We can look everything and anything up on our devices with relative ease, but sometimes just knowing a rough number is useful for a quick calculation in your head. Here's a list of numbers that have stuck around in my head, and some more that I'd like to get stuck in my head.

Work

• 1 year ≈ 2000 working hours
• salary_annual ÷ 2000 ≈ salary_hourly

Speed

• 1 m/s ≈ 2 mph
• Speed of light ≈ 3 × 10⁸ m/s

Distance

• c_earth ≈ 40,000 km
• Sun → Earth ≈ 8 light-minutes
• Earth → Moon ≈ 400,000 km (≈ 1 light-second)
• r_moon ≈ 1,000 miles
• r_mars ≈ 2 × r_moon
• r_earth ≈ 2 × r_mars
• r_sun ≈ 100 × r_earth

Time

• 1 year ≈ π × 10⁷ seconds

Temperature

• 0ºF ≈ ⁻20ºC
• 100ºF ≈ 40ºC

Misc

• 2¹⁰ ≈ 10³
• 2²⁰ ≈ 10⁶
• 2³⁰ ≈ 10⁹

Symbols

• r_x = Radius of x
• c_x = Circumference of x