Approximate calculation is a convenient way to get an idea or estimate of a number or value. But, if you want to avoid using this term, there are many synonyms that you can use. For example, an estimation, a rough calculation, or a ballpark figure. Other synonyms include an approximation, an educated guess, a close guess, or a proximate value. Additionally, you can also describe an approximate calculation as a rough estimate, an imprecise calculation, or a tentative calculation. In summary, there are many ways to describe an approximate calculation without using this specific term and these synonyms can help you to communicate a rough estimation with greater clarity and precision.