Generally speaking you cannot convert between imperial and metric unless you are a very able mathematician.
Imperial.
A tonne is a metric measurement.
I'm guessing by customary, you mean imperial. if so inches are customary/imperial
A metric tonne is lighter than the imperial equivalent by roughly 16 kg. The imperial ton weighs 2,240 lbs (or 1,016 kg), while the metric tonne is 1,000 kg.
Both imperial and metric is currently used in the UK. The metric system is exclusively taught in UK schools. So, eventually, as children become adults, the metric system will become the norm. Resistance to using the metric system is usually from adults who were taught the imperial system when they were schoolchildren.
Here is a complete list of imperial/metric conversions (i.e. feet to meters): http://www.thetipsbank.com/convert.htm
NoYes they do. In science classes it is actually more likely to see metric conversions than customary conversions. This is because most of the world uses metric.
0.138 what? Without knowing the units of the 0.138 (I assume in some imperial measurement) it can't be converted to metric.
It may be simple but imperialtometric.com will allow you to make conversions between imperial and metric units. You may want to invest in a proper conversion table to do this though if this is business related.
A gallon is certainly not metric. It is imperial.
There are many references to convert meter to feet. One can find these references and calculators at Metric-Conversions, Wikihow, and The Calculator Site.
There are free, online calculators that can easily do this for the user. Some of these are the websites named "convertunits" , "metric-conversions" and "onlineconversion". There are even more, but just one of the three mentioned is more than sufficient for this job. All mentioned sites can also perform other metric to imperial conversions, and vice versa.
Imperial.
Mostly just tradition. People are used to the old system, so they don't want to change to unfamiliar metric, even though the conversions are easier.
Acre is an imperial measurment. Hectacre would be an analogous metric measurment.
Metric tons can't be converted to imperial gallons. Metric tons measure mass, while imperial gallons measure volume.
There is no such thing as a metric gallon. The metric measurement is litres. There are 4.54609188 litres to an imperial gallon.