I'm still going to answer the question, but technically, pounds can't be converted to milligrams. A pound is a unit of weight, which is the force of gravity pulling things down. A milligram is a unit of mass, which is what the force acts on. But because gravity pulls the same everywhere on Earth, we generally say that they can be interchanged.
The conversion rate between pounds and kilograms is 2.2lb = 1kg. Again, this technically means that on Earth, one kilogram has 2.2 pounds of force acting on it. Using a bit of algebra, we can see that 1lb = 0.45kg, or 1 = (0.45kg / lb). Now, seeing as you asked for milligrams, we need to know how many milligrams in a kilogram. This conversion rate is 10 6mg = 1kg. We can stick this into our other conversion rate and say that 1 = 4.5x10 6mg / lb. This means that we can multiply the number of pounds we have by 4.5x10 6mg / lb, and it will give us our milligrams.
That's all you need to know, but if you want a fun fact, one kilogram actually only weighs 0.37 pounds on the moon. On Jupiter, that same kilogram would weigh about 5.6 pounds. Gravity is different if you leave Earth, and so if you one day go to the moon (it could happen), you'll have to re-calculate your conversion rate.
