# Rule of thumb

A rule of thumb (also a rule of thumb ) is a method for the rapid determination of a mathematical or technical value without precise technical calculations to be performed. The origin of the name is not clear. The synonym rule of thumb is a loan translation from English rule of thumb .

The calculations that a rule of thumb contains are always so simple that they can also be determined quickly using mental arithmetic . Most rules of thumb are based on experience.

A distinction can be made between rules of thumb that characterize empirical values ​​without an exact calculation method at all, and rules of thumb that allow estimates to be made if the exact calculation would take too long or would not be feasible in the head (saying "pi times the thumb"). Scientifically, rules of thumb in cognitive science , artificial intelligence and computer science are examined under the term heuristics .

## Examples

### Minutes until sunset

Take the palm of your hand with your inside to your face and stretch your arm out. The little finger is placed on the horizon. The number of fingers that fit between the sun and the horizon with an outstretched arm are counted. The result is multiplied by 15 minutes. Only applies in the afternoon / evening and in mid-geographical latitudes.

### Distance estimation using the jump of the thumb

The thumb jump is a rough method of estimating the distance between the person measuring and a stationary object whose width is known.

### Removal of a lightning strike

The rule of thumb (a third of the distance between lightning and thunder in seconds) for the distance of a lightning strike in kilometers is:

${\ displaystyle {\ rm {Distance \ approx {\ frac {number \ of \ seconds} {3}} \; km}}}$

The result deviates from the actual distance depending on the ambient temperature, as the speed of sound is not exactly 1/3 of a kilometer per second.

### Automotive physics

A rule of thumb for the braking distance , which is explained in the article there, is:

${\ displaystyle s \ approx ({\ rm {Speed ​​\ in \ km / h: 10) ^ {2}}}}$

The rule of thumb makes use of the fact that the speedometer shows the speed in steps of 10, making the calculation particularly easy.

### chemistry

In the chemical is often (sometimes more ironically) stated that all (inorganic) of white powder as the density 2.3 g / cm³ have and all flammable liquids having a density of 0.8 g / cm³.

Van't Hoff's rule , which states that the reaction rate of a chemical conversion roughly doubles for every 10 K increase in temperature, is also meant as a rule of thumb .

### Economy

In business considerations that can Pareto rule or 20/80 rule be used, stating that in cause-and-effect relationships often been the pronounced to about 20% cause strength deployed 80% of the possible effect. Examples: 80% of sales are generated with 20% of customers, 20% of products are responsible for 80% of complaints, 20% of the causes of errors result in 80% of errors, 20% of the means of production cause 80% of the costs. Based on these considerations, optimization processes are usually limited to the main causes.

#### Compound interest

The 72 rule states that the value of an investment will double after 72 years, divided by the interest rate in percent.

Instead of the interest rate, z. B. the assumed inflation rate for the term can be used to predict the loss of value. According to the 72 rule, an amount of money without interest would lose half of its purchasing power in 7.2 years with 10% inflation.

### Decision theory

With the 37% rule , a point in time should be found for the decision to find the optimal element.