IWM vs MDY: 11% overlap
iShares Russell 2000 ETF and SPDR S&P MidCap 400 ETF Trust share 11.4% of their portfolios by weight — computed as Σ min(wA, wB) over every holding both funds own.
You would hold two tickers and get the diversification of 1.9. Two funds that shared nothing would score 2.0; two identical funds score 1.0.
Top shared holdings
IWM and MDY hold 78 of the same securities. The ten contributing most to the overlap figure:
| Holding | in IWM | in MDY | Δ |
|---|---|---|---|
| MOG-A | 0.4% | 0.3% | 0.1% |
| CYTK | 0.3% | 0.3% | 0.0% |
| BTSG | 0.4% | 0.3% | 0.1% |
| CTRE | 0.3% | 0.3% | 0.0% |
| UMBF | 0.3% | 0.3% | 0.1% |
| ONB | 0.3% | 0.2% | 0.0% |
| HQY | 0.3% | 0.2% | 0.0% |
| TEX | 0.3% | 0.2% | 0.0% |
| CMC | 0.2% | 0.2% | 0.0% |
| FLR | 0.2% | 0.2% | 0.0% |
Where IWM and MDY differ
Sector weights as a share of each fund, ordered by the size of the gap rather than the size of the sector — two funds that overlap heavily still usually diverge somewhere.
| Sector | IWM | MDY | Δ |
|---|---|---|---|
| Industrials | 14.7% | 25.4% | −10.7pp |
| Health Care | 19.9% | 9.7% | +10.2pp |
| Financials | 19.0% | 14.8% | +4.2pp |
| Materials | 4.2% | 5.4% | −1.2pp |
| Energy | 6.1% | 4.9% | +1.2pp |
| Cash & Other | -0.0% | 1.1% | −1.1pp |
| Communication Services | 2.5% | 1.5% | +1.0pp |
| Information Technology | 12.8% | 13.8% | −1.0pp |
Δ is IWM minus MDY, in percentage points. Sector data is not published by every issuer, so “Unclassified” is a real category rather than a gap in the table.
What each fund holds alone
86.7% of IWM by weight, held in no part of MDY.
87.5% of MDY by weight, held in no part of IWM.
A large count with a small weight is the usual shape: the positions one fund adds are typically its smallest ones.
Concentration and cost
| IWM | MDY | |
|---|---|---|
| Top 10 holdings | 3.3% | 7.6% |
| Positions held | 1,967 | 400 |
| Cost per $10,000 held, per year | $19 | $23 |
Cost is the expense ratio applied to $10,000; it excludes any brokerage commission, bid-ask spread or tax consequence. Figures are facts about the funds, not a suggestion about which to hold.
Frequently asked
How much do IWM and MDY overlap?
IWM and MDY overlap 11.4% by portfolio weight — largely distinct portfolios. They share 78 holdings. The figure comes from current issuer holdings files (IWM as of 2026-07-23, MDY as of 2026-07-23).
How is the overlap between IWM and MDY calculated?
As the sum of the smaller weight in each fund, over every holding both funds own: Σ min(wA, wB). If IWM holds a stock at 6% and MDY holds the same stock at 4%, 4% counts as shared. The result is bounded between 0% and 100%, and it is computed after looking through any fund-of-funds to the underlying holdings.
Is it redundant to hold both IWM and MDY?
That depends on your goals — we state facts, not advice. At 11.4% overlap, each fund still contributes meaningfully distinct exposure.
What does IWM own that MDY doesn't?
IWM holds 1,967 positions and MDY holds 400; 1,888 of IWM's positions don't appear in MDY at all, and they are 86.7% of IWM by weight.
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Related comparisons
Sources: issuer holdings files, IWM as of 2026-07-23, MDY as of 2026-07-23. Weights are fractions of each fund. Diagnostics, not advice.