What is Cost Basis?

Cost basis is what you paid for an asset, adjusted over time for events that change it. It is the number subtracted from your sale proceeds to determine your capital gain or loss, which makes it the difference between an accurate tax bill and an inflated one.

Buy a stock for $100 a share with a $5 commission and your basis is $105. Sell at $150 and you owe tax on $45, not $50.

Why Cost Basis Gets Complicated

For a single purchase held and sold in one piece, basis is trivial. It becomes difficult in the ordinary cases: buying the same holding repeatedly over years, reinvesting dividends, surviving stock splits, or inheriting assets.

Each of those either creates multiple tax lots with different basis figures, or changes the basis of what you already hold. Getting it wrong usually costs you money: if you cannot substantiate basis, zero becomes the practical fallback and the entire proceeds are treated as gain.

Methods for Calculating Basis

When you sell part of a position, which shares you are deemed to have sold determines the gain.

First in, first out (FIFO) is the default. The oldest shares are sold first. In a position that has appreciated over time, this sells your lowest-basis shares and produces the largest gain, though it also reliably produces long-term treatment.

Specific identification lets you name the exact lots to sell. This is the method that gives you control: sell high-basis lots to minimize the gain, or deliberately sell losing lots to harvest a loss. You have to identify the shares with your broker at or before the sale, not afterward on your return.

Average cost pools all shares into a single average basis. It is available for mutual fund shares and for shares acquired through a dividend reinvestment plan, and it trades precision for simplicity. Once elected for a holding, changing methods has restrictions.

Note that last in, first out is not among them. LIFO is an inventory method for businesses; for securities the IRS permits FIFO, specific identification, and average cost where eligible. Selling your most recent shares is accomplished through specific identification rather than a LIFO election.

Adjustments That Change Your Basis

Reinvested dividends are the most commonly missed. Every reinvested payment is a purchase, and every purchase adds to your basis. Someone who has reinvested dividends for fifteen years and reports only the original purchase price will overstate the gain substantially and overpay.

Inherited and Gifted Assets

These follow entirely different rules, and the difference is large enough to shape estate decisions.

Inherited assets receive a step-up in basis to fair market value on the date of death. A stock bought for $20,000 and worth $200,000 at death passes to heirs with a $200,000 basis, and the $180,000 of embedded gain is never taxed.

Gifted assets generally carry over the giver’s basis. The same stock given during life arrives with its $20,000 basis intact, and the recipient owes tax on the full gain when they sell. From a capital gains standpoint, this is why appreciated assets are often better left to heirs and cash is the simpler thing to give during life.

Gifts that have lost value follow a dual-basis rule instead. If the asset was worth less than the giver’s basis on the date of the gift, you use the giver’s basis to figure a gain and the gift-date value to figure a loss. Sell somewhere between the two and you report neither gain nor loss, which means a built-in loss cannot be transferred by gift.

Which assets you spend and which you leave behind depend on their embedded gains, your bracket, and your heirs’. You can model how asset location affects what heirs actually receive in ProjectionLab.

Broker Reporting

Brokers have been required to report cost basis to the IRS in phases: stock acquired on or after January 1, 2011; mutual fund and eligible dividend reinvestment shares on or after January 1, 2012; options and simpler debt instruments from 2014; and more complex debt from 2016. These are covered securities, and the basis appears on your 1099-B.

Anything older is noncovered. The broker may show a basis for your convenience, but it is not reported to the IRS and it is not necessarily right, particularly for positions transferred between firms. Records for older holdings are worth keeping independently.

Frequently Asked Questions

What happens if I do not know my cost basis? You are still required to make a reasonable, documented estimate. Historical price data, old statements, and transfer records can reconstruct it. Reporting zero is the costly fallback, because it taxes the entire proceeds as gain.

Does cost basis matter in a retirement account? Generally no. Gains inside an IRA or 401(k) are not taxed as capital gains, so tracking basis for capital gains purposes is unnecessary. Basis does still exist in a few places: nondeductible contributions to a traditional IRA, after-tax contributions to a 401(k) or 403(b), and nonqualified Roth distributions all rely on basis to determine what portion comes out tax-free.

Do stock splits change my cost basis? Your total basis stays the same; the per-share figure changes. Own 100 shares at $60 basis and a 2-for-1 split leaves you with 200 shares at $30. Nothing is gained or lost, and no tax is due.

How does cost basis work for cryptocurrency? The same principles apply, since the IRS treats digital assets as property. Basis is the purchase price plus fees, and specific identification is available if you can document which units you sold. Reporting has since caught up: brokers now report gross proceeds on Form 1099-DA, and basis reporting applies to covered digital assets acquired after 2025. Independent records still matter for anything transferred between platforms, self-custodied, or acquired earlier.