guesstimate

A guesstimate is an informal estimate derived from limited information by combining guesswork and estimation techniques. It is not a precise measurement and is typically used to inform planning, prioritization, or quick decision making when data is scarce, time is limited, or the cost of obtaining exact data is prohibitive. Guesstimates are usually presented with stated assumptions and, sometimes, a range or confidence level.

Etymology and usage have made the term common in business, engineering, and everyday discourse. The exact origin is unclear, but the blend of “guess” and “estimate” reflects its purpose: a provisional figure offered to move a discussion forward rather than to deliver a final answer.

In practice, a guesstimate provides a starting point for budgets, project timelines, or resource needs before formal models are available. A typical guesstimate includes clear assumptions, and may specify a bracket, such as “roughly 2–4 weeks, assuming two developers and no major blockers.”

Techniques used to form guesstimates include order-of-magnitude calculations, using analogies with known cases, scaling from a similar project, triangulation from partial data, or back-of-the-envelope methods. The emphasis is on transparency about the assumptions and the level of uncertainty.

Limitations include susceptibility to bias, optimism or pessimism, and potential misapplication in high-stakes decisions. Guesstimates should be treated as provisional and updated as more information becomes available, ideally accompanied by quantified ranges and a plan for validation.

See also: estimation, rough estimate, back-of-the-envelope calculation, planning poker, Delphi method.