Wikipedia does a pretty good job at explaining how different calculators treat percentage differently. For example Google or Excel will say: 50 + 10% = 50.1 ................ (== 50 + (10/100)) ...but Soulver or Microsoft Calculator will say: 50 + 10% = 55 ................ (== 50 + (50*10/100)) I understand the motivations for both systems pretty well. The former is for consistency with mathematics generally, while the latter is surely the 'popular' way, (maybe somewhat unfortunately). More than an explanation (since I know reasoning can be applied to both of the above systems), I'd like some opinions, since I'm making my own multi-line calculator - a very good one I might add. At the moment, I'm leaning towards the 50 + 10% = 55 system. But then there's stuff like this: 2+3+5+10% = 5.5 2+3+5+10% = 11 Unfortunately again, the second way is the popular one I think. If people type the question, 11 is what they'd expect right? Are we talking 70% of people, or maybe 50 or 90%? How about: 2+3*4+10% Would most people (we're talking average Jane Bloggs up to pro mathematician here) generally tend to read this as: a: ((2+3)*4)+10% ....or.... b: (2+(3*4))+10% ....or.... c: 2+(3*(4+10%)) ....or.... d: 2+((3*4)+10%) Any advice would be appreciated. In the end, I value consistency with the rest of math's operator precedence rules, but I also (grudgingly) value what people actually use, as they'll be the judges of my software, not me.