It depends on the context. [*calculation*][1] can mean 

>  careful planning or forethought, esp for selfish motives

or 

> an estimation of probability; forecast

It isn't necessarily redundant, per se, but I'd say that you'd be hard-pressed to produce an example in which it wasn't redundant if the context of math is understood. If a math context wasn't previously understood, then it likely isn't redundant (there is possible ambiguity as to the aforementioned usages). In those situations, though, it's likely that qualifying with "mathematical" is just excessive or pedantic. I'd only use it if it's truly useful to. 

[1]: http://www.thefreedictionary.com/calculation