# Arithmetic/Geometry » Rules of thumb

In mathematics you have to use rules of thumb when estimating.

Sometimes these are given in the exercise.

It is good practice to write down assumptions before you use them in a calculations to show why you are using this nummer. For example:

"I assume the walking speed is 6 km/h."

Slight drawback:

Numbers and Space has other rules of thumb than Modern Mathematics.

Rules of thumb that are the same.

- Height of a story/floor: 3 metres

- Cycling speed: 18 km/h

- Measurements of a door: 1 by 2 metres.

Modern Mathematics:

- Distance pink-thumb: 20 cm

- Length of an adult: 1.80 m

- Big step: 1 m

- Walking at a pace: 6 km/h

Numbers and Space:

- Length of a step: 70 cm

- Length of an adult: 1.75 m

- Length of a car: 4 m

- Walking: 5 km/h