Well, it's enclosed in an IF() function, so there are three parts to it...

**IF condition test**:

ISBLANK(B12)

**IF true condition**:

" "

**IF false condition (1st)**:

IF(MOD(B12,500) = 0, VLOOKUP(B12,TV1,2,1), VLOOKUP(ROUNDDOWN(B12/500,0) * 500,TV1,2,1) + ((VLOOKUP((ROUNDDOWN(B12/500,0)+ 1) * 500,TV1,2,1) - VLOOKUP(ROUNDDOWN(B12/500,0) * 500,TV1,2,1)) * (MOD(B12,500)/500)))

.. breaking this one down further ..

**IF condition test**:

MOD(B12,500) = 0

**IF true condition**:

VLOOKUP(B12,TV1,2,1)

**IF false condition**:

VLOOKUP(ROUNDDOWN(B12/500,0) * 500,TV1,2,1) + ((VLOOKUP((ROUNDDOWN(B12/500,0)+ 1) * 500,TV1,2,1) - VLOOKUP(ROUNDDOWN(B12/500,0) * 500,TV1,2,1)) * (MOD(B12,500)/500))

So far this is pretty straight forward. I think you're probably trying to get a grip on this last IF section (a False return on the first condition check and another False return on the second condition check). If this is the case, let's break down this last section a little more..

**ROUNDDOWN(B12/500,0)**
This will round the number down from B12 (towards the integer of zero) expressed as a whole number

**VLOOKUP(ROUNDDOWN(B12/500,0) * 500,TV1,2,1)**
This will take the rounded down integer and multiply it by 500 again (apparently you are needing to take out any of the trailing decimals) and perform the Vlookup in TV1, return the second column under a non-exact match.

I'm hoping that

**TV1** is a named range and you're not actually using the range TV1 (cell). Of course you would have to have version 2007 to do this as 2003 and prior do not even have this cell range as an option. But I felt I should mention it.

**(VLOOKUP((ROUNDDOWN(B12/500,0)+ 1) * 500,TV1,2,1)**
This Vlookup is taking the same rounded down integer, but adding a single digit to it and performing the same lookup. Basically it's getting the next highest number in the list.

**(MOD(B12,500)/500)**
The Mod function is kind of like a divisor function. This is the most confusion part of the function for me, that is if you are looking at using the numbers from column A (I'm assuming A, but the data which you posted) which are all in units of 500, which is your

*divisor* part of the syntax of your Mod function. What that means in plain english is that the formula (Mod) will

*always* return 0 if this is the case. This of course makes no sense, and if it is the case you may as well just have 0.

I'm further assuming that since you are dividing it by 500 that you wouldn't intentionally set a 0 as the numerator portion of a division, hence you would get a #DIV/0 error. So you're more than likely

*not* using a divisor of 500, but if you do that is where the error would be coming from.

So the entire (what I'm referring to as) False/False portion of your formula looks like..

**VLOOKUP(ROUNDDOWN(B12/500,0) * 500,TV1,2,1) + ((VLOOKUP((ROUNDDOWN(B12/500,0)+ 1) * 500,TV1,2,1) - VLOOKUP(ROUNDDOWN(B12/500,0) * 500,TV1,2,1)) * (MOD(B12,500)/500))**
So you're looking at the first entry found for your rounded down integer (second column returned value that is), then you're looking at taking the next highest value (from the second column again) and taking that away from the previous returned value (in other words finding the difference).

This is the second part which doesn't make sense to me. If you are taking the first occurance and adding the difference between the first occurance and the second occurance, then you're basically returning the second occurance. I may be misunderstanding that, but if it is the case, you only need to return the second occurance (basically eliminating two Vlookup formulas you have there).

Once you have that number you are multiplying it by the return of the Mod function.

Hope this explanation helps.