5.8 grams is 1/5 ounce.
You should convert metric measurements to standard US measurements when you need to communicate with someone who is more familiar with the US system, or if the specific context or requirements you are dealing with call for the use of standard US measurements.
6 liters = 1.58503231 US gallons
to get the right measurements of ingredients for making more or less of what you are cooking
You can convert measurements using online services such as onlineconversion.com. Also, many scientific calculators have programs which convert measurements.
7cent how many squre feet
It depends on what the measurements are.
To convert measurements from millimeters to decameters (da units), divide the measurement in millimeters by 1000. This will give you the equivalent measurement in decameters.
You simply can't convert between totally unrelated types of measurements.
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You need to know the density of the substance.
A gram is a unit of mass. A millilitre is a unit of capacity. Without some unit of density to compare, the two units are therefore incompatible.
We need standard measurements because it helps us to find the measurements of things.