Metric & Imperial calculations for BMI in Excel - Help!

To be pedantic, grams (and kilograms) are units of mass, not weight.

Also, I noticed you used this formula:
=IF(D8="pounds",C8/C10*2*703,C8/C10*2)

But you probably want this formula:
=IF(D8="pounds",C8/C10^2*703,C8/C10^2)

You're not using the correct operand, I believe.

EDIT:

I banged out a BMI calculator, compare it to yours.