Numbers worth knowing by heart
Good estimators are not better at arithmetic than you. They are carrying a small set of anchor numbers, and almost every rough calculation eventually lands on one of them.
There is a persistent myth that people who are good at estimating have unusually fast mental arithmetic. They mostly don’t. What they have is a stocked pantry: a few dozen quantities they know cold, which serve as the starting point for everything else.
The list below is not trivia. Each entry earns its place by being the thing you reach for when a question starts with “how many” or “how much.” Approximate values are given deliberately — you want the round number that is easy to multiply, not the precise one.
People
| Quantity | Round it to | Why it matters |
|---|---|---|
| World population | 8 billion | The denominator for anything global |
| US population | 350 million | Per-person costs, market sizes |
| People per household (US) | 2.5 | Converts people to homes |
| Births worldwide per year | 130 million | Anything age-cohort related |
| Human life expectancy (global) | 73 years | Lifetime totals of anything |
The household figure is quietly one of the most useful numbers on this page. An enormous share of consumer questions — how many televisions, refrigerators, driveways, lawnmowers — run through households rather than individuals.
Time
| Quantity | Round it to | Why it matters |
|---|---|---|
| Seconds in a day | 86,400 | Rate conversions; call it 100,000 |
| Seconds in a year | 32 million | The classic π × 10⁷ approximation |
| Hours in a year | 8,760 | Energy, wages, uptime |
| Working hours in a year | 2,000 | Salary ÷ 2,000 ≈ hourly rate |
| Waking hours in a day | 16 | Anything about attention or habits |
That last trick is worth internalising on its own: a $100,000 salary is about $50 an hour, and a $50,000 salary is about $25. The conversion runs both ways and turns abstract compensation into something you can reason about.
Earth and space
| Quantity | Round it to | Why it matters |
|---|---|---|
| Earth’s circumference | 40,000 km | Travel, satellites, cables |
| Earth’s land area | 150 million km² | Agriculture, forests, cities |
| Distance to the Moon | 380,000 km | The standard “how far is far” yardstick |
| Distance to the Sun | 150 million km | Light takes 8 minutes |
| Speed of light | 300,000 km/s | Anything involving signals |
| Speed of sound | 340 m/s | Thunder: 3 seconds per km |
Physical stuff
| Quantity | Round it to | Why it matters |
|---|---|---|
| Mass of 1 litre of water | 1 kg | The gateway between volume and mass |
| Human body mass | 70 kg | Scaling anything biological |
| Daily food energy | 2,000 kcal | Agriculture, metabolism, diets |
| A car | 1.5 tonnes | Traffic, materials, emissions |
| A commercial airliner | 150 passengers | Air travel questions |
| Sheet of paper thickness | 0.1 mm | Stacks, reams, folding puzzles |
The water fact is the single most productive line in this table. One litre is one kilogram; one cubic metre is a thousand litres, and therefore one tonne. That chain converts almost any volume question into a mass question and back, which is why physicists reach for it constantly.
Powers of ten, felt rather than counted
Memorising the scales matters less than having a physical sense of them:
- 10³ — a thousand. A large lecture hall. Countable in an afternoon.
- 10⁶ — a million. A mid-sized city. A million seconds is eleven and a half days.
- 10⁹ — a billion. A seventh of humanity. A billion seconds is nearly 32 years.
- 10¹² — a trillion. Beyond intuition entirely; only meaningful as a ratio to something else.
How to actually use them
Don’t try to memorise this page. Learn the five or six that keep appearing in questions you care about, and let the rest arrive through use.
The deeper habit is building your own anchors from personal experience. You know roughly how much you spend on groceries, how long your commute takes, how many people were at the last event you attended. Those are calibrated data points nobody had to publish, and in a surprising number of estimates they are more reliable than a half-remembered statistic.
Learning these is the fastest way to improve at Zoggle, where most puzzles are one or two steps away from a number on this page.