Short answer: start from the job your buyer does, not from your product category. List candidate subreddits from a finder tool and from the places your problem already comes up, read a month of posts in each, and check the rules. Then watch three to five for two weeks and keep the ones where people ask for help with the problem you solve. Member count is the weakest signal on the list.
1. Describe your buyer's job, not your category
If you sell a SaaS for bookkeepers, r/SaaS is full of other founders, not your buyers. Your buyers are in the communities about their own work: bookkeeping, their software, their clients. Write down the job, the tools they use today and the words they use to complain about it.
2. Make a list of candidates
- Paste your product page into Website to Subreddits. Every name it returns is checked against Reddit, with live member counts.
- Start from a list for your industry: SaaS, indie games, mobile apps, developer tools, ecommerce or AI tools.
- Search Google for site:reddit.com plus a complaint in your buyer's words, and note which subreddits the results come from.
- In a thread that fits, look at the other communities the people asking post in.
3. Read a month of posts in each
Sort by New and scroll back a month. Count the posts where someone asks for help with your problem. A community of 30,000 with two such questions a week beats one of 3 million where they never come up.
4. Check the rules before you join in
Karma minimums, account age, flair and promotion days differ from one subreddit to the next. Read them in the subreddit rules checker, and if your account is new, see when it clears the bar with the karma calculator.
5. Watch three to five for two weeks
Visit each shortlisted subreddit every few days and answer what you can. After two weeks, drop the ones that produced no question worth answering. When you are ready to look for the threads themselves, use how to find Reddit threads asking for your product.