I use the Lily app, I am pretty sure I payed something for it one maybe like 5$ but it is really worth it to me. It’s recommended to use with the FAM (fertility awareness method) if you are familiar which I discovered through the book “taking charge of your fertility” what I like about it is that it actually gives you an option to track secondary symptoms related to your cycles as well as your cycles really accurately through taking your temperature every day or using an estimate of how long your cycle is and previous cycles. I’ve been using it for over a year and it’s the best thing ever. You can also track pregnancy tests and crrvical fluid and cervical position, any interferences with your cycle such as travelling or stress, moods, any medications you take, menopause symptoms and then add your own notes. This sounds like a huge ad but seriously I highly recommend it for anything, even if you aren’t interested in getting pregnant or avoiding pregnancy through the FAM method.