holdings could destabilize a money market fund.
The rule also imposes a separate 120-calendar-day ceiling on weighted average life (WAL), which measures the average time until principal repayment rather than final maturity. The two limits exist because variable-rate securities are treated differently in each calculation. For WAM, a variable-rate government security that resets its interest rate at least every 397 days is treated as maturing on its next reset date rather than its final maturity date.1eCFR. 17 CFR 270.2a-7 – Money Market Funds Floating-rate securities with maturities under 397 days are treated as having a maturity of just one day. These shortcuts make WAM reflect rate sensitivity, because a bond whose rate resets next week behaves like a very short instrument even if its final maturity is years away. WAL ignores the reset-date shortcuts and captures when principal actually comes back.
Money market funds must report both figures on SEC Form N-MFP and post the data prominently on their websites for at least six months.2SEC.gov. Form N-MFP – Monthly Schedule of Portfolio Holdings of Money Market Funds1eCFR. 17 CFR 270.2a-7 – Money Market Funds If you invest in money market funds, both numbers should be easy to find.
Why WAM Is Not the Same as Duration
Investors sometimes treat WAM and duration as interchangeable. They are not, and confusing them can lead to poor risk estimates. WAM only considers when principal is due. Duration, specifically modified duration, considers the timing and size of every cash flow a bond produces, including coupon payments, and discounts them to present value at the bond’s yield. A bond paying a 7% coupon returns more of its economic value to you earlier than a bond paying a 2% coupon, even if both mature on the same date. Duration captures that difference. WAM does not.
Modified duration gives you a direct estimate of price sensitivity: a portfolio with a modified duration of 5 will lose roughly 5% of its value for every 1% rise in interest rates. WAM offers no such translation. Two funds can share an identical WAM and have meaningfully different durations if their coupon rates, yields, or embedded options differ.
Embedded options are where WAM falls furthest behind. Callable bonds, which the issuer can redeem before maturity, will likely be called when rates drop, shortening the bond’s effective life. Putable bonds, which the investor can sell back to the issuer, behave differently again. Effective duration models how cash flows change under different rate environments. WAM ignores all of that and uses the final stated maturity date.
For a portfolio of plain, non-callable, fixed-rate bonds, WAM and duration tend to move in the same direction and a high WAM reliably signals high rate sensitivity. For anything more complex, especially portfolios holding callable corporates, mortgage-backed securities, or floating-rate notes, duration is the sharper tool. Treat WAM as a useful first filter and duration as the instrument you reach for when precision matters.
WAM Versus Weighted Average Life
WAM and WAL answer different questions. WAM asks when the contract says the bond matures. WAL asks when the principal actually comes back. For a standard corporate bond that pays only interest until a single lump-sum repayment at maturity, the two numbers are identical. The distinction becomes critical for any security that returns principal gradually over time.
WAL is calculated similarly to WAM, but instead of using the final maturity date, it multiplies each expected principal payment by the time until that payment occurs, sums the results, and divides by the total initial principal balance.3SEC.gov. Weighted Average Life of the Notes (Outstanding) Because amortizing securities send principal back to investors throughout their life, WAL is always shorter than WAM. The gap can be enormous.
Take a pool of 30-year residential mortgages packaged into a mortgage-backed security. The WAM is 30 years by definition, since that is the contractual term. But homeowners make monthly principal payments and frequently prepay by refinancing or selling. Once typical prepayment behavior is factored in, the WAL shortens dramatically. A Ginnie Mae REMIC trust prospectus illustrates the effect across various prepayment speeds for a 30-year pool:
- 0% PSA (no prepayments): WAL of about 20.5 years
- 100% PSA (standard baseline): WAL of about 11.4 years
- 250% PSA (moderate prepayments): WAL of about 6.5 years
- 425% PSA (fast prepayments): WAL of about 4.2 years
At the commonly used 100% PSA baseline, WAL is roughly 11 years. Under faster prepayment assumptions, it drops to the mid-single digits. The WAM stays at 30 years regardless. An investor relying on WAM to judge the interest rate exposure of an MBS portfolio would overstate the risk by a factor of three or more.
Extension and Contraction Risk
The sensitivity of WAL to prepayment speed creates two distinct risks for MBS investors. When rates fall, homeowners refinance faster, shortening the WAL and returning principal sooner than expected. That sounds good until you realize you have to reinvest the cash at lower rates. This is contraction risk.
Extension risk works the other way. When rates rise, refinancing slows to a crawl. Homeowners hang on to their low-rate mortgages, principal comes back more slowly, and the WAL stretches out. The Ginnie Mae figures show the effect clearly: at 0% PSA the WAL nearly triples compared with the 250% PSA scenario. An investor who bought an MBS expecting a 6-year WAL could end up holding what effectively behaves like a 15-year or 20-year bond if rates spike and prepayments dry up. That bond is now losing value precisely because rates rose, and the investor is locked in longer than planned.
The prepayment assumption most commonly used to project WAL is the conditional prepayment rate (CPR), which expresses prepayments as an annualized percentage of outstanding principal. Analysts model CPR using factors like interest rate levels, loan age, and seasonal patterns. Different CPR assumptions produce different WAL estimates, which is why MBS prospectuses and research reports typically show WAL at multiple prepayment scenarios rather than a single point estimate. If you are evaluating an MBS fund, look for the prepayment assumption underlying the reported WAL and consider how your own rate outlook might push actual prepayments higher or lower than that assumption.