Width of your thumbnail ≈ 1 cm. Hair grows about that per month.
8.2 billion
People alive today
Write it out: 8,200,000,000. That's 8.2 × 10⁹.
108 billion
Humans ever born, all of history
Today's living = ~7.6% of this total
385,000/day
Babies born per day
≈ 4.5 per second
170,000/day
People who die per day
≈ 2 per second
Part E · test yourself
1. You read your hair 3 years without cutting. Roughly how long would it be?
About 45 cm — roughly shoulder length. 1.25 cm/month × 36 months = 45 cm. In practice slightly less, because individual hairs shed and restart.
2. How many people were alive when the Roman Empire was at its peak (around 100 AD)?
About 300–400 million total worldwide. That's less than the current population of the USA alone (330M). Today's world has 25x more people than existed then.
3. In one hour, how many babies are born worldwide?
About 16,000. (385,000 per day ÷ 24 hours ≈ 16,000 per hour.) In the time it takes to watch a movie, roughly 32,000 babies are born.
4. What percentage of all humans who ever lived are alive right now?
About 7.6%. With 8.2 billion alive and ~108 billion ever born: 8.2 ÷ 108 ≈ 7.6%. More humans are alive today than in any previous era — yet we're still less than 1 in 12 of everyone who ever existed.
5. The world gains 80 million people per year. Which country has a population close to that number?
Germany (~84M) or Turkey (~85M) are the closest. So the world adds roughly one Germany's worth of people every single year. Within your lifetime (assuming ~70 more years), that's potentially 5–6 billion more people added — approaching another planet's worth.