Multiple online platforms offer a wide variety of Old School RuneScape Skill Calculators. They have what we call the OSRS Calculators for Combat, the Herblore Training, the Construction xp, the max hit and a Crafting calculator amongst many more.
The idea is to create a site by using the Lunagang calculator, especially the Prayer calculator one in particular. Now, you can go ahead and try out all the calculators that are available for combat, herb or any of the other 40 tools provided.
In order to calculate your OSRS combat level, you need to simply add your Strength and Attack levels together and then multiply them by the figure 0.325. You can now add this to your base combat level and you have your melee combat level.
So, by multiplying this value by 0.325 and adding the result to your base combat level calculated before, you get your magic combat level.