What is the default TCP window size?

Without window scaling, the maximum TCP window is 64 KB (2^16). With window scaling (standard on modern OS), windows can reach 1 GB. Most OS auto-tune the window size, but may not optimize for high-BDP paths without configuration.

Why does latency limit throughput?

TCP requires acknowledgments. The sender can only have window_size bytes in flight. Once the window is full, it waits for ACKs. With 50 ms RTT and 64 KB window: 64 KB / 50 ms = ~10 Mbps, regardless of available bandwidth.

How do I tune TCP buffers on Linux?

Set sysctl values: net.core.rmem_max and net.core.wmem_max (max buffer), net.ipv4.tcp_rmem and net.ipv4.tcp_wmem (auto-tuning range: min, default, max). Set max values to at least the BDP for your highest-bandwidth, highest-latency path.

Does TCP window scaling work with load balancers?

Most modern load balancers and proxies support window scaling. However, some older hardware load balancers may strip or ignore window scaling options. Test end-to-end to verify. This is a common cause of unexpectedly low throughput.

What about UDP performance?

UDP has no window concept — it sends as fast as the application allows. QUIC (used by HTTP/3) uses its own congestion control with window-like mechanics but avoids TCP's head-of-line blocking. BDP is still relevant for QUIC buffer sizing.

How does packet loss affect the window?

TCP congestion control (Cubic, BBR) reduces the window on packet loss. High loss rates collapse the effective window, severely limiting throughput. A 1% loss rate can reduce throughput by 50–75%. Monitor loss alongside window tuning.

TCP Window Size Calculator

Calculate optimal TCP window size and maximum throughput based on bandwidth and round-trip latency (bandwidth-delay product).

BandwidthLink capacity

Mbps

Round-Trip TimePing latency

Packet Loss0 = ideal, 0.1% typical WAN

Max Segment Size

Custom Window SizeTest a specific window size

bytes

Bandwidth-Delay Product

5.96 MB

Optimal window = 6,250,000.00 bytes (4,281.00 segments)

Window Scale Factor

Max window = 8,388,480.00 bytes (2^7 × 64 KB)

Default Window (64 KB)

10.5 Mbps

0.01% utilization — no window scaling

Optimal Throughput

1,000.00 Mbps

Window = BDP — 100% link utilization

Custom Window

10.5 Mbps

65,536.00 byte window — 0.01% utilization

Loss-Limited (Mathis)

9.0 Mbps

With 0.1% loss — 0.01% effective utilization

Link Utilization

Default (64 KB)0.01%

Custom (64 KB)0.01%

Loss-Limited0.01%

Optimal (BDP)1.00%

Throughput Comparison

Configuration	Window Size	Throughput	Utilization
Default (no scaling)	64 KB	10.5 Mbps	0.01%
Custom	64 KB	10.5 Mbps	0.01%
Loss-Limited (Mathis)	—	9.0 Mbps	0.01%
Optimal (BDP)	5.96 MB	1,000.00 Mbps	100%

Common Scenarios Reference

Scenario	Typical RTT	BDP (1 Gbps)	Scale Factor	Notes
Local LAN	< 1 ms	~0.6 KB	0	No scaling needed
Same Region	5–15 ms	~0.6–1.8 MB	4	Window scaling helps
Cross-Country	30–70 ms	~3.6–8.4 MB	7	Scaling required
Transatlantic	80–150 ms	~10–18 MB	8	Large buffers critical
Satellite (GEO)	500–700 ms	~6–8 MB	7	BDP dominates

Planning notes, formulas, and examples

About the TCP Window Size Calculator

TCP throughput is fundamentally limited by the TCP window size and network round-trip time (RTT). The Bandwidth-Delay Product (BDP) determines the optimal window size: the amount of data that can be in flight (sent but not yet acknowledged) to fully utilize the available bandwidth.

If the TCP window is smaller than the BDP, the connection cannot fill the pipe: the sender must wait for acknowledgments before transmitting more data, leaving bandwidth unused. This is especially impactful for high-bandwidth, high-latency links (WAN, satellite, cross-region cloud).

This calculator computes the BDP for your link parameters and determines the required TCP window size to achieve maximum throughput. It also estimates the maximum achievable throughput with the default and optimal window sizes.

When This Page Helps

Default TCP window sizes are often too small for high-bandwidth or high-latency links. This calculator determines the optimal window size to maximize throughput on your specific network path.

How to Use the Inputs

Enter the link bandwidth in Mbps.
Enter the round-trip time (RTT) in milliseconds.
Review the bandwidth-delay product and required window size.
Configure OS TCP buffers to at least the BDP value.

Formula used

BDP (bytes) = bandwidth_bps × RTT_sec / 8
BDP (bits) = bandwidth_bps × RTT_sec
Max Throughput = window_size × 8 / RTT_sec (bps)

Example Calculation

Result: BDP: 6.25 MB, optimal window: 6.25 MB

BDP = 1,000 Mbps × 50 ms = 1,000,000,000 × 0.050 / 8 = 6,250,000 bytes (6.25 MB). A TCP window of 6.25 MB is needed to fully utilize a 1 Gbps link with 50 ms RTT. Default 64 KB window would limit throughput to ~10 Mbps.

Tips & Best Practices

Enable TCP window scaling (RFC 7323) for windows larger than 64 KB.
Set OS socket buffer sizes (SO_RCVBUF, SO_SNDBUF) to at least the BDP.
Modern Linux auto-tunes window size — verify with ss -i.
WAN links (cross-region cloud) benefit most from window tuning.
Satellite links (500+ ms RTT) require windows of 10s of MB.
Monitor TCP retransmissions alongside window size for complete picture.

The Bandwidth-Delay Product Explained

Imagine a pipe: bandwidth is the pipe's width, RTT is its length. The BDP is the pipe's volume — the amount of data needed to fill it. If you send less than BDP, the pipe has empty space (wasted bandwidth). The TCP window must be at least as large as the BDP to keep the pipe full.

Real-World Impact

A 10 Gbps link between US East and US West (RTT ~60 ms) has BDP = 75 MB. Default 64 KB windows would achieve only 8.5 Mbps on a 10,000 Mbps link — 0.085% utilization. Proper tuning achieves near-wire speed.

Modern TCP Congestion Control

BBR (Google's TCP congestion control) probes for optimal window size automatically, adjusting based on measured bandwidth and RTT. It performs better than traditional Cubic on high-BDP links. Consider enabling BBR on servers (net.ipv4.tcp_congestion_control=bbr).

Sources & Methodology

Last updated: February 8, 2026

Frequently Asked Questions

Without window scaling, the maximum TCP window is 64 KB (2^16). With window scaling (standard on modern OS), windows can reach 1 GB. Most OS auto-tune the window size, but may not optimize for high-BDP paths without configuration.