What Is Cost Basis?

Cost basis is the original value of an investment for tax purposes — typically what you paid for it, including any fees, adjusted over time for events like reinvested dividends or stock splits. It’s the number that matters when you sell: your taxable gain or loss is the difference between the sale price and your cost basis, not the difference between the sale price and whatever you think you paid.

Why it’s rarely just “the purchase price”

The simplest case is a single lump-sum purchase: buy 100 shares at $50, pay a small commission, and your cost basis is $5,000 plus that commission. Sell later at $70 a share, and your taxable gain is $7,000 minus your $5,000-plus-fees basis. But basis gets more complicated as soon as an investment involves more than one event:

• Reinvested dividends increase your basis, because each reinvestment is effectively a new purchase — you already paid tax on that dividend as income, so it can’t also be taxed again as part of your gain when you eventually sell.
• Stock splits don’t change your total basis, but they redistribute it across more shares. A 2-for-1 stock split on a position with a $5,000 basis and 100 shares leaves you with 200 shares and the same $5,000 total basis, or $25 per share instead of $50.
• Return of capital distributions (common in some funds and REITs) reduce your basis rather than counting as ordinary taxable income, since they’re treated as giving you back part of your own investment rather than a profit.
• Corporate actions like mergers, spin-offs, or reorganizations often require basis to be split or reallocated across the resulting securities, following rules specific to the transaction.

Multiple purchases, multiple basis lots

Buying the same stock at different times and prices creates separate tax lots, each with its own basis and purchase date. If you bought 50 shares at $40 last year and another 50 at $60 this year, you’re not holding “100 shares with a $50 average basis” for tax purposes by default — you’re holding two distinct lots, and which one you sell from affects both your taxable gain and whether that gain qualifies for long-term treatment.

Brokers generally offer a few methods for choosing which lot a sale draws from:

• FIFO (first in, first out) — sells the oldest shares first. This is the common default and tends to realize more long-term gains as a position ages.
• LIFO (last in, first out) — sells the most recently purchased shares first.
• Specific identification — you choose exactly which lot to sell, letting you deliberately realize a smaller gain, a larger loss, or optimize for long-term versus short-term treatment.
• Average cost — used automatically by many funds, especially mutual funds, which blends all lots into a single average basis per share rather than tracking them separately.

Specific identification gives the most control and is the method behind deliberate tax-loss harvesting — choosing to sell a lot sitting at a loss to offset gains elsewhere, while continuing to hold a different lot in the same security.

Why long-term vs short-term status depends on basis records too

Basis tracking isn’t only about the dollar amount — it’s paired with the acquisition date of each lot, which determines whether a sale qualifies for long-term capital gains treatment (generally taxed at lower rates) or short-term treatment (taxed as ordinary income). Selling from the wrong lot, even at an identical price, can mean the difference between a short-term and long-term holding period. This is one of the more common places investors overpay unintentionally: letting a broker’s default lot-selection method decide, rather than checking whether specific identification would produce a better outcome for a given sale.

Cost basis vs related concepts

	What it measures
Cost basis	What you paid, adjusted for fees, splits, and reinvestments
Market value	What the position is worth right now
Capital gain/loss	Sale price minus cost basis
Unrealized gain/loss	Current market value minus cost basis, before any sale

An investment can have a large unrealized gain purely because its cost basis is low relative to today’s price — this is common in long-held positions, and it’s part of why some investors are reluctant to sell even strong performers: the tax bill on the gain is calculated entirely from the gap between sale price and basis, not from the position’s current size.

Why brokers, not just investors, track this

Since 2011, brokers in the US have generally been required to report cost basis to both the account holder and tax authorities for “covered” securities purchased after that point — which is why modern brokerage tax statements typically show basis directly rather than leaving investors to reconstruct it themselves. Older positions, transfers between brokers, and certain security types can still fall outside automatic tracking, which is why keeping personal purchase records remains useful even when a broker handles most of it automatically. Errors here compound: a wrong basis on record can overstate or understate a gain for years until it’s corrected.

The takeaway

Cost basis is the reference point every capital gain or loss calculation starts from — not just the purchase price, but that price adjusted for fees, reinvested dividends, splits, and any return-of-capital distributions along the way. Because positions built from multiple purchases create separate tax lots with their own basis and holding period, which lot you sell from is a real lever on the tax outcome of a sale, not just an accounting detail — worth checking before accepting whatever lot a broker’s default method picks for you.