Interface Queue Calculator
Calculate network interface queue length needed to avoid packet drops under burst load.
About Interface Queue Calculator
The txqueuelen parameter sets how many packets the kernel can buffer for a network interface before the queueing discipline (qdisc) starts dropping new arrivals, and correctly sizing it means balancing two failure modes: too small drops packets during legitimate bursts or high-latency paths, while too large introduces bufferbloat — packets sit queued for so long that added latency hurts interactive and real-time traffic even though nothing is technically dropped. A reasonable target ties queue depth to the bandwidth-delay product divided by average packet size, giving just enough buffer to absorb one RTT's worth of in-flight data.
Formula
Optimal queue depth
optimal_queue_packets = BDP / avg_packet_size
- BDP
- bandwidth-delay product in bytes
- L_{pkt}
- average packet size in bytes
Worked example
With the default values below, Interface Queue Calculator returns Optimal txqueuelen (packets, BDP/MTU) of 1,667. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Configured txqueuelen (packets)minimum 0
- 1,000
- Interface Bandwidth (Mbps)minimum 0.1
- 1,000
- Round-Trip Time (ms)minimum 0.1
- 20
- Avg Packet Size (bytes)minimum 64
- 1,500
- Incoming Packet Rate (pps)minimum 0
- 90,000
Results
- Optimal txqueuelen (packets, BDP/MTU)
- 1,667
- Current Sizing Assessment
- Well-sized
- Queue Overrun Risk?
- true
- Excess Rate Beyond Drain Capacity (pps)
- 6,666.7
- Time to Fill Queue at Excess Rate (ms)
- 150.00
Step by step
- 1Values used
- 2Optimal queue depth
- 3Optimal txqueuelen (packets, BDP/MTU)= 1,667
- 4Current Sizing Assessment= Well-sized
- 5Queue Overrun Risk?= Yes
- 6Excess Rate Beyond Drain Capacity (pps)= 6,666.7
- 7Time to Fill Queue at Excess Rate (ms)= 150.00
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 do I check and change txqueuelen?
`ip link show <iface>` displays the current qlen value; change it with `ip link set <iface> txqueuelen <N>` (temporary, resets on reboot unless persisted via a network manager profile or udev rule).
What is bufferbloat and how does it relate to txqueuelen?
Bufferbloat is excessive queuing delay caused by oversized buffers that let packets sit queued far longer than necessary before being dropped or transmitted, defeating TCP's congestion control feedback loop — modern fixes like fq_codel and CAKE actively manage queue depth and latency rather than relying purely on a large static txqueuelen.
Does a higher-bandwidth interface always need a larger txqueuelen?
Not necessarily on its own — queue sizing should scale with bandwidth-delay product (which factors in RTT, not just bandwidth), so a high-bandwidth but low-latency LAN link may need a smaller queue than a lower-bandwidth but high-latency WAN link.