That's simple but also could be tricky. Your gain is the difference between the selling price and your adjusted cost basis.
If you bought the home before the marriage, then your basis is $140,000. You paid $40,000; your spouse acquired half ownership on marriage, then you paid your ex $100,000 for their share.
If you bought the home with your spouse, then your basis is $120,000; your basis in your half was $20,000, plus the $100,000 you paid for your spouse's share.
You can then increase your cost basis by any permanent improvements that are still part of the home, such as remodeling, new roof, new furnace. (If you replaced the furnace in 1990 and again in 2015, only include the cost of the second replacement that is still part of the property.
You must also reduce your basis if you used the home in business, or ever took a deduction for a casualty loss.
Also, if you had previously owned a home in the 80s, and you postponed the gain from that home when buying this home, you must reduce your basis by the postponed gain. The gain postponement rule was eliminated in 1997, but you must account for previously postponed gain if you have any.
As I mentioned, the gain postponement rule was eliminated in 1997. Now, it is much more simple. If you owned your home at least 2 years and lived in it as your main home at least 2 of the past 5 years, you can exclude the first $250,000 of capital gains, or $500,000 if married filing jointly. Under any calculation of cost basis, your gain would be less than $250,000 and won't be taxable if you have lived in the home as your main home.
If you lived other places as your main home (making this home sometimes a rental or second home), then your gain may still be partly taxable. This is a very complicated topic and we would need more details to walk you through it.
