Why do we pack maize in 90kg bags instead of 100kg?


The Sunday Nation


19 November 2023


Why is it that, in school, Kenyans are only taught to use metric measuring units (metres, kilograms etc.) yet, when trading, they prefer to use imperial units? This switching of units is usually in length measurements (feet and inches) but not in other quantities – mass, volume etc.

We trade land in acres instead of the metric hectare. Our artisans take measurements in feet and inches instead of metres and centimetres – even though they buy working material in metres.

I still remember how our geography teach would keep converting rainfall measurements from millimetres to inches, yet all other subjects were being taught in metres, centimetres and millimetres. I would later learn that he was actually making a conversion from an extremely convenient system to a cumbersome (and meaningless).

It turns out that the reason meteorologists use millimetres of rainfall instead of centimetres is that 1mm of rain is equivalent to exactly one litre of water collected over one square metre of ground surface. Thus, instead of reporting, say 50mm rainfall, one can equally accurately say that the rain fell at the rate of 50L of water per square metre.

This equivalence is easy to demonstrate: one metre has 1,000mm; so, an area of one square metre is equal to 1,000mm x 1,000mm = 1,000,000 square mm. If 1mm of water is poured on such an area, the volume will be 1 x 1,000,000 = 1,000,000 cubic mm.

How much is that in litres? Well one litre is a volume measuring 10cm x 10cm x 10cm = 1,000 cubic cm (also shortened to ‘cc’). But 1cm = 10mm, so one litre is also 100mm x 100mm x 100mm = 1,000,000 cubic mm. QED!

I think it will take a long time for us to go metric fully, but along the journey, we shall have to contend with some awkward packages of goods; like the 90kg bag of maize. Why isn’t it 100kg (a nice round number)? I suspect that it started off as 200 pounds under the imperial system. This was then converted to metric and it came to 90.72kg, which was rounded to 90kg.

Meanwhile, beware when making unit conversions: in 1983, an Air Canada Boeing 767 jumbo jet ran out of fuel midflight and crash landed – all due to wrong unit conversion. Two decades later in 1999, a NASA spacecraft crushed on Mars instead orbiting the planet because it was sent instructions using the wrong units.

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