ISOs also are called statutory or qualified stock options. Incentive stock options usually expire after 10 years.
Strike price: the price set by employer when ISOs are granted
exercise ISO: When the vesting period expires, the employee can purchase the shares at the strike price
Sale price: the fair market value when the employee sells the shares
ISOs are taxed in two ways. The first method is on the spread, and
the second is on any increase (or decrease) in the stock's value when it
is sold.
The spread between the fair market value of the stock exercised and the
option's strike price is considered income for AMT (Alternative Minimum Tax) purposes if you exercised ISO shares and did not sell them in the same calendar year. If the shares were exercised and sold in the same calendar year (i.e.
same-day sale), the sale price and sale date are indicated on statement
of taxable income, and AMT need not be calculated.
The income from ISOs is subject to regular income tax and
alternative minimum tax, but it is not taxed for Social Security and
Medicare purposes.
A qualifying disposition for an ISO is taxed as a capital gain at
long-term capital gains tax rates and on the difference between the
selling price and the cost of the option.
For qualifying disposition, the shares must be held for more than one year from the date of exercise and two years from the time of the grant. Both conditions must be met for the profits to count as capital gains rather than earned income.
Take an example:
Strike price: $3
Fair market value on exercise day: $63
Sale price: $163
Case 1: same-day sale
$63 - $3 = $60 will be considered as ordinary income and report on W-2, no AMT
Case 2: exercise and hold to be qualifying disposition (1 year after exercise, 2 years after grant)
$63 - $3 = $60 will be subject for AMT on the exercise year
$163 - $63 = $100 will be considered as long-term capital gain (need to double confirm)
See tax tips from turbo tax with all cases
There is some risk of making a big enough profit from the sale of ISOs to trigger the federal alternative minimum tax. That usually applies only to people with very high incomes and very substantial options awards. However, you will also generally earn an AMT credit in that year. You can use the credit to lower your tax bill in later years. Unfortunately, there are limitations on when you can use an AMT credit.
See https://www.thebalance.com/incentive-stock-options-3192970 for reference