How does apple health app calculate calories burned from steps?

fittyinthemaking

I dont have an apple watch but i use my iphone to track my steps as its the most accurate thing ive come across so far, i also run it through mfp so it shows me approximate cals burned from steps. does anyone know how these cals are worked out? it must be to do with how many steps you do in a certain amount of time/distance? divided by some clever equation?