Every number on airiya.org comes from a specific calculation. This page explains exactly how overlap is measured, where the data comes from, how often it is updated, and what the limitations are.
Most overlap calculators simply count the number of securities held by both funds. That approach is misleading — a security at 0.01% weight contributes almost nothing to your real exposure, but counts the same as Apple at 7%+ of the S&P 500.
airiya.org uses a weight-adjusted overlap. For each security held by both funds, we take the smaller of its two weights, then sum those minimums across every shared holding. The result is the proportion of your combined portfolio that is genuinely duplicated.
When you compare more than two ETFs, every pair is calculated independently and shown as a list, sorted from most to least overlap. There is no single "portfolio overlap" number — pair-by-pair is more honest because it shows exactly which combinations are redundant and which complement each other.
To show the calculation in practice, here are the top 5 holdings of VOO (S&P 500) and VTI (Total US Market) using the actual Q1 2026 data we ship:
| Security | VOO | VTI | min(wA, wB) |
|---|---|---|---|
| NVDA | 7.31% | 6.16% | 6.16% |
| AAPL | 6.62% | 5.88% | 5.88% |
| MSFT | 4.95% | 4.40% | 4.40% |
| AMZN | 3.43% | 3.04% | 3.04% |
| GOOGL | 3.06% | 2.73% | 2.73% |
| Top-5 contribution to overlap | 22.2% | ||
VOO holds 504 stocks (the S&P 500 plus a few related listings). VTI holds 3,503 stocks (the total US market). They share 502 securities — almost every S&P 500 component appears in VTI as well. Because VTI is market-cap-weighted, those 502 large-caps dominate its weight too, which is why the overlap is so high despite VTI holding 7× more stocks.
When you click any pair on the homepage, we show the 5 stocks contributing the most to the overlap. The number shown is each stock's min(weight in A, weight in B) — its contribution to the overlap weight, not its standalone weight in either fund.
This matters because the same stock can have very different weights in different funds. For example, NVIDIA might be 8% of CSPX (S&P 500) but only 4% of VWCE (FTSE All-World). The overlap contribution is the smaller number — 4% — because that is how much of NVIDIA exposure is actually duplicated when you hold both.
To make this explicit, the panel shows three numbers for every stock:
All holdings data comes directly from each fund provider's official disclosures. We do not use third-party aggregators as a primary source, because aggregators introduce their own normalisation errors and update lag.
| Provider | Funds covered | Format |
|---|---|---|
| Vanguard | VOO, VTI, VXUS, VT, VWCE, VWRL, VGWD, VEUR, VFEM, VGS, VAS, VHY | CSV |
| iShares (BlackRock) | IWDA, IUSQ, EMIM, CSPX, SXR8, CSNDX, IEUR, WSML | CSV |
| BetaShares | A200, DHHF, NDQ | CSV |
| Schwab | SCHG, SCHD | CSV |
| Invesco | QQQ, FWRA | CSV |
| SPDR (State Street) | SWRD | CSV |
| Amundi | WEBN, CEU2 | CSV |
Each file is downloaded, parsed, and normalised into a common schema. Tickers are cleaned (e.g. VTI UP → VTI, CBA AT → CBA) and used as the primary identifier. For funds that disclose company names but not tickers (Amundi WEBN/CEU2, Invesco FWRA, SPDR SWRD), we match by normalised company name against tickers from other ETFs covering the same securities.
ETF holdings are updated quarterly by most fund providers, though some publish monthly or daily snapshots. Our database is re-verified on the following schedule:
airiya.org currently covers 30 ETFs across three jurisdictions. The list is curated for breadth (US/UCITS/ASX) rather than completeness — we cover the funds global investors actually ask about most.
In scope:
Not covered:
If you want a fund added, request it via the contact page. We add the most-requested tickers each quarterly update.
Transparency about limitations is more useful than hiding them. Here is what to keep in mind: