How do you perform order-of-magnitude calculations?

Question

How do you perform order-of-magnitude calculations?

Answer 1

Order-of-magnitude calculations means that you don't care about the exact value, you're just trying to get a quick approximation.  One order-of-magnitude is a power of 10, so 10, 100, 1000, 10000, etc.

For example, how many calories are eaten by all the people in my city each day?  It would be very difficult to measure and calculate this exactly, but getting an order-of-magnitude answer is easy.

Around 1,000,000 people, each eating around 1,000 calories = 1,000,000,000 total calories.  The real answer may be 2 billion, 3 billion, or half a billion, but that's not what I'm trying to get with an order-of-magnitude calculation.