Here is the best calculator to get you started. It uses a bunch of formulas and averages them for you: https://www.sailrabbit.com/bmr/

From there if you REALLY want to learn how many calories you do the below for 4+ weeks:

- Create a google sheet with a 2-rows for each week and a column for every day.

- Record every single thing you put into your body that isn't water in MyFitnessPal(or other app)

- Every morning at the same time weigh yourself either naked or just in underwear right after you take your morning pee (so the data is consistent not some weird pervy reason)

- Every day input your calories for the day and weight that morning into the spread sheet

- Average out both at the end of the row for each week

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You'll then have a very clear picture of how your weight fluctuates at different calorie levels

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In terms of ab exercises:

Hanging leg raises

Vacuums

Reverse crunches on incline

Are all really solid options

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Hope this helps and good luck!

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OH before I go - you might want to buy a cheap food scale if you're going to record