https://www.irs.gov/publications/p523/ar02.html#en_US_2015_publi

3.	Determine any taxable gain.
a.		Because you did not meet both tests in line 2, read the following tests carefully. Part of your gain is not eligible for exclusion. Did you qualify for depreciation? If you used all or part of your home for business or rental between May 6, 1997 and the date of sale, total all the depreciation deductions you could have taken because of that use, whether or not you took them. If you have good records to show the IRS did not allow you to take that much depreciation, total the amount you were actually allowed to take. Keep in mind depreciation related to business or rental use of your home before May 6, 1997 doesn't count.
		Your total depreciation		a.
b.		Subtract line 3a from line 1a. If the result is less than zero, enter -0-		b.
c.		Did you have non-residence gain? If there were times after 2008 when neither you nor your spouse (or a former spouse) used your home as a main residence, and did not meet the situations under the Second test, above, do the following calculation:
		(1) Enter the total number of days after 2008 when neither you nor your spouse (or former spouse) used the home as a main residence. This is the non-use days
		(2) Enter the total number of days you owned your home (counting all days, not just days after 2008). This is the days owned
		(3) Divide the non-use days by the days owned. Calculate to 3 decimal places. This is your non-residence factor
		Multiply line 3b by your non-residence factor. This is your non-residence gain		c.
		For example, if you owned your home for 10 years (3,650 days), did not use it as your main residence for 500 days after 2008, and had a $20,000 gain, your calculation would look like this. Calculation of non-residence factor: 500 ÷ 3,650 = 0.137 Calculation of non-residence gain: $20,000 × 0.137 = $2,740
d.		Add your total depreciation (line 3a) to your non-residence gain (line 3c). You might have one but not the other. See alsoRecapturing Depreciation.
		This is your ineligible gain