I left out a few steps. Here's some additional help.

LEN([reference]) returns the total number of characters in a cell.

If A1 is "10 ft. 5.125 in"
LEN(A1) will return the number 15.

FIND("X",[reference]) returns a number that represents the first character of a text reference.

If A1 is "10 ft. 5.125 in"
FIND("ft."A1) will return 4. Note that spaces count.

LEFT([reference],[number]) will return a string of text characters.

If A1 is "10 ft. 5.125 in"
LEFT(A1, 4) will return "10 f".
LEFT(A1, 2) will return "10".
VALUE(LEFT(A1,2) will return the number 10.

ISERROR([reference]) will return TRUE if an error message occurs, and FALSE if not.

ISERROR(FIND("CAESAR","10 ft. 5.125 in")) will return TRUE.

ISNUMBER("10 ft. 5.125 in") will return FALSE.

IF([reference],[operation 1],[operation 2]) if [reference] resolves to TRUE, then perform operation 1, else it'll perform operation 2.

If our formula for B1 were:
IF(A1="",0,A1)

And A1 is blank, B1 will be 0. If A1 isn't blank, then B1 will be A1.

So combining the above information, let's say we don't know for sure what's in A1, but it's one of the following values:

1. Nothing
2. Some number plus "in" (e.g. 5.125 in)
3. Some number plus "ft." (e.g. 10 ft.)
4. Some number plus "ft." with some number plus "in" (e.g. 10 ft 5.125 in)

Step 1: B1 will check to see if it's blank. If it's not blank, then push it on to C1. If it is blank, resolve it to the value 0.
B1 = IF(A1="",0,A1)

Step 2: If it's 0, push it forward, else, check to see if it's case 2 by determining that it's not case 3 or 4.

C1 = IF(B1=0,0,IF(ISERROR(FIND("ft.",B1),VALUE(LEFT(B1,FIND("in",B1)-2)/12))

Step 3: If C1 is a number, push it forward, else, determine if it's Case 3 by omitting Case 4 as a possibility. We divide by 12 to convert to feet.

D1 = IF(ISNUMBER(C1),C1,IF(ISERROR(FIND("in",C1),VALUE(LEFT(C1,FIND("ft.",C1)-2))))

Step 4: If D1 is a number, push it forward, else you need to convert 10 ft. 5.125 in. into a number that represents calculated feet.

I'll let you figure that one out, but I recommend that you use as many columns as you need, as well as to use RIGHT() coupled with LEN() at some point.