What should I wear to a classical music concert?

Various Settings

While the stereotypical image of a classical music concert that comes to mind is a formal concert hall where people are dressed up in formal evening wear, the reality is that classical music is performed in various settings. I've attended performances wildly different venues: outdoor events, multipurpose halls without air conditioning, a restaurant converted into a performance space for the evening, and of course, concert halls. As you would expect, the choice of attire for each of these settings may vary. 

Getting In

The best thing to do to make sure you don't wear something that gets you denied entry to the concert in the first place, is to check your tickets to the event or Google the venue for any dress code or restrictions of any kind. For example, as part of their general booking information, the Dewan Filharmonik Petronas has a smart casual dress code and clearly states several types of clothing that are not allowed such as sports attire and flip flops.  

But what if there isn't anything explicitly stated?

Blending In

In my experience, smart casual attire will get you in anywhere. The exact definition of smart casual, though? I'm not even entirely sure how to define that, and neither could I find any consensus on the internet. 

A personal benchmark I like to use is "What would I wear to work on a day that I'm giving a presentation?". This generally works for me in conjuring up the right amount of looking put together without being too loud. Not exactly the most precise of benchmarks, I know. But realistically, going to a concert is like attending any other event. You want to somewhat blend in and look occasion-appropriate, but still allow for your personal style and comfort.  

And Not Blending In Too Much

One thing to note about classical music being performed live: performers, especially as part of an ensemble, traditionally and still often do, wear black. Make of that what you will with your own outfit choices. 

I've personally worn black as an audience member before, and it's not a big deal at all. However, when I am attending a performance in a more intimate setting, I do like to avoid it so performers stand out.