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.

I wouldn't aim for "exact" per se, but it comes down to having a solid brand positioning first and foremost, aimed at those you WANT to reach, that you feel are the best product/service-to-market fit for what you built your business for in the first place. This won't always be "clean" demographic categories, but rather they may very well overlap into what we focus on here: unique buying tribes (especially if you are trying to reach a large segment of Millennials, who are highly tribal in their nature, digital and offline).

So it may be "oldest half of Millennials who are pet owners in the midwest", or "Retired Baby Boomers who are also homeowners in Southern California", and so on.

Having this set out at the beginning of launching a new startup, or relaunching an existing business into the market with a heavy marketing push, will you help fast-track your marketing and advertising efforts immensely as you'll know more "exactly" who you're aiming at, and it'll be very helpful when picking targeting criteria on whatever marketing/advertising channels you're utilizing (Twitter, OTT, programmatic, YouTube, Google AdWords, and so on).

Of course, throughout all of this will be lots of research...you can try your hand at that on your own using solid resources like Pew Research (https://www.pewresearch.org) and/or starting with Google, which will help guide you to many other solid resources (dependent on the quality of your searches of course).

More than likely, with well (but again...they won't be "exact") defined personas and sticking to 2-3 people groups (not 10), you'll have plenty of folks you can market to, probably more than your marketing & advertising budget can stomach :-)

Happy to discuss this further with you, so feel free to reach out any time!

Demographics describe who we are as individuals, for example: ethnicity, age/generation, gender, income, marital status, education, and homeownership. These and other characteristics categorize us without describing our personality. Many demographic characteristics cannot be changed and are causally related to our physical being. A recognized definition is: "The characteristics of human populations and population segments, especially when used to identify consumer markets". Retailers will find that most of the demographic data they need to make business decisions can be found on the U.S. Census website.
To determine exact target demographics that will help you in launching your product must investigate the following points:
1. Demographics: Generations
Members of the lawn and garden industry have promoted container gardening and the use of raised garden beds as a way for 'Baby Boomers' and 'Mature' generations to garden without having to bend over. Evidence suggests that the volume of wine purchase annually by members of Generation Y has a greater impact on the economic viability of the wine industry than some other generations. Terms, such a 'Gen X,' 'Generation Jones,' and 'Baby Boomers', were created as a way of classifying "a group of individuals, most of whom are the same approximate age, having similar ideas, problems, attitudes, etc.". Marketers have discovered that they can often work with producers and retailers to provide goods and services that would appeal to a majority of members of a particular generation, based on the general needs and wants of consumers within the group. See the sidebar for examples.
2. Demographics: Ethnicity
For the past several years, the population of certain ethnic groups has increased, particularly those who are of Asian and Hispanic or Latino descents. These changes present opportunities for many agricultural businesses. It is necessary for agricultural businesses to understand that within the Asian ethnic group, that those of Chinese, Korean, and Japanese descent will have different languages and cultures. The same is true for groups within the Hispanic ethnic group.
1. Agricultural retailers or producers who offer foods could produce vegetables, flours, grains, and other ingredients used by ethnic groups on a day-to-day basis and when preparing meals for Chinese New year (mushrooms, bamboo shoots, and bean sprouts). If this approach is pursued, it will be important to know which vegetables are commonly used by each ethnic group. It will also be important to learn about the meaning that certain ingredients convey. Examples include: Chinese lettuce (prosperity) and turnip (good omens).
2. Those who grow plants and flowers can also market their products to ethnic groups who wish to incorporate these items into traditional ceremonies.
It will be necessary to determine that there is a large enough population of the target ethnic group in order to support the retailer's or producer's efforts. Sources such as Holidays on the Net describe the occasion, ethnic group(s) that celebrate a given event, and what products make the celebration authentic.
3. Demographics: Income
Disposable income (or net income) is the amount of money that an individual has available to pay for expenses minus taxes and deductions. Discretionary income is the amount of money that remains after consumers pay for "needs" (food, rent/mortgage, insurance, etc.) that are required to sustain a reasonable livelihood. In general, the higher the level of net income the greater the amount of discretionary income that would be available for a consumer to purchase 'wants,' or items not necessary to maintain their life but rather their lifestyle.
1. If the 'core' customer has a limited income, then goods and services marketed to them should be within a price range that is affordable.
2. Products marketed to more affluent consumers, however, would be manufactured, packaged, and promoted in a manner that would reflect their level of quality and prestige and would be priced accordingly. Agricultural businesses that offer food items classified as specialty foods might be considered as supplying a 'want' and would have to develop a marketing strategy to appeal to consumers with a higher income level.

4. Demographics: Geographical
The location where a consumer lives (e.g., the southeast vs. the northwest U.S. and/or in a metropolitan area versus a rural environment) can greatly influence his or her needs, wants, and access to goods and services. Cultural 'tastes' and traditions can affect food preparation, ingredients commonly used, and availability of specialty cuisine. East Coast consumers may include seafood as a staple in their diets, while consumers living in the southwest may frequently use chili peppers. Food retailers should be aware of entrees and recipes commonly prepared in their area and provide as many essential ingredients as possible. As a greater number of consumers travel and experiment with new cuisines, they may desire to recreate the dish in their own kitchen upon their return. Specialty or gift shop retailers can offer gift baskets that include these food ingredients or sell baking mixes that customers can prepare with ease.
The availability and variety of food ingredients can also differ based on consumer's proximity to large metropolitan areas. Consumers residing in metropolitan areas, such as New York and Philadelphia, have access to a large variety of products and outlets from which to purchase. Rural consumers, however, may have fewer options. Retailers in these areas will need to become educated about current trends and predictions and what is needed to provide consumers with ingredients and information necessary to cook a popular meal or to plant a fashionable garden. Becoming a member of a national association may be a wise decision. Subscribing to free e-newsletters published by marketing associations and related sources will also provide retailer with useful information.

5. Demographics: Homeownership
Knowing the level of homeownership within a market area is particularly important for businesses that sell plants, vegetable seeds, and lawn and garden services. Consumers who are homeowners may be more willing than renters to: a) invest in the establishment and upkeep of their lawn and garden and b) purchase outdoor plant material and landscape services.
1. Income level and amount of outdoor space may have an influence on the amount of money spent and the types of goods and services purchased.
2. The types of plant material grown and marketed should not only be appropriate for the 'target' market's location but should also be the right dimensions for the consumer's outdoor lawn or garden space. Horticultural business that find that there are several apartment complexes or condominiums, with limited yard or no yard at all, within their market area should consider marketing plant material that can be grown in containers, in raised beds, or have a more vertical growth habit.

Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath