Put Back-of-the-Envelope Numbers in Perspective

Back-of-the-envelope calculations (BOTECs) are quick, approximate estimations typically performed on paper. While not intended to yield precise results, they provide a preliminary evaluation of a system’s feasibility and critical parameters.

For example, to estimate a neighborhood’s population, you might count houses in a sample area and multiply by the average household size. This is faster than conducting a full census. Similar calculations can validate data or test assumptions.

BOTECs in system design

A request may pass through web, application, cache, and storage components. In an interview, estimate the resources that could constrain the proposed design rather than modeling every machine. Keep the workload and capacity assumptions visible so you can revise them when the design changes.

Common estimations include:

• Concurrent TCP connections a server can support.

• Requests per second (RPS) a web, database, or cache server can handle.

• Storage requirements for a service.

We use BOTECs to abstract hardware specifics, latencies, and throughput rates. We will first examine server types and latencies to understand the system’s reality. Then, we will simplify these details to estimate RPS, bandwidth, and storage capacity.

Types of data center servers