The metric system and English system, also called the imperial system of measurements, are both common systems of measurement used today.

The main difference between imperial and metric units are that metric units are easier to convert between because those conversions require only multiplying or dividing by powers of 10. There are 10 millimeters in a centimeter, 100 centimeters in a meter, and 1,000 meters in a kilometer. To convert between these units, you only need to move the decimal place. For example:

The same is true for metric mass units – there are 1,000 grams in a kilogram.

Converting imperial units is much less straightforward. Take imperial length units, for example. There are 12 inches in a foot, 3 feet in a yard and 1,760 yards in a mile. Converting 520 feet to miles would go something like this:

Another difference between imperial and metric units is where they are commonly used. In the United States, imperial units are used for most everyday purposes, whereas almost everywhere else in the world, metric system units are more common.

## Conversion Between Metric System and English System Units

The following is a list of some of the relationships between imperial and metric system units:

- 1 inch = 2.54 cm
- 1 ft = 30.48 cm
- 1 mile = 1.609 km
- 1 pound = 0.454 kg
- 1 gallon = 3.785 L

## The International System of Units

The difference between imperial and metric units becomes particularly relevant when talking about base units. The International System of Units (SI), the official system of measurement used throughout the world, especially in scientific applications, is based upon the metric system units. All SI units can be formed by a combination of seven base units.

## What Are the Seven Basic Units of Measurement?

You are likely familiar with using a ruler to measure length, a stopwatch to measure time or a scale to measure mass, but have you ever wondered how accurate these devices are, and how you can be sure that all rulers and stopwatches and scales are measuring equally well? And how were the associated units defined in the first place?

If you think about a wooden ruler, for example, it is subject to minor variations in length due to expansion and contraction resulting from humidity and temperature. In fact, all materials vary slightly in size due to environmental conditions and are subject to scratches, impurities and changes over time. Ultimately, in order to enable extremely accurate scientific measurements, we need precise ways to define units of measurement.

All SI units can be derived from seven base units of measurement, each of which are defined in terms of fundamental scientific constants as described in the following sections. Note that no such equivalent set of fundamental definitions exists for any imperial units. Rather, imperial units are derived as unit conversions from SI units.

## Time

Originally, time was measured in the passing of days. Eventually these days were broken into 24 hours, the hours broken into 60 minutes and each minute into 60 seconds.

Mechanical clocks built in medieval Europe were some of the first devices that made for consistent and uniform time measurements. But now we are capable of considerably more accuracy. The SI unit of time is the second, and 1 second is defined as the time it takes for a cesium-133 atom to oscillate 9,192,631,770 times.

## Length

Length is a measure of linear distance. The SI unit for length is the meter, but the formal definition of 1 meter has changed over the years. Originally, 1 meter was defined as the unit of length equivalent to 10^{-7} of the Earth's quadrant passing through Paris.

Later, a platinum iridium prototype rod was made, and copies distributed that were regularly compared to it. But now the meter is defined in terms of the constant speed of light in a vacuum, c = 299,792,458 m/s.

## Mass

Mass is a measure of an object’s inertia, or resistance to changes in motion. The SI unit of mass is the kg. 1 kg has also been officially defined differently over the years. Originally 1 kg was equal to 1 cubic decimeter of water at the temperature of maximum density.

Later, just as with the meter, 1 kg was defined as the mass of the International Prototype Kilogram, a cylinder made of platinum iridium alloy. Now it is defined in terms of the fundamental Planck’s constant, h = 6.62607015 × 10^{-34} kgm^{2}/s.

## Amount of Substance

This concept is just what it sounds like. It is how much of something you have – the number of apples on a tree or the number of atoms in an apple. While you might expect that the SI unit would be simply the numerical count of something, it is actually another unit called the mole.

1 mole of a substance contains exactly 6.02214076 × 10^{23} elementary items. This number, also known as Avogadro's number, is exactly equal to the number of atoms in 12 grams of carbon-12, and it is often very close to the number of nucleons (protons plus neutrons) in one gram of any type of ordinary matter.

## Current

It might seem counterintuitive that current, a measure of the rate of charge passing through a point, is considered a fundamental unit instead of charge itself. But the reason for this is that current had previously been easier to measure than charge, and the accuracy of all units relies on our ability to accurately measure the base units.

The SI unit for current is the ampere. Originally, one ampere was defined as the constant current required for two parallel conductors of infinite length and negligible cross section placed 1 meter apart in a vacuum to exert a force of 2 × 10^{-7} N on each other per unit length. Now it is defined in terms of the elementary charge e = 1.602176634 × 10^{–19} C.

## Temperature

Temperature is a measure of the average energy per molecule in a substance. Units of Fahrenheit and Celsius have been used for hundreds of years to measure temperature. On the Fahrenheit scale, water freezes at 32 degrees and boils at 212 degrees, and this defines the degree increments. On the Celsius scale, water freezes at 0 degrees and boils at 100 degrees.

The fatal flaw in these units, however, is that they don’t start at 0. The fact that it is possible to have negative temperature values on these scales quickly makes things confusing when you consider what it might mean for something to be twice as hot as something else. What is twice as hot as 0 degrees?

The SI unit for temperature is the Kelvin, where 0 Kelvin is defined as being absolute 0, or the coldest possible temperature something can be. The size of an increment in the Kelvin scale is the same as an increment in the Celsius scale, and 0 Kelvin = -273.15 degrees Celsius. The Kelvin is formally defined in terms of the fundamental Boltzmann constant k = 1.380649 × 10^{– 23} J/K.

## Light

The fundamental unit for luminous intensity is the candela (cd). A common candle emits about 1 cd. The official, precise definition is defined in terms of the luminous efficacy of radiation of frequency 540 × 10^{12} Hz.