Let's assume the plate address you want to convert is in the Excel cell A1. The conversion is done in Excel as shown below:
= (CODE(LEFT(A1,1))-65)*12 + RIGHT(A1,LEN(A1)-1)
The formula assumes the well format is one character denoting the row followed by one or more integers which denote the column. The left half of the formula before the '+' converts the character into an integer using the CODE function. Since "A" has a code of 65, we subtract this off before multiplying by the row length. The portion of the formula after the '+' simply extracts the integer portion of the well address and adds it to get the well number. The formula works with padded (A01)and unpadded (A1) representations.
To go back from a numbered address to the letter-number format, use the following:
= CHAR(INT((A1-1)/12)+65) & (MOD(A1-1, 12)+1)The portion of the formula left of the '&' handles the row calculation. The CHAR function performs the opposite duty of CODE, taking a number and converting it back to a character. The portion to the right of the '&' calculates the integer (column) portion of the well address.
It's quite easy to do 384-well or other formats. Just replace the red 12 with the correct number of columns. In the case of a 384-well format, you'd replace it with 24.
It is less common to see plate wells converted into numbers from top to bottom and then left to right, although it does happen. In this case, B1 maps to well 2 and H2 to 16 for a 96 well plate.
The conversion from plate well to number in this case is:
= (CODE(LEFT(A1,1))-65) + 1 + (RIGHT(A1,LEN(A1)-1)-1)*8
And to go back:
= CHAR(MOD((A1-1),8)+65) & (INT((A1-1)/8)+1)
Note that instead of using the number of columns (12), we're now using the number of rows (8). It's left as an exercise to the student to tweak the formula for 384-well formatted plates ;)