A custom index fundis an index you wrote yourself. Same mechanics as the S&P 500 — a rule that picks stocks and a formula that weights them — except the rule is yours, you hold every underlying share directly, and you can change or exclude anything you want. Until recently this was a product for family offices charging 50+ bps. It isn’t anymore.
The one-sentence definition
A custom index fund is a rules-based portfolio of individual stocks held directly in your own account, built around a thesis you define. No fund wrapper, no ticker, no fund manager — just the rule, the stocks, and whatever rebalance cadence you pick.
Three parts: universe, selection rule, weights
Every index — the S&P 500 included — is three decisions on one page:
- The universe. The pool of candidate stocks your rule operates on. Russell 3000, FTSE All-Share, a thematic basket you defined, anything.
- The selection rule. The filter that picks the final list. Market cap above $500m. Positive free cash flow. Revenue exposure to AI above 20%. Whatever you can defend on a page.
- The weights. How much of each. Equal-weight, market-cap-weighted, factor-weighted, inverse-volatility, or a custom formula.
Plus a rebalance cadence (monthly, quarterly, annual) and optional caps (no single name over 8%, no single sector over 30%). That’s the whole specification — and it’s the same whether you’re BlackRock or a retail investor with a Schwab account.
Custom index vs ETF vs mutual fund
| Custom index | ETF | Mutual fund | |
|---|---|---|---|
| Ownership | Underlying shares directly | Fund shares | Fund shares |
| Rule | Yours — editable | Fund's — fixed | Fund's — fixed |
| Exclusions / overrides | Per-stock, any time | None | None |
| Tax-loss harvesting | Per holding, continuous | Only the whole position | Only the whole position |
| Typical fee | 0–30 bps | 3–80 bps | 30–150 bps |
| Minimum viable size | ~$1k with fractional shares | 1 share (~$50–$500) | $1k–$100k |
| Transparency | Full — you wrote the rule | Published periodically | Published quarterly (US) |
The biggest non-obvious difference: an ETF is a wrapper around a basket; a custom index is the basket. No wrapper means you see every position, can exclude any one of them, and the IRS treats each stock as a separately taxable lot.
What a custom index unlocks
- Exclusions. Drop every tobacco name, every firearms manufacturer, every company on a sanctions list — without giving up the rest of the benchmark.
- Tilts. Start from SPX-500 and overweight companies with AI revenue exposure. Or underweight anything trading above 40× earnings. Your call.
- Tax-loss harvesting. When any one stock falls, swap it for a statistically similar name, bank the loss, and stay in the market. At $50k+ this adds 30–100 bps of after-tax return a year in taxable accounts.
- Concentration management.If you already own a lot of one employer’s stock, build the rest of the portfolio deliberately light on its sector.
- Thematic precision.“Profitable European defence primes and their top-tier suppliers” is a real thesis. No ETF tracks it exactly. A custom index can.
Who it’s actually for
The economics of direct indexing have shifted three times in a decade: zero-commission trading killed the cost of owning 40 stocks, fractional shares killed the minimum account size, and AI tooling is killing the research overhead. The result:
- $10k–$50k accounts that want exclusions or thematic exposure without the tracking error of a niche ETF.
- $50k–$500k accounts in taxable brokerage where tax-loss harvesting starts paying for itself several times over.
- Institutional / HNW accounts that always had this — now the same workflow runs on a phone.
Who it’s not for
- Tiny accounts with a one-fund set-and-forget plan. VT or VWRL is fine — custom indexing adds complexity you don’t need.
- Day traders and single-stock pickers. A custom index is a long-horizon rule, not a way to express a weekly opinion.
- Tax-sheltered accounts where tax-loss harvesting has no value (ISAs, SIPPs, Roth IRAs). You can still run a custom index here — you just give up one of the biggest benefits.
How Arithmos builds one from a sentence
The pipeline is deliberately boring on purpose — every step prints a rule you can read. Paste “Profitable US software businesses at reasonable valuations” and the agent:
- Picks a universe (US mid/large-cap software NAICS codes).
- Applies the filter (positive FCF, gross margin > 60%, EV/Sales under 12×).
- Chooses a weighting scheme (equal-weight with an 8% per-name cap, by default).
- Backtests 25 years against SPY with honest survivorship and rebalance handling.
- Writes the selection rule in plain English so you can audit it line-by-line.
You can tweak any of those decisions before you execute. The point of a custom index isn’t that it’s clever — it’s that every decision is on one page.
FAQ
Is a custom index the same as direct indexing?
Yes. Direct indexing is the industry term; custom index is the plainer-English name. Both mean “own the underlying stocks, run a rule, rebalance on a schedule.”
What’s the smallest viable account?
With fractional shares and zero commissions, a 40-name custom index runs sensibly on around £1,000 / $1,000. Tax-loss harvesting starts materially paying for itself around $50k in a taxable account.
How is this different from picking stocks?
Stock picking makes one discretionary decision per name. A custom index writes one rule that makes every decision for every name, on schedule, consistently, with a backtest. The unit of edit is the rule — not the ticker.
Can I change the rule later?
Yes — that’s the whole point. The more important question is whether you should. The most valuable thing about a rules-based portfolio is the discipline of sticking to the rule through a drawdown.
Do I need to understand quant finance to build one?
No. Arithmos specifically exists to compress the quant-team workflow into a prompt and a reviewed plan. If you can write a sentence that describes what you want to own, the agent handles the mechanics — and shows you the rule afterwards so you can sanity-check it.