Throughput Calculator Using Bandwidth and Delay

Formula Explanation

The actual throughput on a network is limited by two main factors: the link’s **bandwidth** and the limitations imposed by the **TCP window size** and **latency**. The maximum achievable throughput is the lesser of these two values.

1. Throughput limited by TCP Window: `Throughput = Window Size / Round-Trip Time`
2. Final Throughput: `Actual Throughput = min(Bandwidth, Window-Limited Throughput)`

What is Throughput?

In networking, throughput refers to the actual measured rate of successful data transfer over a communication channel. While ‘bandwidth’ represents the theoretical maximum data you can push through a link, ‘throughput’ is what you actually get in the real world. Think of bandwidth as the number of lanes on a highway and throughput as the number of cars that actually get through per hour, considering traffic jams (latency) and other delays. When you want to **calculate throughput using bandwidth and delay**, you are trying to find this real-world performance figure.

Throughput Formula and Explanation

The primary formula used to estimate the maximum possible TCP throughput considers the TCP receive window size and the round-trip time (RTT), but this is always capped by the link’s bandwidth.

The core formulas are:

• `Round-Trip Time (RTT) = 2 * One-Way Delay`
• `Bandwidth-Delay Product (BDP) = Bandwidth * RTT`
• `Max Throughput from Window = TCP Window Size / RTT`
• `Actual Max Throughput = Minimum(Bandwidth, Max Throughput from Window)`

Variables in Throughput Calculation

Variable	Meaning	Unit (Typical)	Typical Range
Bandwidth	The maximum data carrying capacity of the link.	Mbps, Gbps	10 Mbps – 100 Gbps
Delay (Latency)	Time for a signal to travel from sender to receiver.	Milliseconds (ms)	1 ms – 500 ms
TCP Window Size	Maximum data sent before an ACK is needed.	Kilobytes (KB)	64 KB – 16 MB
Throughput	The actual rate of data transfer achieved.	Mbps, Gbps	Varies based on inputs

Practical Examples

Example 1: High-Speed Trans-Continental Link

Imagine a company has a private 1 Gbps fiber link between New York and London. The one-way delay (latency) is 35 ms.

• Inputs: Bandwidth = 1 Gbps, Delay = 35 ms, TCP Window Size = 64 KB.
• Calculations:
  • RTT = 2 * 35 ms = 70 ms (0.070 s).
  • Throughput (Window Limited) = 64 KB / 0.070 s ≈ 7.3 Mbps.
• Result: Despite having a 1 Gbps link, the maximum throughput is only **7.3 Mbps**. The small TCP window size is the bottleneck. This is a classic “long fat network” problem. For a better outcome, check out our guide on TCP window scaling.

Example 2: Local Area Network (LAN)

Consider a typical office network with a 1 Gbps Ethernet connection to a local server. The latency is very low, around 0.5 ms.

• Inputs: Bandwidth = 1 Gbps, Delay = 0.5 ms, TCP Window Size = 64 KB.
• Calculations:
  • RTT = 2 * 0.5 ms = 1 ms (0.001 s).
  • Throughput (Window Limited) = 64 KB / 0.001 s = 512 Mbps.
• Result: The maximum throughput is **512 Mbps**. In this case, throughput is not limited by the TCP window, but it’s still not the full 1 Gbps due to protocol overhead and other factors not included in this simple calculation. Our Online data transfer calculator can help estimate file transfer times.

How to Use This Throughput Calculator

1. Enter Bandwidth: Input the advertised speed of your network link and select the correct unit (e.g., Mbps).
2. Enter Delay: Input the one-way latency, typically found using a `ping` command. Ensure the unit is milliseconds.
3. Enter TCP Window Size: Input the system’s TCP receive window size. 64 KB is a common default, but modern systems often use larger values. Select the correct unit (KB or MB).
4. Calculate and Interpret: Click “Calculate”. The main result shows your maximum achievable throughput. The intermediate values help you understand the bottleneck: if “Throughput (Window Limited)” is much lower than your bandwidth, your window size is too small for the link’s latency. Understanding the difference between Latency vs. Throughput is key.

Key Factors That Affect Throughput

• Latency: As latency increases, the round-trip time gets longer, reducing the rate at which data can be acknowledged and thus lowering throughput.
• Bandwidth: The raw capacity of the link is the ultimate speed limit. Throughput can never exceed bandwidth.
• TCP Window Size: This determines how much data can be “in flight” at any one time. On high-latency links, a small window size will severely limit throughput.
• Packet Loss: When data packets are lost, they must be re-transmitted, which consumes time and reduces the overall throughput. This calculator assumes zero packet loss. You can explore this with our packet loss tool.
• Network Congestion: If many devices share the same link, it can lead to queues and dropped packets, reducing throughput for everyone.
• Protocol Overhead: TCP, IP, and Ethernet headers add size to the data being sent, consuming a portion of the bandwidth and slightly reducing the ‘goodput’ (application-level throughput).

Frequently Asked Questions (FAQ)

What is the difference between throughput and bandwidth?

Bandwidth is the theoretical maximum capacity of a link. Throughput is the actual, measured data transfer rate, which is almost always lower due to factors like latency, packet loss, and protocol overhead.

How do I find my latency?

You can use the `ping` command in your computer’s terminal or command prompt. For example, `ping google.com`. The time value shown is the round-trip time (RTT). Divide it by two for the one-way delay.

Why is my throughput so much lower than my bandwidth?

Usually, this is because of high latency combined with a TCP window size that is too small for that latency. This is what the Bandwidth-Delay Product explained article covers in detail.

Can I have high bandwidth but low throughput?

Yes, absolutely. A 10 Gbps satellite link with high latency (e.g., 500 ms) and a small TCP window size will have very low throughput despite its massive bandwidth.

How can I improve my throughput?

If latency is the issue, you can try to reduce it (e.g., use a server closer to you). If the TCP window is the bottleneck, you can sometimes tune operating system settings to allow for larger window sizes (TCP Window Scaling).

What is the Bandwidth-Delay Product (BDP)?

The BDP calculates the maximum amount of data that can be in transit on the network at any given time. It is calculated as `Bandwidth * RTT`. To achieve maximum throughput, your TCP window size should be at least as large as the BDP.

Does this calculator account for packet loss?

No, this is a simplified model that assumes a perfect network with zero packet loss. Real-world throughput will be lower if packet loss is present.

What is a “long fat network”?

It’s a network with both high bandwidth and high round-trip delay (a large BDP). These networks require very large TCP window sizes to be utilized effectively.

Related Tools and Internal Resources

Explore our other tools and articles to get a full picture of your network’s performance.

• Online Data Transfer Calculator: Estimate how long a file transfer will take.
• Packet Loss Impact Calculator: See how packet loss affects performance.
• What is Latency vs. Throughput?: A deep dive into these two critical concepts.
• Understanding TCP/IP: Learn about the protocols that run the internet.
• How to improve network speed: A guide to optimizing your web performance.
• Bandwidth-Delay Product Explained: Understand this key concept for network tuning.