What Is a Section 423 Employee Stock Purchase Plan?

A Section 423 employee stock purchase plan is an IRS-qualified program that lets you buy your employer’s stock at a discount through after-tax payroll deductions, with no income tax owed on the discount until you sell the shares. The discount can run as high as 15% off fair market value, and plans with a “look-back” feature price the stock using the lower of two dates, which can push the effective discount well past 15%. The tax code rewards holding: sell too soon and more of your profit is taxed as ordinary income; wait long enough and most of the gain qualifies for lower long-term capital gains rates.

How the Discount Works

The purchase price under a qualified plan can be as low as 85% of the stock’s fair market value. That 85% floor applies at either the beginning of the offering period (the grant date) or the end (the exercise date), whichever value is lower.¹

The look-back is what turns a 15% discount into something potentially much more valuable. Rather than pricing shares on the day you buy them, the plan compares fair market value on the first day of the offering period to the value on the purchase date and applies the 15% discount to the lower number.

Consider a stock trading at $50 on the grant date that rises to $85 by the purchase date. Without a look-back, the plan would price the shares at 85% of $85, or $72.25. With a look-back, the plan uses the $50 grant-date price, so you pay 85% of $50, which is $42.50 per share. You’ve effectively bought $85 worth of stock for $42.50. That’s why the look-back is the single most valuable feature in most plans.

Enrolling and Buying Shares

You enroll during a designated window and choose a percentage of your gross pay to contribute. Section 423 doesn’t set a statutory maximum percentage; each company picks its own cap, commonly 10% to 15% of pay. Contributions come out of your paycheck after taxes throughout the offering period.

Most plans break the offering period into shorter purchase periods, often three or six months. At the end of each purchase period, your accumulated deductions buy shares at the discounted price, and the shares land in a brokerage account the company designates.

The $25,000 Annual Limit

No employee can accumulate the right to purchase more than $25,000 of stock per calendar year across all of the employer’s Section 423 plans combined.¹ That $25,000 is measured using fair market value on the grant date, not the discounted purchase price or the value on the day you actually buy.

When an offering period spans more than one calendar year, the limit applies separately to each year the option is outstanding. An offering that runs from May 2025 through April 2027 crosses three calendar years and allows up to $75,000 in total eligible stock value. Unused capacity from one option can’t carry over to a different option granted under the plan.¹

No Tax When You Buy

When the plan transfers shares to you at the discounted price, you owe nothing to the IRS. Section 421(a) provides that no income results from the transfer of stock when an employee exercises an option under a qualified plan.² Your cost basis equals the cash you actually paid through payroll deductions, not the market value on the purchase date. In the earlier example, your basis is $42.50 per share even though the stock was worth $85.

That deferral is the whole point. If your employer handed you the same $42.50 benefit as a cash bonus, you’d owe income tax immediately. With the plan, the tax bill waits until you sell.

Tax When You Sell

How much you owe, and at what rate, depends entirely on how long you hold the shares after buying them. The IRS draws a sharp line between two types of sales.

Qualifying Dispositions

A qualifying disposition gets you the best tax outcome. You must hold the shares for at least two years from the grant date and at least one year from the purchase date.¹ Both clocks must run out.

When they do, the ordinary income you recognize is limited to the lesser of two amounts: your actual gain on the sale, or the discount that was built into the grant-date price.¹ Everything above that is a long-term capital gain.

Using the same numbers: you bought at $42.50, the grant-date fair market value was $50, and you sell at $150 after meeting both holding periods. The grant-date discount is $7.50 (the difference between $50 and $42.50). Your actual gain is $107.50 ($150 minus $42.50). The ordinary income piece is the lesser of the two: $7.50. Your basis rises by that $7.50 to $50.00, and the remaining $100.00 is taxed as long-term capital gain.¹

Long-term capital gains rates for 2026 are 0%, 15%, or 20% depending on taxable income. Most employees fall in the 15% bracket. Ordinary income rates can reach 37%, so the difference is real money.

