Full year ROI... if you have excel, use the XIRR formula...

31/12/2013 (10,000)








31/12/2014 11,000 10.00%

The above figures are in cells A1, B1, A10, B10 and C10.
C10 has this formula: =XIRR(B1:B10,A1:A10,0.01)

- what is important is the total values of the portfolio at 31st Dec.
- Any new investments and withdrawals, add the dates and their values in between the above figures.
- Investments are in negative values

where got 'scientific'
got calculator, use calculator... got pc, use spreadsheet...

explained simple & quick estimate also no point, if people still ask 'formula'

yeah, easy & quick to estimate by roughly assuming all the new investments are at mid year and thus divide by 2... still get the 17.4% as checked out by XIRR formula.

31/12/2013 (10,000)
1/7/2014 (3,000)
31/12/2014 15,000 17.48%