What Is Cost Basis?

Cost basis is the amount you have invested in an asset for tax purposes. In the simplest case it is the price you paid plus any commissions or transaction fees. If you buy 10 shares at 100 dollars each with a 5 dollar commission, your basis is 1,005 dollars. When you later sell, your taxable gain or loss is the sale proceeds minus that basis: sell those shares for 1,600 dollars and your taxable gain is 595, not the full 1,600.

This is why basis is worth caring about. The gain is what gets taxed, and the gain is measured entirely against basis, so an inaccurate basis directly distorts your bill. Overstate it and you underpay tax, which invites a correction; understate it and you overpay on a gain that was never really that large. Basis is the quiet anchor under every capital gains calculation, which is why the rest of this guide is about keeping it right. This is general information, not tax advice.

One of the most common and expensive basis mistakes involves reinvested dividends. When a stock or fund pays a dividend and you have it automatically reinvested, that payout was already taxed as income in the year you received it. The reinvested dollars then buy new shares, and that amount becomes part of your cost basis in those new shares. If you ignore this, you will overstate your gain when you sell and end up paying tax twice on the same money.

Over years of automatic reinvestment, these small additions accumulate into a meaningfully higher basis. A fund position you have held for a decade with dividends reinvested has a basis well above your original purchase, and treating it as if you only paid the initial amount would overstate the taxable gain substantially. Reinvested dividends are taxed each year regardless, as covered in how stocks are taxed; tracking their addition to basis is what prevents the double count. This is not tax advice.

A stock split changes how many shares you hold but not how much you have invested, and your basis follows the same logic. In a 2-for-1 split, each share becomes two, so your share count doubles and your per-share basis is cut in half; your total basis is unchanged. If you held 100 shares with a 50 dollar basis each, after the split you hold 200 shares with a 25 dollar basis each, still 5,000 dollars in total.

Because the total does not change, a split is not a taxable event on its own; you owe nothing simply because a company split its stock. The only thing to keep straight is the new per-share figure, so that when you eventually sell some of the shares you use the adjusted basis rather than the pre-split number. Reverse splits work the same way in the other direction, raising per-share basis as the share count falls. This is general information, not tax advice.

Once you have bought the same stock at different prices over time, each purchase is a separate lot with its own basis, and a question arises when you sell only part of the position: which shares did you sell? The default answer is FIFO, first in first out, which treats you as selling your oldest shares first. FIFO is simple, but those oldest shares often have the lowest basis and therefore the largest gain, which is not always what you want.

Specific-lot identification is the alternative that gives you control. It lets you name exactly which lots to sell, so you can pick high-basis shares to keep a gain small, choose long-term lots to get the lower rate, or deliberately realize a loss for tax-loss harvesting. The catch is that specific-lot selection usually must be elected at the time of the sale, with your broker, and properly documented. This is exactly the kind of decision where the details matter, so confirm the mechanics with your broker and a tax professional. This is not tax advice.

You do not have to track all of this by hand for most modern holdings. Brokers are required to track and report cost basis for covered securities, which includes most stock and fund shares purchased in recent years. At tax time your broker issues Form 1099-B, which lists your sale proceeds, your cost basis, and the resulting gain or loss, and sends a copy to the IRS. That form is the starting point for reporting your investment sales.

The gap to watch for is non-covered securities: shares bought long ago, transferred from another institution, or otherwise outside the reporting rules may show a missing or unverified basis, and you are responsible for supplying it. It is always worth checking the 1099-B against your own records, especially for older lots and anything with years of reinvested dividends. Your brokerage forms, not any app, are the official record. This is general information, not tax advice.

Basis works differently when you did not buy the shares yourself. Inherited stock generally receives a stepped-up basis: your basis is reset to the market value on the date of the original owner's death, rather than what they paid. If someone bought a stock decades ago for 5,000 dollars and it was worth 50,000 when they passed, an heir's basis is generally the 50,000, so a large unrealized gain can effectively disappear for the heir.

Gifted shares follow a different rule: you usually take over the giver's original basis, so the built-in gain comes with the gift. These rules have important nuances around holding periods, joint ownership, and estate circumstances, and they are precisely the kind of situation where a small detail changes the outcome. Treat this as an overview and confirm the specifics with a tax professional. This is general information, not tax advice.

The through-line is that basis is a living number, not just your first purchase price. Reinvested dividends raise it, splits redistribute it, returns of capital lower it, and inheritance can reset it entirely. Keeping it accurate is what makes your eventual capital gains calculation correct. Every rule here can change and depends on your individual circumstances, so verify current details with the IRS or a licensed professional before making any decision.

Your official cost basis lives with your brokerage and its Form 1099-B, and that is where any tax figure should come from. What an AI assistant that can read across your accounts adds is a plain-language view of how each position has moved, which is the context around basis rather than the basis itself. Walnut lets you connect any major US broker, then chat through Claude, ChatGPT, or its built-in AI to see how each holding has performed and how long you have held it. You can build baskets around a plan, track every position against the S&P 500, and place trades that Walnut only sends to your broker after you approve them. It reads your accounts by default and does not move money on its own. Walnut is not an investment adviser or a tax adviser, it is not the record of your cost basis, and it does not tell you which lots to sell or what to buy.