if it wasn't used as your principal residence at any time in the 5 years before sale the exclusion is zero
otherwise
1) number of days (or months) used as your principal residence in the 5 years before sale. do not use more than 730 days or 24 months
2) divide the number on line 1 by 730 if you used days or 24 if you used months (can't be more than 1 round to at 3 decimal places)
3) multiply the ratio on 2 by $500,000. this is the exclusion
also note that depreciation recapture applies before any exclusion
