Know What YouActually Own.

ETF overlap is when two or more funds in your portfolio hold the same underlying stocks, so you own those companies several times over without seeing it. It matters because overlapping funds quietly concentrate your portfolio: the S&P 500 makes up most of a total US market fund by weight, so holding an S&P 500 ETF alongside a total-market ETF adds very little diversification — the same mega-cap stocks dominate both. A brokerage statement can’t show this, because it lists each fund as a single line. To find your real exposure you have to look through each ETF to its constituent holdings, multiply each stock’s weight inside the fund by the fund’s weight in your portfolio, and sum across every position — which is exactly what a PortLens scan automates. Or start with the numbers for popular fund pairs.

Look-through exposure is your portfolio measured at the level of underlying companies rather than tickers. Each fund is replaced by its constituent holdings at their effective weights — a 20% position in an ETF that holds 7% Apple contributes 1.4% Apple to you — and exposures to the same company are summed across every fund and direct holding. The result answers questions a holdings list can’t: how much of your portfolio is really in one stock, one sector, or one country once every wrapper is opened. Institutions have run this analysis for decades; PortLens computes it in about a minute, along with effective holdings — a concentration measure showing how many equally-weighted positions your portfolio actually behaves like.

Count your independent sources of risk, not your tickers. A useful check has three steps: look through your funds to underlying companies (overlapping ETFs can make ten funds behave like one), measure concentration with effective holdings (1 ÷ the sum of squared weights — a portfolio of one 50% and two 25% positions behaves like 2.7 equal positions, not 3), and check the spread of that looked-through portfolio across sectors and geographies. A free PortLens scan runs all three and returns a 0–10 diversification score built from holdings count, sector and industry spread, look-through concentration, and geography — with the formula public on the methodology page.

There’s no universal threshold — what matters is whether overlap concentrates you beyond what you intended. Two S&P 500 trackers from different providers overlap almost completely, which is harmless duplication; a growth fund stacked on a tech fund and a Nasdaq-100 fund is riskier, because the same handful of mega-caps can quietly become a third of your portfolio. The practical test is your top-10 looked-through weight and your effective holdings count: if opening up your funds reveals far fewer independent positions than your account statement suggests, the overlap is doing real damage to your diversification. PortLens shows both numbers for your actual portfolio, so the “too much?” question gets a measured answer instead of a rule of thumb.

Yes — the core scan is free and doesn’t require an account. Enter your holdings and you get the full look-through: true exposure, ETF overlap, hidden concentration, portfolio beta, and the diversification score. A Pro tier adds return projections (five independent models, including Monte Carlo simulation and a Fama-French factor regression on your actual holdings). Portfolio data stays private: scans aren’t published anywhere, and competitions only ever share the stats you explicitly choose to share.

Market data comes from Financial Modeling Prep (US equities and ETF constituents) with Yahoo Finance as the international fallback and CoinGecko for crypto; the risk-free rate is the 1-year US Treasury yield from FRED; factor data is the daily Fama-French series from the Ken French Data Library. Featured investor portfolios come from SEC 13F filings. Every source, refresh schedule, and known limitation — including exactly where ETF constituent coverage is weakest — is documented on the methodology page.