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Asked: 2026-05-21T11:32:50+00:00

I want to attempt to calculate how much data (bytes) I send/receive over the

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I want to attempt to calculate how much data (bytes) I send/receive over the network. I send/receive both TCP and UDP packets, so I need to be able to calculate the size of these packets including their respective headers. I looked at this questions: Size of empty UDP and TCP packet and it lists the minimum size of the header, but is that libel to change? Should I just add the number of bytes I send in the packet, but the size of the minimum header? Also, I know at some point (n bytes) the data would be too big to fit in just one packet.

One other thing, the client is a mobile device, so it may receive over cellular or wifi. I am not sure if there is a difference in the packet size between the two, but I would probably just want to assume what ever is larger.

So my questions are, assuming the data is n bytes long:

1) How big would the TCP packet be, assuming it all fits in one packet?

2) How big would the UDP packet be, assuming it all fits in one packet?

3) Is there an easy way to determine the number of bytes it would take to overrun one packet? For both TCP and UDP.

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1 Answer

  1. Editorial Team

    Lets assume we’re only talking about ethernet and IPv4

    Look at your interface MTU, which has already subtracted 
    the size of the ethernet headers for the OS I can 
    remember (linux and FreeBSD)

Subtract 20 bytes for a normal IP header (no IP options)

Subtract 20 bytes for a normal TCP header

Or

Subtract 8 bytes for a UDP header

    That is how much data you can pack into one IPv4 packet. So, if your TCP data is n bytes long, your total ethernet payload is (n + 20 + 20); your ethernet payload for UDP is (n + 20 + 8).

    EDIT FOR QUESTIONS

    RE: MTU

    Your interface MTU is the largest ethernet payload that your drivers will let you encapsulate onto the wire. I subtract because we’re assuming we start from the MTU and work up the encapsulation chain (i.e. eth -> ip -> tcp|udp); you cant send TCP or UDP without an IP header, so that must be accounted for as well..

    RE: Calculating application overhead

    Theoretical calculations about the overhead your application will generate are fine, but I suggest lab testing if you want meaningful numbers. Usage factors like average data transfer per client session, client hit rate per minute and concurrent clients can make a difference in some (unusual) cases.

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