Good resource for understanding real estate cap gains taxes on primary res?

[Mu206Stach]

I've never sold real estate before (I've owned my current place for 20+ years).

It has appreciated from what I paid for it (150K) to roughly 525K today (give or take).

I still have a mortgage on it (9 years left).

Now, if I sold it today, and didn't buy anything else (just rent)... do I pay cap gains on the difference?  (375k)

I've heard of the 250K rule, but don't really understand how it fits in, and if that is only if you buy another place within a year?

Now part 2 of my question, and this is where it gets interesting. 

What if I subdivide my property, and sell half of it -- for say... 275K, and keep the other half and live in it.  How do I calculate my cap gains?

And then, if 5 years later I sell the remaining half... for say 450K... what is the cap gains rule?

I'm sure I'd need a pro to help figure this all out, but thought I'd see if folks here have insights and pointers to good resources (besides asking me to read and digest all of the tax code...)

[MDM]

I'm sure I'd need a pro to help figure this all out, but thought I'd see if folks here have insights and pointers to good resources (besides asking me to read and digest all of the tax code...)

Well, not all of the tax code, but how about http://www.irs.gov/taxtopics/tc701.html?

[Cathy]

For whatever reason, the IRS publications on this topic do not reflect the current law and should not be relied on. According to IRM § 4.10.7.2.8, IRS publications are intended to explain the law in plain English, but are not guaranteed to be correct, "should not be cited to sustain a position", and are nonbinding. Treat IRS publications as a starting place for your research, but not as a definitive source of answers.

The actual law on this topic is found in 26 USC § 121. For an excellent summary in plain English, see this article: https://www.kitces.com/blog/limits-to-converting-rental-property-into-a-primary-residence-to-plan-for-irc-section-121-capital-gains-exclusion/