Ask most people what they will be taxed on when they sell an investment and they will say "the money I get." That is wrong, and the difference can be large. You are taxed on your gain, and your gain is the sale price minus your cost basis. Basis is the unglamorous number that decides how much of a sale is profit and how much is just getting your own money back. This explainer follows the Internal Revenue Service's own guidance.

Basis is usually what you paid

The starting point is simple. In the IRS's words, "basis is generally the amount you paid for the asset." For something you buy, cost includes "the amount you pay for it in cash, debt obligations, and other property or services."

Crucially, it is not just the sticker price. For stocks and bonds, the IRS says basis is "the purchase price plus any additional costs such as commissions and recording or transfer fees." Those add-on costs raise your basis, and a higher basis means a smaller taxable gain later. The same idea applies to a house, where basis includes purchase-related expenses, not only the price on the contract.

Adjusted basis: the number moves over time

Basis is not frozen at purchase. It gets adjusted up and down over the years you own the asset, producing what the IRS calls adjusted basis.

It goes up when you put money into improving the asset. Capital improvements that add value, a new roof or an extension on a house, are added to basis. It goes down through things like "allowable depreciation and insurance reimbursements for casualty and theft losses." For an ordinary investor the most common upward adjustment is reinvested improvements on property; for landlords, depreciation steadily lowers basis, which is why a rental sold years later can carry a surprisingly large taxable gain.

The practical lesson is to keep records. Every commission, every improvement, every adjustment changes the basis, and therefore the tax, and the burden of proving basis falls on you.

Inherited property: the powerful step-up

Here is the rule that matters most in family finances, and it is generous. When you inherit an asset, your basis is not what the person who died originally paid. Per IRS Publication 551, the basis of inherited property is generally "the FMV of the property at the date of the individual's death", its fair market value on that date.

This is the "stepped-up basis," and its effect is dramatic. Suppose a parent bought shares, or a house, decades ago for $50,000 and it is worth $500,000 when they die. If they had sold it themselves, the gain would have been $450,000. But an heir's basis resets to the $500,000 value at death, so if the heir sells soon after for $500,000, the taxable gain is close to zero. A lifetime of appreciation is effectively wiped clean for tax purposes at death. It is one of the most consequential provisions in the tax code for ordinary families.

Gifts: basis carries over

Gifts work in almost the opposite way, and people are often surprised. When you receive property as a gift, the IRS says "your basis is the donor's adjusted basis at the time you received the gift." The old basis carries over to you; there is no step-up.

So giving an appreciated asset away during life does not erase the built-in gain the way dying with it does. The recipient inherits the low basis along with the asset, and owes tax on the full appreciation when they sell. There is a special twist for assets that have lost value: if the gift's fair market value is below the donor's basis, you use that lower value for calculating a loss, but the donor's higher basis for calculating a gain. (There is also a rule letting you add part of any gift tax paid to your basis.)

This gift-versus-inheritance gap is why the timing of passing on an asset is a genuine financial decision, not just a sentimental one.

Why it matters

Basis is quiet, but it is where a lot of tax is won or lost. Three things are worth carrying away.

First, your basis is more than the price: fees and improvements count, and every dollar of basis you can document is a dollar of gain you will not be taxed on. Second, keep records for the whole time you own something, because you, not the IRS, have to prove what your basis is when you sell. Third, inheritance and gifts follow opposite rules: inherited assets step up to value at death, while gifted assets carry the donor's old basis, and that difference can be worth thousands.

None of this is tax advice for your specific situation, and the details, especially around inherited and gifted property, run deeper than one article; the IRS points to Publication 551 and the Schedule D instructions for the specifics. But the core idea is simple and universal: you are taxed on the gain, the gain is measured from basis, and knowing your basis is how you make sure you pay tax on profit and not on your own money coming back.