TCP Slow Start Calculator
Track how the congestion window doubles each round trip during slow start and when it reaches ssthresh.
About TCP Slow Start Calculator
Slow start acknowledges one window and sends two, so the congestion window doubles every round trip — exponential growth in time, measured in RTTs rather than seconds. Once cwnd reaches ssthresh the stack switches to congestion avoidance and adds roughly one segment per RTT instead. Short HTTP responses never leave slow start, so the initial window and the RTT — not the link speed — decide how fast a page loads over a long path.
Formula
TCP Slow Start
cwnd = initial × 2^rounds, and rounds to ssthresh = ceil(log2(ssthresh ÷ initial)).
- initial
- Initial window in segments (IW10 on most modern stacks)
- rounds
- Number of round trips completed
- ssthresh
- Slow start threshold where growth turns linear
Worked example
With the default values below, TCP Slow Start Calculator returns Congestion Window After N Rounds of 160. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Initial congestion windowaccepts 1–100 segments
- 10 segments
- Maximum segment sizeaccepts 88–9,000 bytes
- 1,460 bytes
- Round-trip timeminimum 0.1 ms
- 30 ms
- Round trips elapsedaccepts 0–30 RTTs
- 4 RTTs
- Slow start thresholdminimum 1 segments
- 64 segments
Results
- Congestion Window After N Rounds
- 160 segments
- Congestion Window In Bytes
- 228 KiB
- Round Trips To Reach ssthresh
- 3
- Time To Reach ssthresh
- 90 ms
- Throughput At That Window
- 62.29 Mbit/s
Step by step
- 1Values used
- 2TCP Slow Start
- 3Congestion Window After N Rounds= 160 segments
- 4Congestion Window In Bytes= 233,600
- 5Round Trips To Reach ssthresh= 3
- 6Time To Reach ssthresh= 90 ms
- 7Throughput At That Window= 62.29 Mbit/s
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is TCP Slow Start calculated?
cwnd = initial × 2^rounds, and rounds to ssthresh = ceil(log2(ssthresh ÷ initial)). Slow start acknowledges one window and sends two, so the congestion window doubles every round trip — exponential growth in time, measured in RTTs rather than seconds. Once cwnd reaches ssthresh the stack switches to congestion avoidance and adds roughly one segment per RTT instead.
Why does TCP Slow Start matter?
Short HTTP responses never leave slow start, so the initial window and the RTT — not the link speed — decide how fast a page loads over a long path.
What values do I need to enter?
This calculator takes 5 inputs: Initial congestion window, Maximum segment size, Round-trip time, Round trips elapsed, Slow start threshold. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.