When you freelance, you're likely not receiving equity in your client's company, getting paid time-off, or health insurance. So it's on you to account for all of these things in your rate.
1. Divide by 52
The resulting number is your weekly rate.
2. Research full-time market rates for your area
Services like LinkedIn, Glassdoor, and others allow you to find such numbers. Take into account your experience level. If you're still junior, don't use a \Senior Design Director\ salary in the following steps.
3. Charge weekly
It sets up better expectations with you and your client. You stop forcing out those 2 hours of \work\ at the end of the day. Your client doesn't feel the need to micro-manage you (imagine how they feel when you go to the bathroom. Literally $ lost). Instead, set weekly expectations that you know you can meet. Manage your time wisely and always deliver.
4. Take the base salary, then 1.2 - 1.5x it
The salary you found in the previous step is for full-time employees, but you're not. Multiplying by 1.2 - 1.5 accounts for the things I mentioned above (no equity, no paid time-off, no health insurance).