Commonly Asked Questions about Imperial and Metric Systems

We regularly receive queries regarding imperial and metric dimensions. Read our latest blog to find answers to many of the most commonly asked questions about metric and imperial measurements.

There continues to be widespread confusion over metric and imperial measurements, especially here in the UK. While we use metric kilograms for weight we still use ‘stones’ when weighing ourselves. We measure distance and present road signs in miles but measure fuel efficiency in litres per 100km as well as miles per gallon (mpg) and our recipe ingredients are defined in grams while beer is served in pints.

Imperial to Metric Conversion Questions

The most commonly asked questions tend to be about imperial to metric and metric to imperial conversion.

How do you convert inches to centimetres?

Converting inches into centimetres is relatively easy.

1 inch = 2.54 centimetres.
So 10 inches = 25.4 centimetres.

Since 1959 an inch has been officially defined as exactly 2.54 centimetres by international agreement. A quick and easy approximation that’s good enough for non-precision measurements is 1 inch is equivalent to about 2.5 centimetres.

How many millimeters are in an inch?

This question is related to the previous one. Since there are 10 millimetres in a centimetre and 1 inch is equivalent to 2.54 centimetres - therefore 1 inch equates to 25.4 millimetres.

How many centimeters are in a foot?

Since we know that 1 inch is exactly 2.54 centimetres - a foot, or twelve inches, is therefore:

 

 

12 x 2.54 centimetres = 30.48 centimetres.

The US and the UK have officially defined a foot as exactly 30.48 centimetres. But in most practical applications people consider 30cm to be equivalent to 1 foot, unless precision is essential.

 

 

What is the metric equivalent of a mile?

The metric unit used for distance equivalence in miles is the kilometre (km).

1 mile = 1.60934 km.

 

 

Considering 1 mile to be equivalent to 1.6km is accurate enough for everyday use. This makes 5 miles neatly equivalent to 8km.

The measurement of a mile dates back to Roman times when a mile was considered to be 1,000 paces. The kilometre is based on the length of 1 metre which is 1/10,000,000 of the distance between the equator and the north pole. The twosystems were never devised to align.

 

How do you convert feet and inches to meters?

People often need to convert distances and measurements defined in imperial feet and inches into the metric equivalent in metres and centimetres. Since we know that 1 inch = 2.54 centimetres and there are 100 centimetres in a metre, converting from feet and inches into metres can be achieved in two or three steps.

 

1. Firstly, convert the imperial measurement into the total number of inches
2. Secondly, convert the total number of inches into centimetres based on 2.54cm to the inch.
3. Thirdly, convert the total number of centimetres into metres.

For example:

• 5 feet 10 inches equates to 70 inches.
• 70 x 2.54 = 177.8cm
• And 177.8 cm = 1.778 metres or approximately 1.78 metres.

 

How many inches are in a meter?

Since we know that 1 inch is exactly 2.54 centimetres and there are 100 centimetres in a metre, dividing 100 by 2.54 tells us exactly how many inches there are in a meter = 39.37.

A reasonable approximation is 1 metre is about 3ft 4 inches or 40 inches. Another useful approximation technique is to keep in mind is that 10cm is closely equivalent to 4 inches.

 

 

What is the difference between meters and yards?

As we have just demonstrated, a metre is approximately 3ft and 4 inches or 1 yard and 4 inches. The yard was considered to be the average stride length of King Henry 1 of England, allegedly.

 

How do you convert miles to kilometers?

Converting miles to kilometres is based on this equivalence:

 

1 mile = 1.609344 kilometres.

 

So, multiply the number of miles by 1.609344 to derive the number of kilometres.

 

For example:

30 miles = 48.28032

Using 1 mile = 1.6km is close enough for most day to day conversion requirements.

Using the Metric or Imperial System

Not surprisingly, people are often confused regarding which system to use and when. Here are some of the questions asked.

 

When should I use metric vs. imperial?

 

The choice between using either the metric of imperial system of measurement depends on a number of factors; where you are, what you are doing or measuring and who you are communicating with.

 

The metric system should be used if:

 

• You are outside the USA. Nearly every other country uses metric measurements for daily life.
• Working in science, engineering or medical. Metric is the global standard used in pharmaceuticals, lab measurements and much more.
• Traveling and driving abroad. While the UK and USA continue to use miles and yards in road signage, almost everywhere else uses metric measurements.
• International cooking. Recipes tend to use grams and millilitres, avoiding confusion with cups and tablespoons.
• Need to avoid calculation inaccuracies. The metric base-10 system is far simpler and potentially more accurate than the imperial fraction based system.

