Quick answer
TCP throughput is limited to window × 8 ÷ RTT. A 64 KiB window with a 20 ms round trip gives a maximum of 26.214 Mbps.
Enter the TCP window size and the round-trip time. The calculator returns the maximum throughput. Use this network throughput calculator to check your own figures.
Related tools: IP Subnet Calculator: IPv4 network, broadcast, mask and host range from an address and CIDR prefix. Download Time Calculator: Time to download a file at a given speed. Bandwidth Calculator: Bandwidth needed to move a file in a given time.
How to use the Network Throughput Calculator
- Enter tCP window size (KiB) (above 0).
- Enter round-trip time (ms) (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| TCP window size | A number, in KiB | > 0 |
| Round-trip time | A number, in ms | > 0 |
Formula and method
Formula
throughput = window × 8 ÷ RTT
IEEE-754 double precision; shown to the decimals indicated.
Upper bound set by the window; loss and congestion lower it.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: TCP window size = 64; Round-trip time = 20.
| Result | Value |
|---|---|
| Maximum throughput | 26.214 Mbps |
Example 2
Inputs: TCP window size = 256; Round-trip time = 50.
| Result | Value |
|---|---|
| Maximum throughput | 41.943 Mbps |
Example 3
Inputs: TCP window size = 1024; Round-trip time = 100.
| Result | Value |
|---|---|
| Maximum throughput | 83.886 Mbps |
The first line is the headline result (maximum throughput). IEEE-754 double precision; shown to the decimals indicated.
Worked example
65,536 bytes × 8 = 524,288 bits per round trip, and ÷ 0.020 s = 26.2 Mbps. Doubling the latency halves the limit.
Window scaling
Without window scaling the window is capped at 64 KiB, which is why long-distance links were slow. Modern stacks scale the window, so the limit then depends on the buffer size.
How the result changes with tCP window size
Holding every other input at the first worked example, this table changes only tCP window size. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| TCP window size | Maximum throughput |
|---|---|
| 32 | 13.107 Mbps |
| 64 | 26.214 Mbps |
| 128 | 52.429 Mbps |
| 256 | 104.858 Mbps |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- TCP window size: entering "0" shows "TCP window size must be greater than 0." beside that field and no result.
- Round-trip time: entering "0" shows "Round-trip time must be greater than 0." beside that field and no result.
Choosing between the Network Throughput Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [IP Subnet Calculator](/technology/ip-subnet-calculator/) | iPv4 address and cIDR prefix length | Network |
| [Download Time Calculator](/technology/download-time-calculator/) | file size, size unit and connection speed | Time |
| [Bandwidth Calculator](/technology/bandwidth-calculator/) | data size, size unit and time allowed | Bandwidth required |
| [Data Transfer Calculator](/technology/data-transfer-calculator/) | speed, speed unit and duration | Data transferred |
| [Latency to Distance Calculator](/technology/latency-to-distance-calculator/) | round-trip latency and signal speed as a fraction of c | Approximate distance |
Background: networking and display
Network speeds are quoted in bits per second and file sizes in bytes, and a byte is 8 bits, so a 100 Mbps link moves at most 12.5 megabytes per second; real throughput is usually below the advertised rate. Pixel density is the diagonal in pixels divided by the diagonal in inches: a 1920×1080 display of 24 inches is about 92 pixels per inch, and a 3840×2160 display of 27 inches about 163.
Assumptions and limitations
- Window scaling needed above 64 KiB
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for tCP window size and round-trip time to confirm this.
Frequently asked questions
How do I raise throughput on a long link?
Use a bigger window.
Does bandwidth matter?
The link rate is another cap.
What is RTT?
RTT is round-trip time, how long a packet takes to reach the other end and for the reply to come back.
What does the Network Throughput Calculator calculate?
Maximum TCP throughput from window size and round-trip time. It returns Maximum throughput.
What formula does the Network Throughput Calculator use?
throughput = window × 8 ÷ RTT. Upper bound set by the window; loss and congestion lower it.
What do I need to enter in the network Throughput Calculator?
You enter TCP window size (KiB) and Round-trip time (ms).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Window scaling needed above 64 KiB.
What is an example of a network Throughput result?
With tCP window size 64 and round-trip time 20, the maximum throughput is 26.214 Mbps.
Where does the method come from?
The method follows: Bandwidth–delay product relation (RFC 1323 / RFC 7323).
Related calculators
- IP Subnet Calculator — IPv4 network, broadcast, mask and host range from an address and CIDR prefix.
- Download Time Calculator — Time to download a file at a given speed.
- Bandwidth Calculator — Bandwidth needed to move a file in a given time.
- Data Transfer Calculator — Amount of data moved at a speed over a time.
- Latency to Distance Calculator — Approximate one-way distance from round-trip latency.
- Packet Loss Calculator — Percentage of packets lost.
Sources
- Bandwidth–delay product relation (RFC 1323 / RFC 7323)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.