Tax-loss harvesting starts with tracking your lots

Here is a quiet truth the brokerage app would rather not dwell on. Every year, inside your portfolio, there are positions in the red that could be doing useful work for you, and most people never see them clearly enough to act. Tax-loss harvesting is the unglamorous move of selling something at a loss on purpose, so that recognised loss can offset gains elsewhere. Boring, real, and worth actual money. But the whole thing falls apart at step zero, because you cannot harvest a loss you cannot see. Forget the idea that this is some clever trade. It is a bookkeeping problem wearing a trench coat.

First, the grown-up disclaimer, said plainly. This is general education, not tax advice. The rules that govern whether a harvested loss actually counts differ sharply between countries, they change, and they have sharp edges. Strata shows you the lots. It does not advise the trade and it does not place it, so check your own jurisdiction’s rules or a professional first. Right, with that out of the way, let’s talk about why all of this rests on your tax lots.

What tax-loss harvesting is, in principle

Strip away the jargon and the idea is almost insultingly simple. You own something now worth less than you paid for it. You sell it, turning a paper loss into a realised loss the tax system can recognise, and that realised loss can be set against gains you took elsewhere, lowering the bill those gains would create. Many places let you carry an unused loss forward too, though that is exactly the kind of detail that varies, so treat it as illustrative.

The catch is that “worth less than you paid for it” is carrying an enormous amount on its back. Worth less than you paid when, at what price, in which lot, bought on what date? A loss is a precise arithmetic fact about a specific parcel of shares. If you do not know the parcel, you do not know the loss.

Why it lives or dies on accurate tax lots

Here is the part everyone skips, and it is the part that decides everything. A tax lot is one purchase. Forty shares, bought on this date, at this price. That is your cost basis for that parcel. Buy the same holding five times over three years and you do not own “a position.” You own five lots, each with its own basis and holding period, each independently winning or losing right now.

Harvesting happens at the lot level, never the position level. A holding can show a cheerful green number overall while hiding a couple of lots deep in the red, the ones you bought at the top. Sell the wrong lots and you might realise a gain you did not want, or a loss far smaller than the one actually sitting there. Which lots you sell is the whole game, and you can only answer that if your book tracks lots properly. This is also where the FIFO-versus-average-cost decision stops being academic. Under FIFO the oldest lots leave first. Under a single blended average, the per-lot losses you wanted to harvest get smeared into one number and vanish. We pulled that apart in FIFO versus average cost basis; an average hides exactly the thing you are trying to find.

Seeing the losing lots before you act

So the practical job is unglamorous. Make the losses visible. Strata tracks your tax lots in FIFO order and derives your realised and unrealised P&L from the ledger you keep, so you can look at a holding and see it broken into its parcels. You see which lots are underwater, by how much, and when you bought them. The unrealised loss sits right there, per lot, instead of being averaged into a single tidy figure that quietly buries it.

That visibility is the whole point. You are not reconstructing what you paid in 2023 from a shoebox of contract notes. You are reading a book you typed and can reconcile against your statements, which is the only kind of number worth making a tax decision on. None of this works on a scraped, unreconcilable feed where you cannot even tell whether a number is yours or the pipeline’s, which is roughly the case we made for keeping the thing manual by design. Harvest from a book you trust, or do not really harvest at all. You just guess.

And because Strata holds every account in one place, you see the losing lots across all of them at once rather than logging into four brokers and squinting. That matters more than it sounds, because the rules about losses often reach across your accounts, not just the one in front of you.

The rules differ, and that is not a footnote

Now the firm caveat, because this is where people get hurt. Seeing a loss and being allowed to claim it are two different things. Most tax systems have anti-abuse rules to stop you selling at a loss purely for the tax break and then buying the same thing straight back. The wash-sale style rules in some countries, and the bed-and-breakfasting style rules in others, disallow or defer the loss if you repurchase the same or a substantially similar holding within some window around the sale.

Please take this seriously. How long that window runs, what counts as substantially similar, whether the rule reaches across your spouse’s accounts or your pension, the size of your allowances, the way losses carry forward. Every one of those differs sharply by jurisdiction, and governments change them without asking. I am deliberately not handing you a number of days or a rate, because any figure I invented would be wrong for most of you and dangerous for the rest. This is exactly where you check your own country’s current rules, or pay a professional an hour of their time, which is cheap next to a disallowed loss.

Strata’s job stops at showing you the lots, the basis and the P&L, clearly and honestly. Whether a given sale is a valid harvest under your rules is a call for you and your accountant. We built the workbench. We do not give the advice.

Make it a habit, not a December panic

The investors who get value from harvesting losses are not the ones who remember it in a sweat on the last working day of the year. They are the ones who glance at their losing lots regularly, the way they reconcile everything else, so that when a position is genuinely broken they already know which parcels are underwater. It folds neatly into a quarterly portfolio review. Read the book, see what is red at the lot level, then check the rules before you act.

So much for the glamour. Tax-loss harvesting is not a trick you pull out at the last minute. You can only do it well if you have kept an honest, reconciled, per-lot book all along. The trade itself is the easy part. The book is the work, and the book is where the edge actually lives. If you would like one that shows you where your losses sit, lot by lot, you can build your portfolio and start keeping the ledger that makes a harvest possible.