Disk spaces to save database

dear all,
if we have 1 data point, we need save data of that data point to
historical database within 1 years.So we need how many disk spaces or capacity of disk to save data of that data point?

This question is not so straightforward. You would need to specify the logging type (and potentially the shape of the data for "When point value changes" logging), backend database, and the polling interval of the data. But, for estimation's sake, let's say a value takes 20 bytes. Let's say we're polling it every 3 minutes, and storing all data.
24*20 == number of 3 minute intervals per day
365.24 == number of days per year
20 bytes == random estimate, you could probably figure out an exact amount for your data type and database24*20*365.24*20 == 3506304
or about 3.5 MB
Of course, if you're logging 3 second data, it would be closer to 210 MB, and 30 ms data would be a whopping 21 GB

Thank phildunlap for your support.
I know it is not so straightforward. But we need it because it is proof to show demo and do estimate for our customer.