On average, most strains have about 10 percent THC. Strains with 15 – 20 percent are above average and the ones with over 30 percent are often medicinal. Be sure to look up your strain online or ask your provider about how much THC the strain contains.
How Much Should You Start With
If your strain has 10 percent THC, then a gram of cannabis contains 100mg of THC. This simple math problem will allow you to know, or to give an educated guess, on how much THC you have per serving. This will grant you some power over your high and prepare you for your edible.
(excerpt taken from: https://thefreshtoast.com/cannabis/calculating-how-much-thc-in-your-homemade-edibles/)