List of examples



The rising sea

If all the ice on Earth melts, by how much would the oceans rise ?

How big is lake Geneva?

Make a quick estimate from two known distances

World How big is the world population?
Coincidences Six degrees of separation?


How do you measure the height of a tree?


2003:  France Telecom has a deficit of 68 G€.  How much is that per person in France?  How much is it per person on the planet ?

Satellite TV

Live interviews via satellite have a noticeable delay between question and answer.  Why?


We are now encouraged to drive with our headlights on, even in daytime, to increase safety.  How much extra power is needed?

In the shops

How much paint to repaint the room?


How much cat food to leave for the holidays?


Our cat eats canned food.  In its lifetime, how much metal will we have thrown away?


How many balloons do we need to lift a child?


Silverware gets black.  The black layer is silver oxide:  silver atoms combined with the atoms of the oxygen in the air.  When you clean it, you remove the black layer, hence you remove some silver too.  Will your silverware disappear if you clean it too often?

Oil consumption

OPEC steps up production.  Is now (January 2003) at 24.5Mbarrels/day (1barrel=159l).  Is that a lot?  If we put the yearly consumption into a single tank, how big is the tank?


The leaning tower of Pisa is open to the public again.  But for safety reasons, the number of people is limited.  How long would it take to let the entire world population visit the tower?

Robbing a bank

On 2003-03-18, Saddam Hussein took 1 billion out of the Iraq state bank, in 100$ and 100€ notes. Could he put it in his suitcase?

Spending power

If you had a billion to spend, how much would you have to spend per day if you lived for 70 years?


Does it take less energy to fly than to drive?

Publicity cost

A corporate restaurant has real samples of the day's dishes on display at the entrance so customers can choose before entering.  Does that add to the cost of your meal?

Exchange rates

Confused?  So am I.


2003-01-07 EDF reaches a record 78GW of electric power. What if every person on the planet had the same electric comfort as people in France?

Personal power

What is the power output of an average person?

Oil addiction again

2004-06-01 the world consumes 78 million barrels of oil per day. 1 barrel = 159l

next planned revision: 2004-06-01