Estimation is a guess, not a measurement. Estimates are uncertain, otherwise, they would be observations (or measurements!).
Description
Therefore, estimate in intervals, giving a lower and upper bound to your estimate. The difference between the two shows your confidence in the estimate. If this difference is relatively small, it shows high confidence.
Caution: Be aware of the anchoring effect http://en.wikipedia.org/wiki/Anchoring_effect
Good estimates ensure that the estimated value will be contained in the interval with a very high probability.
Estimates often rely on assumptions - which should be Explicit Assumption.