 

Use imperial measurements if:

 

• You are in the USA. The United States continues to use imperial measurements for everyday use.
• Working in US construction and trades. Construction item dimensions, such as lumber and pipework, are all defined in inches. Also, areas of land are measured in imperial units (acres).
• Working with or talking to older generation people in the UK. Many of the UK’s older generation are far more familiar with the imperial system of measurement than metric.
• Cooking with USA recipes. American recipes and cookbooks tend to use Fahrenheit for temperature along with cups and tablespoons for recipe ingredients.

 

Confusingly, in aviation, altitude is measured in feet worldwide, while airport runway lengths are measured in metres, outside the USA.

As a good rule of thumb, the metric system should generally be used for international audiences.

 

Which countries use the imperial system?

Just three countries in the world continue to use the imperial system or a mix of imperial and metric.

 

1. USA
2. Liberia
3. Myanmar

The UK is officially a metric country but continues to use legacy imperial units for some measurements such as road distances and signage, body weight along with beer and milk. People in the UK sometimes say: ‘We’re metric, except when we’re not’.

 

Why does the U.S. still use the imperial system?

A primary reason why the USA is resistant to metric conversion is the cost of retooling and converting infrastructure such as road signage. Another reason is that their imperial system of measurement is deeply entrenched and highly familiar.

 

Back in 1975 US Congress passed the Metric Conversion Act in attempt to align the country with the rest of the world. But this was a voluntary conversion that was ignored by most Americans.

 

There is a very strong cultural attachment to imperial units of measurement in the USA. Sports, cooking, construction and everyday language all routinely use imperial measurements and references. When people ask if the USA will ever go fully metric the response is generally - unlikely, unless aliens force them to.

 

Do engineers and scientists use metric or imperial?

Engineers and scientists tend to use the metric system (SI units) due to its precision, standardisation and ease of calculation, making it less prone to calculation errors.

 

But there are exceptions. For example US metrologists sometimes use Fahrenheit for public forecasts but use Kelvin and Celsius in research. In US civil and mechanical engineering the construction industry uses feet, pounds per square inch (psi) and gallons. Pipe diameters are measured in inches and fasteners, such as SAE (society of automotive engineers) bolts, are defined in fractions of an inch.

 

NASA officially uses metric measurements but older projects use parts defined in imperial units. The 1999 Mars Climate Orbiter crashed due to a metric/imperial unit mix-up.

 

 

Is the UK metric or imperial?

The UK is officially a metric country but imperial measurements continue to be used in some applications, which can cause confusion.

 

While most measurements are metric imperial measurements continue to be used for:

 

• Road signage. Miles per hour and distances in miles and yards.
• Beer and Milk. People like to buy their beer and delivered milk in pints.
• Body weight. People tend to measure their body weight in stones and pounds.
• Body height. Like body weight, people tend to be more familiar with measuring their height in feet and inches.
• Land area measurement. Sports fields, such as football pitches, are still measured in yards and land areas might still be measured in acres.

 

Not surprisingly the British system can be confusing. “We’ll measure our body weight in stones, drive miles to buy a litre of petrol, and drink a pint while complaining about the weather in Celsius.”

Precision and Accuracy of Metric and Imperial Measurements

Precision and accuracy of measurements is critical in science, engineering, design and construction. Here are relevant questions that are often asked.

 

Which system is more accurate: metric or imperial?

The metric system is recognised as inherently more accurate, consistent and easier to use in scientific and technical applications. The metric system is a base-10 numbering system that naturally minimises calculation errors and comfortably deals with high levels of accuracy.

 

The imperial system relies on peculiar conversions with 1 foot having originated (allegedly) from a man`s foot size. 1 foot has 12 inches and there are 5,280 feet in 1 mile. Dealing with peculiar conversions like these, often involving fractions, inevitably results in calculation errors.

 

Even in the USA, where imperial measurements are the norm, they rely on metric units in fields like medicine and automotive engineering, where accuracy is vital.

 

Both the metric and imperial systems can be equally precise if tools are well calibrated. But the metric system avoids the potential for human error by using decimals rather than fractions.

 

Why is the metric system considered easier to use?

The metric system is widely considered to be the easiest to use for a variety of practical, mathematical and scientific reasons.

The metric system is a base-10 numbering system that simply uses multiples of 10, which are easy to calculate and translate. Eg. 0.1m = 10cm = 100mm.

 

Imperial, however, relies on irregular conversions and the use of fractions.

• 12 inches = 1 foot.
• 3 feet = 1 yard
• 1760 yards = 1 mile.

