estimate可以作为动词，也可以作为名词。estimation只能作为动词，当二者为名词时，都有“估计”的意思。如何区别呢？

比如经常说的“x的估计值”the estimate of x，那什么时候用estimate的名词，什么时候用estimation呢？

Linda的email解释最权威

   The word "estimate" can be used both as a noun and a verb.  For example, as a noun, "I will give you an estimate on the amount of time it will take to complete this job".  As a verb, "I will estimate the time it will take to complete this job".  

    "Estimation" is usually used only as a noun.  It is the act or process of estimating.   For example, "The amount of time it will take to complete this job is approximately 4 hours.  This is only an estimation of the time.  The job could take longer." or,  This is only an estimate of the amount of time.

  综上所述，当都作为名词的时候，estimate专指结果，而estimation指过程（process of estimating）。

ex：

estimate of x 

x的估计值

estimation of  x

估计x（的过程）