How do I convert the weight of human ashes to cubic inches?

You'd need to know the density of human ashes. I tried Googling it, and I saw a number of unreliable-looking answers. The closest thing I've seen to a definitive source was here, where the author claims (or quotes somebody else who claims) that the density is 0.5g/cm^3.

In that case, you'd take the weight in grams and divide by the density. That would give you volume in cm^3. To get volume in cubic inches, you can use the conversion factor that there are about 16.4 cm^3 per cubic inch. Therefore, you'd take your answer in cm^3 and divide by 16.4 to get cubic inches.

I'm guessing you probably have the weight in pounds, not grams? If so, multiply the weight in pounds by 453.6 to get grams.

Putting it all together, assuming the figure of 0.5g/cm^3 is correct, you'd have:

weight(lbs) to volume(in^3): V = W*453.6/0.5/16.4 = 55.4W

So each pound of weight corresponds to about 55 cubic inches of volume. That's probably something like 1/6 the volume of a shoe box.