There is only one formula, and it is easy. I do assume that you
meant, "Should I use pennyweights or GRAMS?", unless your pieces are
monumental. We use pennyweights - many use grams. I like the even
number of 20dwt/oz.
First of all, get the price of gold - www.kitco.com is one that lists
it daily. Then multiply that times the gold that you are using - you
say 18kt., so multiply times.750, 14kt. is .585. It's easy to get the
decimal for other karats, because it's just the proportion of gold,
i.e. 18/24 =.750, 14/24=.585, 22/24=.916, etc.
So, after you multiply spot x.750, you have the price of 18kt. gold
per troy oz. Then divide that by 20 for per pennyweight, or 31.1 for
per gram. Then, weigh your ring, and multiply that weight times the
cost/dwt. gives you the cost of the ring.
Example: Spot = $450 x.750 = $337.50 (18Kt./oz) divided by 20
=$16.875 (18kt./dwt) for a ring that weighs 8dwt = $135 cost.
I actually made up a spreadsheet up on the wall of gold and platinum
prices so we can just look at it instead of calculating it all the
time, but it's the same numbers...