You need to consult a vet about this and it could be dangerous if you give your dog a wrong dosage.
I don't recommend giving anything to your dog before you have checked with a vet as it could cause serious side effects.
Never give anything without speaking to a vet first and finding out if it is okay, the correct dosage and things you should watch for. Medication is the number one cause of poisoning in our pets and very often given to them by someone who meant well but didn't call the vet first. In this case benadryl isn't typically used or labled for 'calming' or sedating but something else entirely. There are things that could be used if you actually need them though so just speak to your vet.