CO2 and TIC calculator
Back in the 2008 I created an online calculator for the calculation of dissolved CO2 based on pH and alkalinity. This has now been superseded by better online tools for this purpose.
Make sure you choose the appropriate pH scale in the preferences section. If you are using a standard glass pH electrode with NBS/IUPAC buffers (most calibration buffers are of this type), use the NBS pH scale. This is particularly important if you are working in seawater because there is about 0.15 pH units difference between the NBS scale versus the other pH scales (that can make a significant difference to your final computed CO2 concentration!).
Also, make sure you select the appropriate CO2 constants for fresh ("Salinity = 0...") or saltwater (so many to choose from!) .
PS. Are you aware of the significant limitations of using pH values from standard glass electrodes to infer dissolved CO2 levels in seawater?
The experience of this researcher summarises the issue
"For the past couple of weeks we has used about 15 different pH probes and loads of different buffers and we can’t get anything to read consistently in seawater (and we can’t even test for accuracy). I am not sure how people report accurate values with low variation for calculated pCO2 given that this is based on pH values from glass probes."
I raised this issue in a letter I wrote to the Journal of Experimental Biology.
Moran, D. (2014). The importance of accurate CO2 dosing and measurement in ocean acidification studies. Journal of Experimental Biology 217: 1827-1829.