Disqualifying Dispositions

Sell before satisfying either holding period and the entire spread between the stock’s fair market value on the purchase date and your purchase price becomes ordinary compensation income.²

In the same example, if you sell before meeting the holding periods, your ordinary income is $42.50 per share ($85 minus $42.50). Your adjusted basis becomes $85.00 ($42.50 paid plus $42.50 of ordinary income). The remaining $65.00 is a capital gain. Whether it’s short-term or long-term depends on how long you held the shares after the purchase date: one year or less is short-term (taxed at ordinary rates), more than one year is long-term.

The contrast is stark. The qualifying sale produced $7.50 of ordinary income and $100.00 of long-term capital gain. The disqualifying sale produced $42.50 of ordinary income plus $65.00 of capital gain. Same $107.50 of profit either way, but a bigger tax bill on the disqualifying sale because more of it hits at ordinary rates.

Selling at a Loss

Disqualifying dispositions create a trap when the stock drops after you buy. Even if you sell at a loss, you still owe ordinary income tax on the full purchase-date spread.

Suppose the stock was worth $85 on the purchase date, you bought at $42.50, and the price fell to $60 by the time you sold. You recognize $42.50 per share as ordinary income anyway. Your adjusted basis becomes $85.00, and you record a $25.00 capital loss. You can use that capital loss to offset other capital gains, or deduct up to $3,000 of net capital losses against ordinary income each year. But a capital loss doesn’t offset ordinary income dollar-for-dollar, so the mismatch stings.

A qualifying disposition is kinder here. Because the ordinary income is capped at your actual gain, selling for less than you paid means zero ordinary income and a straight capital loss.

Reporting the Sale Correctly

This is where most participants overpay. Your broker’s Form 1099-B reports a cost basis for your shares, but that basis almost never accounts for the compensation income already included on your W-2. Enter the 1099-B numbers straight onto your return and you’ll pay tax on the same income twice.

The fix runs through Form 8949, which feeds Schedule D. Report the sale proceeds in column (d) and the broker’s reported basis in column (e). In column (g), enter an adjustment that increases your basis by the ordinary income already reported as wages.³ Use adjustment code “B” to indicate the 1099-B basis was incorrect. That reduces your capital gain (or increases your capital loss) to the right amount.

Your employer must furnish Form 3922 after any purchase of ESPP shares. It reports the grant date, exercise date, fair market values on those dates, and the price you paid.⁴ Keep it even in years you don’t sell; you’ll need those numbers whenever you eventually do.

Nothing Is Withheld

Unlike regular wages, your employer does not withhold income tax on the compensation income from an ESPP sale. The statute explicitly waives withholding for both qualifying and disqualifying dispositions.²¹

Your employer should still report the ordinary income as wages in box 1 of your W-2 for the year you sell. If they don’t, you’re still on the hook to report and pay, using line 8k of Schedule 1 on Form 1040.⁵ Because nothing is withheld, a large sale can leave you with a surprise bill in April. Consider making an estimated tax payment in the same quarter you sell to avoid underpayment penalties.

If You Leave the Company

If your employment ends before the next purchase date, most plans refund your accumulated payroll deductions without interest, often in your next paycheck or within a few weeks. The code technically allows the company to use pre-termination deductions to buy shares within three months of your last day (12 months if you left due to disability), but most plans don’t.

Shares you already own aren’t affected. They stay in your brokerage account and you can sell whenever you want. The holding period clocks keep running, so leaving doesn’t restart or freeze your timeline. Quit six months after a purchase and you still need to wait out the full two-year and one-year holding periods from the original grant and purchase dates to qualify for the favorable tax split.

1
Office of the Law Revision Counsel. 26 U.S. Code 423 – Employee Stock Purchase Plans
2
Office of the Law Revision Counsel. 26 U.S. Code 421 – General Rules
3
Internal Revenue Service. Instructions for Form 8949
4
Internal Revenue Service. Instructions for Forms 3921 and 3922
5
Internal Revenue Service. Stocks (Options, Splits, Traders) 5