Around the world, there are many different ways to measure distance and weight, and depending on your source of information the units given may differ even if the thing being measured is the same. You could be looking for a recipe and find that one measures ingredients by the cup, and then check another where the measurements are given in milliliters.

The reason for this comes down to there being two different systems of measurement: Imperial and Metric. And whilst the Metric System is the preferred system of measurement in most countries, there are still some that use the Imperial System.

In this article, we will cover the differences between the Metric and Imperial Systems, what units of measurement each one uses, and how to convert measurements from one system to the other. So, let's jump in!

You have probably come across both the Metric and Imperial systems in everyday life and at some point had to covert a measurement from one to the other. Despite most countries using the Metric System as the official system of measurement, you will likely still come across Imperial measurements and therefore it is useful to be able to identify the appropriate unit of measurement and know how to convert measurements from one system to another.

What is a system of measurement?

A system of measurement is a set of units that work together and are used to quantify phenomena and physical quantities such as length, area, volume, weight, and time. The system of measurement is based on a basic unit of measurement and the conversion of that measurement to other related units. For example, you may want to convert the length of a journey from miles to kilometers which would require the conversion from the Imperial System to the Metric System.

However, before the introduction of these standardized systems, we used to use informal units of measurement such as handspan and cubit — the latter being the length between the elbow and the tip of the middle finger. Understandably, these differed from person to person and so the need for a reliable system of measurement resulted in the development of the British Imperial System, the United States Customary System, and the Metric System.

The first system that was developed was the British Imperial System which was then adopted and altered by the United States of America after they gained independence from Britain. It became what is known as the US Customary System. Both the Imperial System and the US Customary System were deemed too complicated and so the Metric System was developed and became the official measurement system in most countries.

What is the Metric system?

The Metric System, or The International System of Units (SI) as it is formally known, is the standard for measurement around the world. It was first adopted in France in 1795 after the French Revolution resulted in the government requesting that a measurement system be put in place that would unify the country. Although, the origins of the Metric System actually go as far back as 1670 in Lyon, France where church vicar Gabriel Mouton proposed a decimal system which formed the basis on which the full Metric System was then developed.

The Metric System is still a decimal system of units that uses the base unit of a meter. A meter is the equivalent of one-tenth-million of one-half of a meridian — the shortest distance from the North Pole to the equator, passing through Paris. The astronomers and mathematicians Pierre Méchain and Jean-Baptiste Delambre were commissioned by the government to work out this distance in 1792, and without their work, the Metric System as we know it would not be in place.

In the 19th century, the Metric System consisted of three base units for length, mass, and time. However, advancements in technology resulted in additional units being required. In 1960, the General Conference on Weights and Measures (CGPM) launched the International System of Units with six base units and 16 derived units. Later in the 20th century, a seventh base unit was added and the meter was redefined to be based on the speed of light rather than the distance between the North Pole and the equator.

The International System of Units (SI) is now the most commonly used system of measurement in the world and it is the only system with official status in the majority of countries. Not only is it used in science and technology, but it occurs in everyday life too.

Metric Units

As previously mentioned, the SI consists of seven base units:

  • Length - meter (m)
  • Time - second (s)
  • Amount of substance - mole (mole)
  • Electric current - ampere (A)
  • Temperature - kelvin (K)
  • Luminous intensity - candela (cd)
  • Mass - kilogram (kg)

These units measure physical quantities and each unit is based on a power of 10, meaning that one meter is equivalent to 100 centimeters or 1000 millimeters. Additionally, one kilometer is equal to 1000 meters, 100,000 centimeters, or 1,000,000 millimeters.

What is the Imperial System?

The Imperial System is an older system of measurements that originated and was used officially in Britain (hence the name the British Imperial System) from 1824 until the adoption of the Metric System in 1965. The Imperial system evolved from units of measurement used by the Romans, Anglo-Saxons, and Celts.

As previously mentioned, there were units of measurement such as the cubit which was used in Ancient Egypt, as well as the foot which is another unit of measurement that measures an object in relation to the human body.

Traditional units of measurements such as the pound, foot, and gallon were used originally, but their values varied depending on what the measurement was being used for and other factors such as the trade in which the measurement was being used and the place. As a result of this, Royal standards were introduced by the British Empire in the 17th century, whereby units of measurement were standardized.

When the United States became independent from the British Empire, it developed the British Imperial System after the American Revolution. The US Customary System uses the same units of measurement as the Imperial System except when it comes to volume. The US

Imperial Units

Seeing as many Imperial Units have been around for so long, often the relationship between them is not standardized and can be more complicated than the decimal system used for Metric units.

Imperial Units include:

  • Length - mile (mi), yard (yd), foot (ft), inch (in), barleycorn (Bc), chain (ch), furlong (fur), nautical mile (nmi), link, rod
  • Area - perch, rood, acre, square mile (sq mi)
  • Volume - fluid ounce (fl oz), gill (gi), pint (pt), quart (qt), gallon (gal), cup
  • Mass and weight - grain (gr), drachm (dr), ounce (oz), pound (lb), stone (st), quarter (qr), hundredweight (cwt), ton (t), slug (slug)

There is no set pattern between many of the units as they are not based on a strict system like the Metric System. For example, 1 foot is equal to 12 inches but 1 yard is equal to 3 feet.

