An 'estimate' is an approximate calculation or evaluation of something. It is the concrete approximation of some thing that is measurable. An estimation is the act of estimating. It is not an approximate calculation that is a number, but instead is my PROCESS/WAY of thinking to arrive at that 'estimate'. Example: My estimate is there are 20 cows in that field. - Here my 'estimate' is my calculation that I have arrived at. Estimate directly represents a number here (20). In my estimation, there are 20 cows in that field - Here, 'estimation' is not my end number calculation. I am merely saying that 'I think' or 'to my way of thinking' or 'in my thinking' there are 20 cows there.

As nouns. In my estimation, they are a very capable crew. ( “in my estimation” is a set phrase similar to “in my opinion”.) Here is my estimate to complete the work. $6000. (Can’t use estimation)

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.