Case study
The real cost of switching from an index fund to a factor tilt
You've read the Fama-French papers, you've been convinced that small-cap value deserves a place in your portfolio, and you're ready to tilt. There's one problem. Most of your money is sitting in a plain index fund in a taxable account, and it's up a lot.
This is one of the most common questions in factor-investing circles, and most of the answers you'll find are vibes. "Don't sell, you'll pay tax." "Just do it, the premium will make it back." Both might be right. Neither tells you how much the premium has to be, or for how long.
So let's do the math.
The setup
Here's a situation I see all the time, so I'll use round numbers:
- $200,000 in an S&P 500 index fund in a regular brokerage account
- You paid $120,000 for it over the years, so there's an $80,000 gain baked in
- All of it is long-term, taxed at 15% federal plus 5% state, so 20% all in
- You'd hold whatever you own for the next 20 years
- You expect the index fund to return 7% a year, after fees
That 7% isn't a forecast. It's a placeholder. The whole point of this exercise is that you plug in your own assumptions, and the math tells you what they imply.
Step one: the bill you pay on day one
Sell the index fund and you realize the $80,000 gain. At 20%, that's $16,000 in tax, which leaves you $184,000 to put into the new fund.
That $16,000 is gone. It doesn't compound for you anymore. The new fund now has to do more work with less money just to end up where you would have been anyway.
Step two: compare apples to apples
Here's where a lot of back-of-the-envelope math goes wrong. People compare the switch path, which has already paid its tax, against the keep path, which hasn't. But the keep path isn't tax-free. It's tax-deferred. Whenever you eventually sell that index fund, the old gain plus 20 years of new growth gets taxed.
So the fair comparison is: sell both at the end of 20 years, pay the tax on both, and see who has more spendable money. Using the assumptions above:
| After 20 years, after all taxes | Amount |
|---|---|
| Keep the index fund (7%/yr) | $643,150 |
| Switch, if the new fund matches it (7%/yr) | $606,418 |
| Switch, if the new fund earns 8%/yr | $722,893 |
If the factor fund only matches the index, switching costs you about $36,700 by year 20. That's the $16,000 tax bill plus everything it would have grown into. If the factor fund beats the index by a full point a year, you come out about $79,700 ahead.
Step three: the hurdle
Somewhere between those two outcomes is a break-even point. That's the hurdle rate: the return the new fund has to earn for both paths to end up exactly equal.
In this example, the hurdle is 7.33% a year. The factor fund needs to beat the index by about a third of a percentage point, every year for 20 years, net of its higher fees, just to break even.
Honestly, that's lower than most people expect. A $16,000 tax bill sounds huge, but spread over 20 years of compounding it works out to a fairly modest annual hurdle.
Time matters a lot here, though:
| Years you'll hold | Hurdle rate | Edge needed over 7% |
|---|---|---|
| 5 | 7.53% | 0.53 pts |
| 10 | 7.45% | 0.45 pts |
| 20 | 7.33% | 0.33 pts |
| 30 | 7.26% | 0.26 pts |
The shorter your horizon, the less time the new fund has to earn back the tax, so the bar goes up. If you're switching money you'll need in five years, you're asking for a lot more.
The part the hurdle doesn't tell you
Here's the thing I want to be really clear about. A hurdle of 0.33% looks easy on paper. But you have to clear it with an expected premium, not a guaranteed one.
Factor premiums are real in the long-run data, but they're lumpy. Small-cap value went through a long, painful stretch in the 2010s where it trailed the broad market for years. If you'd switched at the start of that and needed the premium to show up on schedule, you'd have been underwater for a long time.
So the honest way to look at the decision is as a range, not a single answer. Here's how far ahead or behind switching leaves you, depending on what the factor fund actually earns:
| Factor fund earns | 10 years | 20 years | 30 years |
|---|---|---|---|
| 7.0% (no premium) | −$12,380 | −$36,732 | −$84,637 |
| 7.5% (half-point premium) | +$1,440 | +$18,934 | +$83,569 |
| 8.0% (one-point premium) | +$15,850 | +$79,743 | +$276,062 |
Notice the asymmetry. If the premium never shows up, you lose the tax and its growth. If it does show up, even a modest one, the payoff over 20 to 30 years can be much bigger than the cost. That's the actual bet you're making.
What changes the math
A few things move the hurdle a lot:
- How big your gain is. With a $180,000 basis instead of $120,000, the tax drops to $4,000 and the hurdle falls to 7.08%. With a $60,000 basis, the tax jumps to $28,000 and the hurdle rises to 7.61%.
- Your tax rate. Drop the state tax (15% all in) and the hurdle falls from 7.33% to 7.25%.
- Whether you'll ever sell. If this money is likely to pass to heirs, the keep path may never be taxed at all, because inherited assets generally get a step-up in basis. In that case, the hurdle in this example rises to 7.45%. Paying tax now on a gain that might otherwise disappear makes switching harder to justify.
- Fees. Factor funds usually cost more than a plain index fund. Whatever return you assume for the new fund should already be net of that.
Options people use instead of a big sale
A full switch is rarely the only way to get a tilt. These are approaches you'll often see factor investors talk about. None is right for everyone, but they're worth knowing about:
- Tilt with new money. Leave the index fund alone and point new contributions at the factor fund. It's slower, but it costs nothing in tax.
- Tilt inside tax-advantaged accounts. Switching inside an IRA or 401(k) generally doesn't trigger tax, so some people hold their tilt there.
- Sell the highest-basis lots first. If you bought at different times, some lots carry much smaller gains. Selling those first can get part of the way there for a fraction of the tax.
- Use losses when they show up. In a down market, lots bought near the top may be at a loss, and selling them can cost nothing or even save tax.
Try it with your numbers
Everything in this article came from the same calculator that's on the ChurnAlpha home page. Your situation will be different, so plug in your own value, basis, tax rate and assumptions and see where your hurdle lands.
Run this example in the calculator
The figures assume a single lot, flat tax rates, returns net of fees, and both paths fully sold at the end of the period except in the step-up case. They're illustrations of the arithmetic, not predictions of what any fund will earn, and not a recommendation to buy or sell anything. Tax rules vary; check your own situation with a tax professional.