Estimating Orders of Magnitude

Many calculations in physics involve many powers of ten. It can be hard to gauge whether a calculation is correct or accurate: is the distance to a certain starorThere may not seem a lot of difference between the powers in this example, but it corresponds to a factor of 10. It is useful to make a rough estimate before using a calculator. This often be done by adding or subtracting powers when numbers are in standard form.


For more practical purposes it helps to know the masses of many objects in terms of SI units. Some are given below.

1 kg – A packet of sugar or a litre of water. A person might weigh from 50 kg upwards.

1 m – The distance between a persons outstretched hands.

1 s - Duration of a heartbeat.

1 amp – The current consumption of a typical computer.

25 degrees celsius – Roughly room temperature. Actually the SI unit of temperature is Kelvin (K), with 1 degree celsius = 1 degree kelvin.

1 mol – the mass of one mol of a substance is typically from a few grams to a few tens of grams. 1 mol of Carbon 12 is about the number of carbon atoms in a pencil lead.

1 m/s – Walking speed. A car might move at 30 m/s on the motorway.

1 N – About the weight of an apple.

1.5 V – A typical small battery voltage.

10 J – The energy used to lift a bag of sugar to waist height. The typical person uses about 10000 J per day.

500,000 Pa – A typical pressure exerted on the ground by a person standing up.

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