Useful Number Facts For Quick Check Calculations

When I'm doing my quick check calculations, I sometimes need some facts which are not given with the numbers. I've collected some of the useful, but not necessarily obvious, number facts which I use for my calculations. There are lots more. If you know of others, write to the Mathemagician to let me know.

There are about 15 thousand possible three-letter combinations.
The volume of a sphere is half the volume of the same size cube
There are a million seconds in two weeks (1,209,600)
There are 100,000 seconds in a day (actually 86,400)
There are 1,000 hours in a month (720 or 744)
There are 10,000 hours in a year (actually 8,760)
There are 250 working days in a year.
There are about 2000 working hours in a year.
There are about 6 billion people in the world (or 5 billion, for the purpose of easier calculation)
People live to be about 100 years old (for easy calculation)
The Earth is about 24 or 25 thousand miles around
The radius of the Earth is about 6000 km
The surface area of the Earth is about 400 trillion square metres
The mass of the Earth is about 6 x 10^24 kg
The mass of the sun is about one million earths
It costs about $1000 a month to rent a billboard.

There is divinity in odd numbers, either in nativity, chance, or death.
- William Shakespeare, "The Merry Wives of Windsor"

Some guidelines for 'quick check' calculations.

Numbers in the real world.

Grannus' circle.

The front gate of Esmerel.

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