TCP Slow Start Calculator
Track how the congestion window doubles each round trip during slow start and when it reaches ssthresh.
Inputs
Congestion Window After N Rounds
160segments
Congestion Window In Bytes
228 KiB
Round Trips To Reach ssthresh
3
Time To Reach ssthresh
90ms
Throughput At That Window
62.29Mbit/s
Step by step
Values used
Initial congestion window = 10 segments; Maximum segment size = 1,460 bytes; Round-trip time = 30 ms; Round trips elapsed = 4 RTTs; Slow start threshold = 64 segments
TCP Slow Start
cwnd = initial × 2^rounds, and rounds to ssthresh = ceil(log2(ssthresh ÷ initial)).
Congestion Window After N Rounds
= 160 segments
Congestion Window In Bytes
= 233,600
Round Trips To Reach ssthresh
= 3
Time To Reach ssthresh
= 90 ms
Throughput At That Window
= 62.29 Mbit/s
How it works
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
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.