 

The metric system uses a consistent, standardised collection of prefixes for all measurements:

 

 

• Kilo = 1000x
• Centi = 1/100
• Milli = 1/1000

 

The imperial system has its own rules for each class of measurement. E.g. Pounds and ounces, inches and feet etc.

 

Metric units make logical sense and don’t rely on any mental gymnastics. For example, water freezes at 0 degrees celsius and boils at 100 degrees. But in fahrenheit water freezes at 32 degrees F and boils at 212 degrees F.

 

Metric is also the global standard used in science, engineering and medicine worldwide, even in countries that continue to use imperial measurements. This standardisation in metric measurement avoids potentially costly and life threatening errors and facilitates beneficial development.

 

Metric is recognised as a universal language that’s consistent, logical, scalable and error-proof while imperial is like a medieval relic - charming but chaotic.

 

Are tape measures in the U.S. in imperial or both?

Most of the commonly found tape measures in the USA are boldly marked in feet, inches and fractions of an inch. Some of today’s tape measures have a reverse side showing metric units, often in smaller print than the primary imperial side. Some tape measures, especially those used in construction and engineering, show both metric and imperial measurements on the same side.

 

How do I read a ruler in inches and centimeters?

Measuring lengths in either inches or centimeters, using a ruler, is very simple.

 

 

Inches on a ruler are marked by longer lines alongside the number. Between these inch markers are shorter lines of varying lengths, denoting fractions of an inch. The next longest line shows the half inch points, the next shows the quarter inch, then the eighth inch and then the 16th inch. Some rulers may even show 32nd inch fractions.

 

To measure something using this imperial scale you need to line the 0 inch line with the start of what you want to measure and then look where the end falls on the scale. If this is between inch markers look at what the fraction is. The ultimate accuracy is determined by the ruler markings and how the ruler is used. For example, if a measured object falls between 2 and 3 inches at around the 5/16 line its dimension would be 2 and 5/16”.

 

Taking metric measurements with a ruler follows the same process. Centimetres are marked by longer lines with 0.5 centimetre marks being shorter with 1mm markings in between. Some metric rulers even have 0.5mm graduations. So if a measured object falls between the 5 and 6 cm lines exactly on the 0.5 cm position its dimension would be 5.5cm.

 

Why are measurements in metric sometimes shown in decimals but in imperial in fractions?

The reason metric measurements are generally defined in decimals is that the metric system is based on a base-10 numbering system using powers of 10. So, for example:

1 metre = 100cm = 1000mm.

This conversion is simply achieved by moving the decimal point. For example 1.5 metres = 1500mm. The question: Is 1500mm bigger or smaller than 1.2m? - is easily answered without any headache.

 

Contrastingly, fractions, although they are intuitive for larger subdivisions (halves, quarters and possibly eighths) are prone to calculation inaccuracy and calculation headaches. For example, is 5/8 bigger or smaller than 11/16? This calculation requires a common denominator to be mentally figured out in order to make the contrast.

 

• 5/8 = 10/16.
• Therefore 11/16 is 1/16 larger than 5/8.

In machining and engineering decimal subdivisions of inches are often used for precision. E.g. 0.125” instead of 1/8.

 

History of the Metric and Imperial Systems

People often want to know where both our imperial and metric systems of measurement originated. Here are their commonly asked questions.

 

Where did the imperial system come from?

The imperial system of measurement has a very long and weird history influenced by royal decrees, ancient trade norms and body part dimensions.

 

The measurement of 1 foot, for example, is supposedly based on the length of a man’s foot which for Romans was around 11.6 inches but for big-footed Anglo-Saxons was apparently around 13.2 inches. It is also anecdotally alleged to have been the size of King Henry I’s foot, but since the measurement was already in use before his reign, this is unlikely.

 

The inch is based on the width of a man’s thumb. And the yard is alleged to be the distance between the nose of King Henry I of England to his thumb tip.

 

The Gallon is a measure derived from medieval wine and ale measurements while the pound weight comes from a Roman libra that weighed 12 ounces.

 

British standardisation in 1824 formalised the imperial system to deal with otherwise chaotic local variations. But this imperial measurement system continued to rely on arbitrary standards such as the brass yardstick that was kept in London.

 

The metric system was developed in France in the 1700s, to replace local units. But Britain stuck to imperial, detailed in the British Weights and Measures Act of 1824. The metre was defined based on the surveyed distance between the North Pole and equator via Paris. It was then standardised to a platinum-iridium bar but is now based on the speed of light, ensuring a constant and precise standard.

 

Who invented the metric system?

