How to track cost basis across multiple accounts

Here is a question that quietly wrecks more tax returns than anyone admits. You own the same ticker in three places. A taxable account, a pension or ISA, and an exchange you opened on a whim one Tuesday. You bought it at different prices, in different years, in wildly different moods. So when you sell some of it, what did it cost you?

If you had to think about that for more than a second, congratulations, you have a cost basis problem. Anyone who has tried to track cost basis across accounts in a single spreadsheet knows the special flavour of dread involved. The mess is fixable, though, and the fix has nothing to do with concentrating harder. You keep one honest book that tracks cost basis across every account for you, lot by lot. Let me show you how the wheels come off, and how to bolt them back on.

Why your cost basis fragments across accounts

Your cost basis does not start out scattered. It gets scattered, one perfectly reasonable decision at a time.

You buy a hundred shares in your taxable brokerage at 40. A year later you top up the same company in your pension at 55, because the thesis still holds and the wrapper is too good to waste. Then a bonus lands and you grab a few more on an exchange at 70, because that is where the cash happened to be sitting. Now you have three accounts holding the exact same thing at three different prices, and none of them knows the others exist.

Each account will helpfully tell you the cost basis for the shares it personally holds. None of them can tell you the truth across all of them, because no broker can see inside the other guy’s books. The dashboard in front of you is missing two thirds of the story and has no idea it is lying. You are the only one in the building who can see the whole position. So you are the only one who can keep the basis straight. Nobody hands you that job, and everybody pretends it happens by itself.

What goes wrong at sale time

Fragmented basis sits there harmlessly for years. It only bites when you sell, which is precisely the moment you cannot afford a wrong number.

Say you sell sixty shares. Which sixty? The cheap ones from the taxable account at 40? The pension ones, where the gain might be sheltered anyway? Some blend you eyeballed at midnight? Your realised profit and loss, and the tax bill stapled to it, swings wildly depending on the answer. Pick the wrong lots and you either overpay the taxman for no reason or, worse, underpay him and find out about it later with interest.

The usual spreadsheet move is to mash everything into one blended average cost across all accounts and call it a day. It feels tidy. It is also wrong in most jurisdictions, because the rules generally care which specific shares left the building, and not the comfortable average you invented for your own peace of mind. A blended number papers over the exact distinction the tax rules are built on. Your realised gain ends up looking plausible, filing cleanly, and being quietly incorrect. That is the worst kind of wrong, because nothing flashes red.

FIFO and the magic of tax lots

The unglamorous bit of machinery that fixes all of this is the tax lot.

A tax lot is one purchase, remembered properly. So instead of “I own 230 shares at some vague average”, you have “I bought 100 on this date at 40 in the taxable account, 100 on that date at 55 in the pension, 30 last month at 70 on the exchange”. That is three lots, each carrying its own date, its own price, its own home. Keep your buys as lots and the muddle evaporates, because nothing got averaged away. The detail is all still there.

Selling then becomes arithmetic instead of guesswork. Under FIFO, first in, first out, the oldest lot goes first. Sell sixty shares and the system retires the 40 lot before it touches anything newer, works out the realised P&L from that specific cost basis, and leaves the rest of your lots standing for next time. No blended fudge, no midnight eyeballing. The cost basis that leaves the book is real, traceable to an actual purchase you actually made.

That is the whole trick to tracking cost basis across multiple accounts. The accounts never have to talk to each other. Every purchase just has to be recorded as its own lot, in one book, so a sell can reach across all of them and consume the right shares in the right order. Strata tracks FIFO tax lots per purchase and derives your realised P&L as you log each sell, so the cost basis stays honest no matter how many accounts the position is smeared across. We have written more about why one reconciled book beats a pile of separate logins in tracking a portfolio across multiple brokerage accounts.

A fair warning, because we are not your accountant. FIFO is the common default, but the lot rules and how gains are taxed differ by jurisdiction, and some places have their own averaging conventions you do not get to opt out of. This is general education, not advice for your specific situation. Know your local rules, or ask someone whose actual job that is.

Reconciling against your statements

A book is only worth keeping if you can prove it is right. So prove it.

Every quarter, or whenever your nerves demand it, hold your book up against the actual statements your accounts send you. For each account, the share count and the lots in your book should match the broker’s reality line for line. When they agree, you have stopped hoping your numbers are correct and started knowing it, because you checked them against the source. A book you can reconcile is a book you can trust. A book you cannot reconcile is just an anxious guess with nicer formatting.

Keeping things by hand quietly wins here too. When you typed every buy and sell yourself, a mismatch is a fact you can chase down. Maybe a dividend reinvestment you forgot. Maybe a fee, or a transfer between accounts. You fix it by logging the missing entry, not by praying some scraper sorts itself out. And when you find a mistake, you do not delete it and pretend it never happened. You void it. It gets flagged and excluded from the maths while still sitting there in the audit log, because the history of how you got the number right is part of the number being trustworthy. It is the same reason accountants do not use erasers. We make the case for doing it this way on purpose in why Strata is manual by design.

So, where does this leave you?

Your cost basis only looks complicated. The real reason is that it currently lives in three accounts that refuse to introduce themselves, plus a spreadsheet that averages away the one detail the tax rules care about.

Pull the same position into one book. Record every buy as its own tax lot. Let FIFO retire the right shares when you sell, then reconcile the whole thing against your statements now and then. That is the entire discipline, and there is no step four. Do it and your realised P&L stops being a hopeful estimate. It turns into a number you can actually stand behind, across every account at once. If you have been weighing the spreadsheet against a proper tool, we laid out that exact choice in portfolio spreadsheet versus app.

This is not for everyone, and that is fine. If you own one fund in one account and never sell, carry on, you do not need any of this. But if your position is scattered across taxable, pension and exchange, and you would rather know your cost basis than approximate it, that is exactly the standard your money deserves. You can build your portfolio and start keeping the basis honest today.