Weighted average maturity, or WAM, is the value-weighted average time until the bonds in a portfolio reach their final repayment dates. Larger positions count more toward the average than smaller ones, so a fund holding mostly 2-year Treasury notes lands near a 2-year WAM, while one concentrated in 20-year corporate bonds lands near 20. The number gives you a quick read on how much interest rate risk a bond fund carries and whether its time horizon lines up with yours.
What WAM Measures
WAM applies mainly to pooled vehicles: bond mutual funds, exchange-traded funds, and structured products like mortgage-backed securities. It compresses a basket of bonds maturing on different dates into one figure by weighting each bond’s remaining time to maturity by its market value in the portfolio. A $50 million position moves the average ten times as much as a $5 million position.
Two mechanics shape what the number captures. First, WAM uses each bond’s final stated maturity date. A 10-year bond counts as 10 years regardless of the coupons it pays along the way. Second, the weighting uses current market value, not par value or original purchase price. If a bond’s price has risen, its influence on the portfolio’s WAM rises with it.
Most bond funds report WAM in years. Money market funds and other ultra-short vehicles report it in days. Regulatory filings require the figure for certain fund types, which makes it one of the easier risk metrics to find when you compare funds side by side.
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, and divide by the portfolio’s total market value.
Here is a three-bond portfolio:
- 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 weighted contribution. Bond A: $20 million × 1.5 = $30 million. Bond B: $50 million × 3.0 = $150 million. Bond C: $30 million × 5.0 = $150 million. Summed, that comes to $330 million.
Total market value is $20 million + $50 million + $30 million, or $100 million. Divide $330 million by $100 million, and the WAM is 3.3 years. On a value-weighted basis, the portfolio’s holdings will reach their final repayment dates in a little over three years on average.
Notice that Bond B and Bond C together hold 80% of the money and mature in 3 and 5 years. They pull the WAM well above Bond A’s 1.5-year maturity. If Bond A were the largest position instead, the WAM would drop considerably. That is the point of value-weighting: the average reflects where the money sits, not just how many different securities the portfolio owns.
Reading WAM for Interest Rate Risk
The practical use of WAM is as a rough gauge of how a fund reacts to interest rate changes. When rates rise, bond prices fall, and the drop hits longer-maturity bonds harder. A fund with a WAM of 12 years will lose more on a rate hike than a fund with a WAM of 2 years, all else equal. If rates drop, the longer-WAM fund gains more. WAM works like a dial: turning it up raises both the potential reward and the potential pain from rate movements.
Bond funds are broadly grouped by average maturity. Short-term funds typically hold securities averaging under about four years, intermediate-term funds fall roughly in the four-to-ten-year range, and long-term funds extend beyond ten years. Those bands let 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 stance than its label suggests.
Short-maturity portfolios face a different problem: reinvestment risk. Proceeds from maturing bonds may have to be reinvested at lower yields if rates fall.
WAM Is Not the Same as Duration
Investors sometimes treat WAM and duration as interchangeable. They are not, and confusing them can produce poor risk estimates. WAM only considers when principal is due. 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 also 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 still 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, tend to get 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 scenarios. WAM ignores all of it and uses the final stated maturity.
For a portfolio of plain, non-callable, fixed-rate bonds, WAM and duration tend to move together, 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 first filter and duration as the instrument you reach for when precision matters.
WAM Versus Weighted Average Life
WAM and weighted average life (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 match. The distinction matters for any security that returns principal gradually.
WAL is calculated much like WAM, but instead of using the final maturity date, it multiplies each expected principal payment by the time until that payment and divides by the total initial principal balance.1SEC.gov. Weighted Average Life of the Notes (Outstanding) Because amortizing securities send principal back throughout their life, WAL is always shorter than WAM, and the gap can be large.
Take a pool of 30-year residential mortgages inside a mortgage-backed security. The WAM is 30 years by contract. But homeowners make monthly principal payments and frequently prepay by refinancing or selling. Under a standard 100% PSA prepayment assumption, the WAL on a 30-year Ginnie Mae pool can come in around 11 years; at faster prepayment speeds, it can drop into the mid-single digits.2Ginnie Mae. Ginnie Mae REMIC Trust 2025-039 The WAM stays at 30 years the whole time. 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.
The takeaway: if a fund holds mortgage-backed or other amortizing securities, look at WAL alongside WAM, and check which prepayment assumption produced the WAL figure.
Money Market Fund Rules
WAM carries 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.3eCFR. 17 CFR 270.2a-7 – Money Market Funds The cap exists to keep 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.
The rule also imposes a separate 120-calendar-day ceiling on weighted average life. The two limits exist because of how variable-rate securities are treated. For WAM purposes, 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.3eCFR. 17 CFR 270.2a-7 – Money Market Funds Floating-rate securities with maturities under 397 days are treated as maturing in one day. Those shortcuts make WAM reflect the fund’s sensitivity to rate movements. WAL ignores the reset shortcuts and measures when principal actually comes back, capturing credit and liquidity risk instead.
Money market funds must report both WAM and WAL figures on SEC Form N-MFP each month.4SEC.gov. Form N-MFP – Monthly Schedule of Portfolio Holdings of Money Market Funds If you invest in money market funds, both numbers should be visible on the fund’s website or in its filings.