It was during the French Revolution that a team of French Scientists aimed to replace thousands of chaotic local units with a universal, decimal based system, not based on the size of a king’s body parts.

 

The French scientists responsible included: Jean-Charles de Borda, Joseph-Louis Lagrange, Pierre-Simon Laplace, and Antoine Lavoisier. They were inspired by the Enlightenment’s love for reason and anti-monarchy sentiment. In 1793 the metre was first defined as exactly 1/10,000,000 of the distance between the North Pole and equator, determined by means of a survey. In 1799 a platinum "Mètre des Archives" rod became the official standard.

 

Other metric units for volume (litres) and mass (grams) were also devised based on logical physical relationships. For example a litre is the volume of 1kg of water and a gram is the mass of 1 cubic centimetre of water at 4 degrees celsius.

 

Importantly, the metric system was highly political, symbolising equality and modernity. Pre-revolution feudal-era units in France included over 800 regional measurement systems that were despised by merchants.

 

What’s the difference between US customary units and British imperial units?

While the US and British imperial systems share common names (gallons, pounds, miles etc.) there are differences. This is primarily due to divergence that occurred after the American revolution.

 

For example a Gallon is 277.4 cubic inches in the British Imperial system but 231 cubic inches in the US Customary system. Similarly a Pint is 20 fluid ounces in the British Imperial system but just 16 fluid ounces in the US customary measurement system. A British Imperial fluid ounce is approximately 28.41 millilitres while the US Customary fluid ounce is 29.57 millilitres.

 

When considering weight measurement a British Imperial pound is 16 ounces or 453.6 grams which is exactly the same as the US Customary measure. But, a British Imperial hundredweight is logically 100 pounds while the US Customary hundredweight is weirdly 112 pounds.

 

The differences primarily occurred due to the revised British standardisation system in 1824 and the post 1776 split between Britain and USA.

 

These notable differences are important when considering measurements such as miles-per-gallon. Since a British Gallon is 20% larger than the US version its important to keep this difference in mind.

Metric and Imperial Dimensional Standards

Questions regarding dimensional standards are often asked in fields like construction, manufacturing and clothing.

 

What is the standard door height in metric and imperial?

 

There are notable differences in standard door sizes. In the UK and Australia the standard height for a door is 6’6” or 1981mm while in the US and Canada the standard is 6’8” or 2032mm. In Europe standard doors are even taller at 6’10.7” or 2100mm.

 

When thinking about door dimensions it can also be beneficial to understand how door width standards vary. In the UK the most common door width is 762mm (30”), with some variations such as 610mm, 686mm and 838mm. For external doors there is even greater variation, although 762mm is very common. The width of external doors in the UK might be 813mm, 838mm or as much as 914mm, where accessibility is essential.

 

Interestingly, in Scotland, internal doors are often slightly narrower at 726mm (28.5”).

In the USA the Americans with Disabilities Act (ADA) requires a minimum interior door width of 813mm (32”) and 914mm (36”) for exterior doors.

 

European standards specify a minimum width for emergency exits of 750mm (29.5”) and 1200mm (47.2”) for disabled access.

 

How are clothing sizes different in metric and imperial?

There is a great deal of confusion in worldwide clothing sizes both metric (used globally) and imperial (primarily used in the US and UK).

 

The US and UK clothing size systems are based on body dimensions in inches while the European system uses body dimensions in centimetres. In Asia, height and weight charts are typically used.

 

For example US dress sizes of: 0, 2, 4, 6, 8… equate to UK sizes of 4, 6, 8, 10, 12…

 

What are common lumber sizes in imperial vs. metric?

Confusion in international construction projects can arise due to lumber size differences between imperial measuring locations (US and Canada) and metric (the rest of the world).

 

A common example is lumber described as 2 x 4 in the US. In reality this wood measures 1.5” x 3.5” which equates to 38 x 89mm. The close equivalent from metric countries measures exactly 45 x 90mm.

 

The reason why US 2 x 4 is not really this dimension is due to historical sawmill practices. Lumber was originally sold at the sawn, full dimensions (2 x 4) before drying and planing reduced its size. But the 2 x 4 label was retained. This means that for US projects its important to always assume that lumber is 0.5” thinner or narrower than labelled. But for EU projects there is no guesswork as 45 x 90mm lumber is the exact measurement of the final product.

 

If you have any questions about reliable security solutions, or if you have any special requirements, remember we are here to help. Give us a call on 01273 092921 and we’ll provide you with free, expert advice.

For more information on Commonly Asked Questions about Imperial and Metric Systems talk to Insight Security