The quiet math of tax-loss harvesting

There's a particular kind of investing advice that lives forever on the internet because it sounds clever: harvest your losses, lower your taxes, get rich quietly. The math is real. The story usually isn't.

What actually happens when you harvest a loss is that you accept a slightly different version of the same investment, you book a paper loss, and you defer — not erase — a small slice of tax. Do that for thirty years, and the deferral compounds in ways that matter. Do it once, and it's mostly noise.

The interesting question isn't whether harvesting works. It's where it earns its keep and where it just generates trades that make your tax preparer mildly annoyed in February.

We rebuilt our internal harvesting model this year on three principles. The first is that bid-ask costs are real and small accounts give them back faster than they save. The second is that wash-sale rules apply to your spouse's IRA whether you remember that or not. The third — and the one most platforms get wrong — is that the right replacement security is rarely the one that maximizes correlation. It's the one you'll still want to own in eleven months.

A concrete example: a client holds $80,000 in a broad domestic equity fund that has dropped 18% in a volatile stretch — a paper loss of roughly $14,400. We sell the fund, immediately buy a comparable fund tracking a different index, and harvest the $14,400 loss. At a 24% federal rate plus New Jersey's 6.37%, that loss is worth somewhere around $4,350 in deferred tax. Not a windfall. But done every year losses are available — which is most years in a diversified portfolio — and the deferred tax dollars stay invested and compound, the accumulated benefit over twenty years can be meaningful without any change to the portfolio's long-run exposure.

The harvest creates a lower cost basis in the replacement fund, which means you owe the deferred tax eventually — when you sell, or when the shares pass to heirs and the estate settles. The strategy is not about escaping tax. It is about choosing the year it gets paid. And choosing a later year is almost always better, because a dollar of tax paid in twenty years costs less in present-value terms than a dollar paid today, and the money that would have gone to the IRS sits in the market earning returns in the meantime.

There is a category of client for whom this matters quite a lot: someone in a high earned-income year who also has realized capital gains from a business sale or a large real estate transaction. A $30,000 harvested loss in that year doesn't just defer tax — it may actually eliminate a chunk of it if gains and losses net to zero at the portfolio level. That's a different conversation from the standard one about steady annual harvesting, and it's the kind of planning that only happens when the investment account and the tax return are being planned in the same room.

The harvest is not glamorous. No one puts it on a slide in an investment review meeting. But over a thirty-year holding period in a taxable account, consistent harvesting — done carefully, with attention to wash-sale rules and replacement quality — tends to be worth more than a half-percentage-point of additional return. For most households, that's a better trade than chasing the half-point.

One more thing the robo-advisors don't mention: harvesting works differently in a portfolio that spans account types. A loss harvested in a taxable brokerage account offsets capital gains in that same brokerage account, and up to $3,000 of ordinary income per year. But IRA accounts — traditional or Roth — don't generate harvestable losses at all, because all gains and losses inside a retirement account are invisible to the tax code until the money comes out. This means the strategy is entirely a taxable-account conversation. If the majority of a household's wealth is in a 401(k) or IRA, the harvest math changes substantially — and the conversation worth having is about Roth conversions and withdrawal sequencing instead.

We track this for every client with a taxable account. Some years the losses are there; some years the market runs and there is nothing to harvest. The point is to have the machinery in place so that when a volatile quarter hands you a window, you act in it — not six months later when the positions have recovered and the opportunity is gone.