Updated July 28, 2026. Quick answer: The two dispositions measure ordinary income off different quantities: disqualifying uses the full spread at purchase, qualifying is capped at the offering-date discount. This computes both on your numbers.
An ESPP has two possible tax outcomes on the same shares, and they measure ordinary income off completely different numbers. This computes both so you can see what holding actually buys.
The asymmetry nobody explains
Both dispositions produce ordinary income and capital gain. What differs is what the ordinary income is measured against.
In a disqualifying disposition it is the full spread at purchase — the purchase-date price minus what you paid — and that figure is fixed regardless of what you eventually sell for. Sell lower and you still report it, taking a capital loss on the way down.
In a qualifying disposition it is capped at the discount measured on the offering date, or your actual gain if that is smaller. Everything above the cap is long-term capital gain.
Why the lookback makes the discount bigger than 15%
A lookback provision prices your purchase at a discount off the lower of the offering and purchase prices. If the stock rose over the period, your effective discount off the current price is far more than the headline number — which is exactly why the qualifying cap, measured on the offering-date price, is often much smaller than the spread at purchase.
What the calculator cannot decide for you
It moves income between two rate buckets. Whether that is worth a year of concentrated exposure to your employer’s stock — on top of your salary already depending on it — is a different question, and usually the more important one.
The holding period is worth real money, and it is a date you can still choose.
Qualifying and disqualifying dispositions of the same shares produce different tax on the same gain. Take both figures to the advisers below and ask which they would choose in your bracket. It is free to you, and it is not the only way to find an adviser.
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Sources
IRC §423(a)(1) (holding periods); IRC §423(b)(6) (15% discount); IRC §423(c) (ordinary income on qualifying disposition); IRC §421(b) (disqualifying disposition).
This states what the cited authority says. It is not tax advice.