Your application needs to perform 5 million writes and 5 million eventually consistent reads per day with item sizes of 1 KB:

Number of writes per day = 5,000,000
Number of writes per second = ceil (5,000,000 / (24 * 60 * 60))  = 58
1 WCU is required for each 1 KB of write. So writes required for each item = ceil(1) = 1
Total WCU required  = 58 x 1 = 1.
Similarly 5,000,000 eventually consistent reads per day  = 58 eventually consistent reads per second.
1 RCU is required for 2 eventually consistency reads. So RCU required for 58 eventually consistent reads  = 29
1 RCU is required for 4KB of read. So reads required for each data item = ceil(1/4) = 1
Total RCU required = 29 x 1 = 29.

This example is based on the pricing example given by Amazon. We will see some more scenarios to make the concept clearer. 

Please note the following:

• The ceil function returns the smallest integer greater than the passed value, for example, ceil (1.5) is 2.
• The AWS Free Tier provides 25 RCU and 25 WCU per second for a whole month. Therefore, if you set and use 58 WCU and 29 RCU for a month, you will be billed for only 33 WCU and 4 RCU.
• The AWS Free Tier provides 25 RCU and 25 WCU per second for a whole month. If you are using DynamoDB only for half a month, you can use approximately 50 WCU and 50 RCU per second. Therefore, if you set and use 58 WCU and 29 RCU for only half of a month, you will be billed for only 8 WCU and 0 RCU.