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Yard Rule

Support for Students Preferring to Use the Imperial System
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A guide to using imperial in school subjects

The first step in being able to use imperial is to have the conversion factors from metric to imperial to hand, so questions can be converted into the appropriate units. Please note the below conversion rates are rounded to a degree sufficiant for most purposes, for the precise conversion rates go to and type in the desired conversion, such as '1 Joule in BTU'. This will work in most cases.

Millimetres - inches
metres - feet
Kilometres - miles
Litres - pints
Millilitres - fluid ounces (UK)
Kelvin - rankine
/9 5ths
Celsius - Fahrenheit
/9 5ths, +32 (roughly double and add 30 for 20-100F)
Kilogram - pound
Gram - slug
Newton - pound force

Newton - poundal

x 7.2330

Joule - BTU


Joule - foot-pound-force


Joule - foot-poundal


It is also helpful to know how to convert between imperial units. Although the numbers are not multiples of ten, this is no harder to do than it is in metric (presuming in some cases you do have a calculator to hand, if you don't your computer will have). To convert in the opposite direction to the conversions shown below (e.g. from feet to miles), just do the inverse of what is shown.

miles - feet
x5,280, or x80, then x66
miles - yards
x1,760, or x80 then x22
feet - inches
yards - feet
tons - pounds
ounces - pounds
fluid ounces - pints
gallons - pints
stones - pounds
Feet/second - MPH
/5280, then x60 twice

A note for chemists: One fluid ounce (fl oz) of water at 62F (521.67R) weighs one ounce (oz) exactly.

Also, 1 gallon (gal) of water at 62F (521.67R) weighs 10 pounds (lb) exactly.


A slight variant on imperial which is better adapted to science and maths is the FPS system. This stands for Foot-Pound-Second, and the system has been around for some considerable time. The system was designed to fit the essential science equation F=MA, or force equals mass times acceleration. It does this by replacing the force unit PoundForce (lbf) with the Poundal (pdl). One poundal is the force required to accelerate an object with one pound (lb) of mass at 1 foot per second per second (ft/s).

The FPS equivalent to the Kelvin scale is the Rankine scale (R). This uses the same degree as Fahrenheit, but starts from absolute zero (-459.67F).

Note: Alternatively, you can replace the weight unit, pound (lb) with a unit called the slug (sl). This is a matter of personal choice, but in my opinion it is better to use the poundal for the sake of uniformity, as it is a little more frequently used, and is better for physics as it is not limited to use on Earth's surface. Also, using the poundal does not upset the fl oz - oz relationship (see above 'note for chemists').


The imperial unit of energy is the BTU, or British Thermal Unit, which is the energy required to raise the temperature of a pound of water by one degree Fahrenheit. This is essentially the same function as the calorie serves in metric. In FPS, you can also use the foot-pound-force or the foot-poundal, depending on whether you use the slug or poundal respectively. These units are coherent, and fit physics equations.


Speed of sound: 1128 ft/s

Speed of light: 753,912,264 ft/s

Acceleration due to gravity: 32.15 ft/s

Atmospheric pressure (ground level, Earth): 68087.256 pdl/ft

Earth-Sun distance (1 Astronomical Unit): Approx. 93 million miles.

Don't use imperial merely out of defiance. Use it because it is better.

Teach imperial, use imperial, save imperial