Calculates required number of Hadoop data nodes on first year

hgp.py

from math import ceil
from tabulate import tabulate

GiB = 1024
TiB = 1048576

def main():
    """
    Calculates cluster growth plan based on numbers listed below
    """
    dailyIngest = 30 * GiB  # average daily ingest rate
    replication = 3         # replication factor (by default 3)
    reserve     = 0.25      # MapReduce temp space reserve (usually 1/3 or 1/4)
    nodeStorage = 6 * TiB   # amount of disk space per node
    monthlyGrowthRates = [0, 0.05, 0.10, 0.15] # growth of ingest rate per month

    results = []
    m = (dailyIngest * replication * 365 / 12.0)
    for r in monthlyGrowthRates:
        s = m * (12 if (r == 0) else (pow(1 + r, 12) - 1) / ((1 + r) - 1))
        total = s / (1 - reserve)
        results.append([r, humanReadableSize(total), ceil(total / nodeStorage)])        
    print tabulate(results, headers=["Monthly growth rate", "Total amount of space", "Number of data nodes"])
    
def humanReadableSize(num):    
    for x in ['MiB', 'GiB', 'TiB', 'PiB', 'EiB']:
        if num < 1024.0: 
            return "%3.1f%s" % (num, x)
        num /= 1024.0
    
if __name__ == "__main__":
    main()

Monthly growth rate	Total amount of space	Number of data nodes
0	42.8 TiB	8
0.05	56.7 TiB	10
0.1	76.2 TiB	13
0.15	103.4 TiB	18

Let's assume that you're planning to build a cluster with 6 TiB data nodes.
If you expect to store 30 GiB daily on average and amount of incoming data is expected to grow 5% per month, then you would need min 10 nodes on first year (with all other options set to default).