【Calculate】 , 【compute】 , 【estimate】 , 【reckon】 mean to determine something (as cost, speed, or quantity) by mathematical and especially arithmetical processes.

【Calculate】 is usually preferred when highly advanced, intricate, or elaborate processes are followed with precision and care and when the result arrived at is not readily proven by a physical confirmation (as by measuring or enumerating).

【Compute】  is preferred where the data are given or the actual figures involved are known and at hand and not arrived at indirectly; it therefore commonly implies the use of simple though often lengthy arithmetical processes.

【Estimate】 carries so strong an implication from its more common sense (see 【ESTIMATE】 ) of an evaluation based on one′s experience and good judgment that even when it implies careful calculation or computation it still connotes a result that is not necessarily exact but approximates the exact result; for he who estimates deals with data or figures that are to some extent unsatisfactory.

Hence 【estimate】 is preferred to 【calculate】 and 【compute】 when the cost of a piece of work to be done is computed at present prices.

【Reckon】 is used in place of 【compute】 and usually connotes simpler mathematical processes especially such as can be carried on in one′s head or aided by the use of counters.