Originally Posted by slykwilli
a dry yeast package has much more yeast than you would get from a single liquid package. Instead of a starter for dry yeast, just hydrate as Jeep pointed out.
A package of liquid yeast will have around 100bn cells if it is fresh. Its viability decreases significantly with time. A month old package would have only about 80% viability or 80bn cells and a 2 month old pack only about 60% viability or 60bn cells. The viability of liquid yeast is more guesswork and hence a starter is used to prove the yeast is viable and increase the quantity to the amount required
Dry yeast on the other hand has about 20Bn cells per gram (around 220bn cells in an 11g pack)and this is less affected by time if stored correctly.
I think the only most unlikely situation in which I would even consider a starter for dry yeast was if there were no additional supplies available of the strain I wanted and the amount I had was well short of the amount required for the batch to be brewed. I would then rehydrate first then pitch into an appropriate sized starter to increase the quantity.