Landmark Numbers for Fermi Questions
A reference set of memorisable quantities — populations, sizes, rates, and densities — that anchor back-of-envelope estimation.
What Landmark Numbers Are
A Fermi estimate is only as good as its anchors. Landmark numbers are the handful of quantities worth memorising — not for trivia, but because they unlock a wide class of estimates by giving you a known reference to scale from.
The goal is a small, orthogonal set: numbers that don’t overlap, cover different domains, and can be combined to reach almost any target.
People
| Quantity | Value |
|---|---|
| World population | 8 billion |
| India population | 1.4 billion |
| USA population | 330 million |
| A large city (Mumbai, Lagos, Jakarta) | ~20 million |
| A mid-size city | ~1 million |
| A small town | ~10,000–50,000 |
| Average household size (India) | ~4.5 people |
| Average lifespan | ~75 years |
| Working life | ~40 years (25–65) |
Time
| Quantity | Value |
|---|---|
| Seconds in a day | ~86,000 ≈ 10⁵ |
| Seconds in a year | ~3.15 × 10⁷ ≈ π × 10⁷ |
| Hours in a year | ~8,760 ≈ 9,000 |
| Age of the universe | ~14 billion years |
| Age of Earth | ~4.5 billion years |
| Human history (writing) | ~5,000 years |
The “π × 10⁷ seconds per year” trick is worth memorising — it’s accurate to 0.5% and comes up constantly.
Size and Distance
| Quantity | Value |
|---|---|
| Earth radius | 6,400 km |
| Earth circumference | 40,000 km |
| Earth–Moon distance | 384,000 km ≈ 30 × Earth diameters |
| Earth–Sun distance (1 AU) | 150 million km |
| Height of a human | ~1.7 m |
| Height of a 10-story building | ~30 m |
| Length of a city block | ~100 m |
| India north–south span | ~3,000 km |
Mass and Density
| Quantity | Value |
|---|---|
| Mass of a human | ~70 kg |
| Mass of a car | ~1,500 kg |
| Density of water | 1,000 kg/m³ |
| Density of air (sea level) | ~1.2 kg/m³ |
| Density of steel | ~8,000 kg/m³ |
| Density of concrete | ~2,400 kg/m³ |
Energy and Power
| Quantity | Value |
|---|---|
| Food calorie (kcal) in joules | ~4,200 J |
| Daily human food intake | ~2,000 kcal = ~8 MJ |
| Human resting metabolic rate | ~80 W |
| A light bulb | ~10 W (LED) |
| A desktop PC | ~200–400 W |
| A car engine | ~100 kW |
| World energy consumption | ~6 × 10²⁰ J/year ≈ 2 × 10¹³ W |
Economics
| Quantity | Value |
|---|---|
| World GDP | ~$100 trillion |
| India GDP | ~$3.5 trillion |
| USA GDP | ~$25 trillion |
| India GDP per capita | ~$2,500 |
| World GDP per capita | ~$12,000 |
| Median Indian monthly salary | ~₹25,000 |
Information
| Quantity | Value |
|---|---|
| Average webpage size | ~2 MB |
| A 2-hour HD movie | ~4–8 GB |
| Human genome | ~3 GB (as raw base pairs) |
| Internet traffic per day | ~500 exabytes |
| Words in a novel | ~80,000–100,000 |
| Words a person speaks per day | ~16,000 |
How to Use Them
Fermi questions are rarely about the landmark numbers directly — they’re about building a chain from what you know to what you don’t. A good landmark set lets you enter that chain at multiple points.
Example: How many barbers are there in India?
- Population: 1.4 billion
- Haircut frequency: once a month → 1.4 billion haircuts/month
- Haircuts per barber per day: ~15 → per month: ~400
- Barbers: 1.4 × 10⁹ / 400 ≈ 3.5 million
The estimate lives or dies on two numbers: population (landmark) and haircuts-per-barber (domain knowledge + common sense). The arithmetic is just scaffolding.
The numbers worth memorising are the ones that appear as anchors in the widest variety of chains — population, lifespan, seconds in a year, energy in food, size of Earth.