Back-of-the-envelope calculation

As a rough estimate is defined as the arithmetic with strongly rounded up or down for reviewing complex calculations. It is used for complex calculations, where a quick check is necessary. In the rollover account is a very imprecise method, which only serves to check results on their credibility. Therefore, one should work very carefully with this method.

Examples of applications

Currency conversion

Many people use to compare rough calculations for currency conversions, for example, a price in Euros with an earlier currency.

In the rollover calculations with rounded factors ( in Germany about 1:2 instead 1:1,95583 in Austria 1:14 instead 1:13,7603 ) rounding errors occur that affect the converted price (where usually the more essential since the introduction of the euro took place devaluation causes a major methodological error).

Slide rule

When they still counted on slide rules, you had to calculate the position of the comma in a rough calculation, since the slide rule does not indicate the magnitude of a number.

Warm-up time of a water boiler

The specific heat capacity of water is 1.16 Wh / kgK. For a rough estimate or impact- even calculation of the value of 1 Wh / kgK enough. To a kettle with 100 liters of water ( about 100 kg, depending on the temperature) to 10 K heat you need over excessively impact expected 1 kWh. A 30 kW gas burner required for " about 2 minutes," a 10 kW burner " about 6 minutes." This yields results that are sufficient for an evaluation. The more or less exact value up to three decimal places, for example, is just not necessary.