Numbers worth knowing
Good estimators do not know more facts. They know a few dozen anchors and how to multiply them.
People
- World population: about 8 billion.
- Largest cities: 20 to 40 million people in the metropolitan area.
- Life expectancy worldwide: about 73 years.
- Births worldwide: about 130 million a year, roughly 4 every second.
Time
- Seconds in a day: 86,400, call it 100,000.
- Seconds in a year: about 32 million.
- A human lifetime: about 2.5 billion seconds.
- Age of the universe: 13.8 billion years. Age of the Earth: 4.5 billion years.
Distances and sizes
- Circumference of the Earth: 40,000 km.
- Earth to Moon: 384,000 km. Earth to Sun: 150 million km.
- Cruising altitude of an airliner: about 10 km.
- Speed of light: 300,000 km per second.
The human body
- Cells in the body: about 30 trillion.
- Heartbeats: about 100,000 a day.
- Breaths: about 20,000 a day.
- Blood: about 5 liters in an adult.
Mass and energy
- A liter of water: 1 kg. A cubic meter of water: 1 tonne.
- A car: 1 to 2 tonnes. A loaded airliner: 70 to 500 tonnes.
- Daily food energy for an adult: about 2,000 kilocalories, close to the energy of a 100 watt bulb left on all day.
Money
- World GDP: about 100 trillion US dollars a year.
- World GDP per person: about 13,000 US dollars a year.
- The largest companies are valued in trillions of US dollars.
How to use anchors
Almost any Fermi question reduces to an anchor multiplied by a guessed rate. How many haircuts happen in the world each year? Eight billion people, perhaps five haircuts each, so about 40 billion. The anchor carries most of the precision, and the guess only needs to be within a factor of a few.
FAQ
Do I need to memorize exact values?
No. One significant figure is enough. Knowing that a year is about 30 million seconds is as useful for estimation as knowing it is 31,557,600.
Which anchor is the most useful?
World population. A surprising share of estimation questions are a per person rate multiplied by 8 billion.