Hey there, I'm going to give you my low cost/ no cost approach. (Large firms have tools and research teams, I do not!) (1) First I'd break down - into a list - all the aspects that make up a demographic target or avatar (e.g. age, race, ethnicity, gender (e.g. male, female, non-binary, geographic location, income level (i.e. both household and individual), education level, marital & family status (i.e. does your target audience have children), occupation). (2) Second, look at the list, write down what you know, and for what you don't know go to step 3. (3) Next, it's time for more research. You may actually know the answers to all those questions, but do you, or your client, know how big that demographic target (or combination of multiple demos) is? Also, everyone in the same demo does not think, believe, or have the same purchase behaviors. So look at the psychographic aspects of your target as well (i.e. interests/ hobbies, media consumption, political beliefs, organizations affiliated with, etc.), 4) Finally, here are my go-to free data resources to calculate the size of my target audience (which is needed to calculate conversion rates and if the market is large enough to make a profit from based on your product variables. 5) Research resources: Pew Research Institute, U.S. Bureau of Labor Statistics, U.S. Census Bureau Quick Facts (obviously I have made the assumption your targets are U.S. based): https://www.pewresearch.org/ | https://www.bls.gov/ | https://www.census.gov/quickfacts/fact/table/US/PST045219
Best of luck! Leslie
I love Leslie's answer, so I will add to hers by going a level above, back to your product. Who is your product built for and what problem are you solving? This seems like a basic question, but when I work with clients on their brand and product development, we spend a lot of time on exploring their target customers.
By developing a key persona or personas (an Avatar, or example of your customer), you will also create a profile that answers this question about your demographics and goes further to roadmap the best way to reach them. Most importantly, what to say to them.
Then, for audience sizing and more specifics, Leslie gives you great guidance in her answer.
I would be happy to talk you through an exercise to get to this strategic answer if you'd like.