Dosages are generally determined, based upon body weight.
 

	Dimenhydrinate 50 mg	Meclizine 25 mg
Baby	1/8 to 1/4 tablet	same, see caution.
Child	1/2 tablet	same, see caution.
Average Adult	1	same
Heavy Adult	1 to 1/2 tablets	same
Frequency	every 4 hours	every 10-12 hours

Cautions: see the notes below.

Note: These dosages for amounts and frequency should match those on the packaging. If not, use that on the package. Warning!! I am not a doctor and these are only suggestions based upon my experience with the medications. As usual, use these at your own discretion.

Children Warning: The literature indicates that Meclizine has not been tested in children under 12. Therefore, it could have side effects which might not be acceptable.

The following dosages for Dimenhydrinate are from more detailed pharmacy books.
                Adult: 50-100 mg every 4-6hours by mouth, (maximum, 400 mg per day)
                Child 6-12 yr: 25-50 mg every 6-8hours by mouth (maximum, 150 mg per day)
                Child 2-6 yr:  less than 25 mg every 6-8hours by mouth  (maximum, 75 mg per day)
Note: there is no "Baby" dosage.

See also Dr. Gahlinger's Seasickness Page
 
 

In the case of Dimenhydrinate, I have found that it is often necessary to give the pills an hour earlier than above, i.e.. every 3 hours instead of 4. Also, that about 20% of victims require 1/2 again as much as recommended, i.e.. for an average adult 1 and 1/2 tablets, or perhaps 2 for a heavy adult. These suggestions for "over dosing" should not interpreted to mean that the over dose should be continued for any longer than necessary. In general, for not more than the first 2 or 3 doses.

In the case of Meclizine, I have never had to use extra to get a cure.

My rule of thumb for giving extra medication is simple. If the victim is not recovering within about 30 minutes they should be given some more. The early failures I had in curing someone already sick, appear to have been due to a reluctance on my part to increase the dosage above the package recommendations. A careful reading of the official instructions indicates that a modest over dose to get the patient stabilized is generally safe. But should be discontinued in subsequent doses, as soon as practical. I have added material from detailed pharmacy recommendations, to supplement my suggestions.
I can't emphasis enough, that failure to use an adequate amount for the patient, considering their condition, will almost certainly result in a failure to cure. And you will have given pills which may have/may not have been absorbed. And the worst part of that is that you may not be able to tell how much has been absorbed. Then you will be in a real quandary as to how much more to give. It is my experience that any patient who shows signs of not becoming cured after giving the pills, has almost certainly swallowed them or somehow not absorbed the pills. Be aggressive to start, then be cautious after you have them stabilized(not sick).