How to compute the storage cost of encrypted data Let's say I have (as an example only) 1 MB of plaintexts. I want to encrypt it using symmetric encryption algorithm (e.g. AES) and store the plaintext and ciphertext in a memory space. Does a ciphertext take the same size as its corresponding plaintext? I mean, in my example, does this mean I need 2 MB to store plaintexts and ciphertexts? Can you provide references?
Your question is too broad in this form.

There are plenty of different symmetric encryption algorithms. Most of them produce 1MB output for 1MB input. Most of them are block-based, so they round up the input size to a multiple of a block-size. If you choose a specific algorithm, you can check the exact details yourselves. You should not choose encryption algorithm based on the overhead though, but rather choose one based on security and performance.

Also, it is usual to compress data before encrypting it, because it is impossible to do so after encryption.