HTTP/3 QUIC Calculator
Compare HTTP/3 over QUIC against TCP plus TLS: handshake round trips, congestion-window growth and total transfer time.
About HTTP/3 QUIC Calculator
QUIC folds the transport and cryptographic handshakes into one exchange and can skip it entirely on resumption, so the setup cost drops from three round trips to one. The rest of the transfer is governed by slow start, which doubles the congestion window each round trip. On a 40 ms path the handshake saving is most of the improvement users actually feel, and because QUIC recovers loss per stream rather than per connection, a lossy mobile link no longer stalls every request at once.
Formulas
HTTP/3 QUIC
datagrams = response ÷ payload size; growth rounds = ceil(log2(datagrams ÷ initial cwnd + 1)); transfer = (setup RTTs + 1) × RTT + rounds × RTT.
- setupRtt
- 0 with QUIC 0-RTT resumption, 1 for a fresh QUIC handshake, 2 for TCP + TLS 1.3
- congestionRounds
- Round trips for slow start to open the window wide enough
TCP + TLS 1.3 baseline
TCP + TLS 1.3 needs one RTT for the TCP handshake and one for TLS before the request, so its floor is 3 × RTT.
Worked example
With the default values below, HTTP/3 QUIC Calculator returns Total Transfer Time of 173.8. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Round-trip timeaccepts 0.1–2,000 ms
- 40 ms
- Response sizeaccepts 0.1–1,000,000 KB
- 60 KB
- QUIC datagram payloadaccepts 576–9,000 bytes
- 1,350 bytes
- Initial congestion windowaccepts 1–200 packets
- 10 packets
- 0-RTT resumption available
- Yes
- Packet lossaccepts 0–50 %
- 1.5 %
Results
- Total Transfer Time
- 173.8 ms
- Time to First Byte
- 40.0 ms
- Congestion-Window Growth Rounds
- 3
- Time Lost to Retransmission
- 13.8 ms
- Saving vs TCP + TLS 1.3
- 80.0 ms
- Datagrams to Send
- 46
Step by step
- 1Values used
- 2HTTP/3 QUIC
- 3TCP + TLS 1.3 baseline
- 4Total Transfer Time= 173.8 ms
- 5Time to First Byte= 40.0 ms
- 6Congestion-Window Growth Rounds= 3
- 7Time Lost to Retransmission= 13.8 ms
- 8Saving vs TCP + TLS 1.3= 80.0 ms
- 9Datagrams to Send= 46
Assumptions and limits
- The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, 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 HTTP/3 QUIC calculated?
datagrams = response ÷ payload size; growth rounds = ceil(log2(datagrams ÷ initial cwnd + 1)); transfer = (setup RTTs + 1) × RTT + rounds × RTT. QUIC folds the transport and cryptographic handshakes into one exchange and can skip it entirely on resumption, so the setup cost drops from three round trips to one. The rest of the transfer is governed by slow start, which doubles the congestion window each round trip.
Why does HTTP/3 QUIC matter?
On a 40 ms path the handshake saving is most of the improvement users actually feel, and because QUIC recovers loss per stream rather than per connection, a lossy mobile link no longer stalls every request at once.
What values do I need to enter?
This calculator takes 6 inputs: Round-trip time, Response size, QUIC datagram payload, Initial congestion window, 0-RTT resumption available, Packet loss. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.