Here are tables with Imperial units and their equivalents within the Imperial System.

Weight:

UnitEquivalent
Pound (lb)16 ounces/7,000 grains
Ounce (oz)16 drams/437.5 grains/0.0625 pound
Dram27.344 grains/0.0625 ounce
Grain (gr)0.037 dram/0.002286 ounce
Stone (st)14 pounds

US Liquid Measures:

UnitEquivalent
Gallon (gal)4 quarts
Quart (qt)2 pints
Pint (pt)4 gills
Gill (gi)4 fluid ounces
Fluid Ounce (fl oz)8 fluid drams
Fluid Dram (fl dr)60 minims
Minim (mi)1/60 fluid dram

US Dry Measures:

UnitEquivalent
Bushel (bu)4 pecks
Peck (pk)8 quarts
Quart (qt)2 pints
Pint (pt)1/2 quart

British Liquid and Dry Measures:

UnitEquivalent
Bushel (bu)4 pecks
Peck (pk)2 gallons
Gallon (gal)4 quarts
Quart (qt)2 pints
Pint (pt)4 gills
Gill (gi)5 fluid ounces
Fluid Ounce (fl oz)8 fluid drams
Fluid Dram (fl dr)60 minims
Minim (mi)1/60 fluid dram

Length:

UnitEquivalent
Nautical Mile (nmi)6,076 feet/1.151 miles
Mile (mi)5,280 feet/1,760 yards/320 rods
Furlong (fur)660 feet/220 yards/1/8 mile
Rod (rd)5.50 yards/16.5 feet
Fathom (fth)6 feet/72 inches
Yard (yd)3 feet/36 inches
Foot (ft)12 inches/0.333 yard
Inch (in)0.083 foot/0.028 yard

Area:

UnitEquivalent
Square Mile (sq mi)640 acres/102,400 square rods
Acre4,840 square yards/43,560 square feet
Square Rod (sq rd)30.25 square yards/0.00625 acre
Square Yard (sq yd)1,296 square inches/9 square feet
Square Foot (sq ft)144 square inches/0.111 square yard
Square Inch (sq in)0.0069 square foot/0.00077 square yard

Volume:

UnitEquivalent
Cubic Yard (cu yd)27 cubic feet/46,656 cubic inches
Cubic Foot (cu ft)1,728 cubic inches/0.0370 cubic yard
Cubic Inch (cu in)0.00058 cubic foot/0.000021 cubic yard
Acre-Foot (ac ft)43,560 cubic feet/1,613 cubic yards
Board Foot (bd ft)144 cubic inches/1/12 cubic foot
Cord (cd)128 cubic feet

What is the difference between the Metric and Imperial Systems?

The main differences between the Metric and Imperial systems are the relations between units, with the Metric System being based on the decimal system rather than the Imperial System having looser connections between units.

Here is a table comparing Metric vs Imperial units:

MeasurementMetric UnitsImperial Units
Length (long)metermile, yard
Length (short)centimeterfoot, inch
Mass/Weightgramounce, pound
Volume (liquid)litergallon, pint, quart
Volume (dry)literbushel, peck, pint, quart

Which countries use the Metric System?

The Metric System is the official system of measurement in most countries. It is commonly stated that only three countries in the world use the Imperial System: the United States; Liberia; Myanmar. Whilst the United States does use the US Customary System, which is based on the British Imperial System, the Metric System is still taught in schools and both the Imperial and Metric Systems are used in everyday life.

Why doesn't the US use the Metric System?

The US still uses the US Customary System due to its prevalence in manufacturing. The system was introduced during the Industrial Revolution and due to the amount of money and effort it would take to adopt the Metric System, industries have campaigned against the change. When it comes to manufacturing, all machinery and equipment are set up using the Imperial System and therefore manufacturers want to avoid unnecessary costs, particularly as the use of the Metric System is not mandatory.

How do you convert Imperial to Metric?

The conversion of units is the conversion of a unit of measurement from one system to another using multiplication or division. This can be done quickly using our online unit converter tool.

Additionally, here are some of the most commonly used Imperial units and their Metric equivalent.

Length:

ImperialMetric
1 inch2.54 centimeters
1 foot30.48 centimeters
1 yard91.44 centimeters
1 mile1609.34 meters
1 mile1.6 kilometers

Mass:

ImperialMetric
1 ounce28.34952 grams
1 pound0.45359 kg
1 pound453.59237 grams
1 tonne0.90718 tonnes

Volume:

ImperialMetric
1 gallon3.785 liters
1 fluid ounce29.57 milliliters
1 cup236.59 milliliters
1 pint473.176 milliliters
1 quart0.946 liters