Units of Measure That Start with F

Several units of measure begin with the letter F, spanning the metric system, imperial/customary systems, and specialized scientific fields. The most common are the Fahrenheit degree (°F) for temperature, the farad (F) for electrical capacitance, the femtometer (fm) for subatomic distances, and the foot (ft) for length. This article explains each unit, its definition, its typical uses, and how it relates to other measures.

Formula

The calculator above converts a temperature from degrees Fahrenheit (°F) to degrees Celsius (°C) and to kelvin (K). The formulas are:

• °C = (°F − 32) × 5/9
• K = (°F − 459.67) × 5/9 + 273.15

Where °F is the temperature in Fahrenheit, °C is the temperature in Celsius, and K is the temperature in kelvin. The Fahrenheit scale sets the freezing point of water at 32°F and the boiling point at 212°F at standard atmospheric pressure.

Worked Example

Using the default value of 98.6°F (normal human body temperature):

1. Subtract 32: 98.6 − 32 = 66.6
2. Multiply by 5/9: 66.6 × 5/9 = 37.0°C
3. For kelvin: (98.6 − 459.67) × 5/9 + 273.15 = (−361.07) × 5/9 + 273.15 = −200.594 + 273.15 = 310.15 K

The calculator returns 37.0°C and 310.15 K, matching these steps.

The Fahrenheit Degree (°F)

The Fahrenheit scale is a temperature scale used primarily in the United States and a few other countries. It was proposed by Daniel Gabriel Fahrenheit in 1724. On this scale, the freezing point of water is 32°F and the boiling point is 212°F, giving a 180-degree interval between them. The scale was originally based on a mixture of ice, water, and ammonium chloride (about 0°F) and the human body temperature (about 96°F, later adjusted). Today, Fahrenheit is defined in terms of the Kelvin scale: exactly 9/5 of a kelvin plus 459.67.

The Farad (F)

The farad is the SI unit of electrical capacitance. It is defined as the capacitance of a capacitor that stores one coulomb of electric charge when a potential difference of one volt is applied across it. In base SI units, 1 F = 1 kg�¹·m�²·s�·A². The farad is a very large unit; most practical capacitors have values in microfarads (µF, 10�� F), nanofarads (nF, 10�� F), or picofarads (pF, 10�¹² F). The unit is named after Michael Faraday, the 19th-century scientist who contributed to electromagnetism and electrochemistry.

The Femtometer (fm)

The femtometer, also called a fermi, is a unit of length equal to 10�¹� meters. It is used to measure distances at the nuclear and subatomic scale, such as the diameter of a proton (about 0.8–0.9 fm) or the range of the strong nuclear force. The name “fermi� honors physicist Enrico Fermi. In particle physics, the femtometer is convenient because it matches the scale of atomic nuclei. One femtometer is one quadrillionth of a meter, or about 10�¹� m.

The Foot (ft) and Other F Units

The foot is a unit of length in the imperial and US customary systems. One foot equals 12 inches, 1/3 of a yard, and exactly 0.3048 meters. It is commonly used for measuring height, short distances, and in aviation (altitude). Other units beginning with F include:

• Furlong – a unit of distance equal to 220 yards (201.168 m), used in horse racing.
• Fathom – a unit of depth equal to 6 feet (1.8288 m), used in nautical contexts.
• Fluid ounce (fl oz) – a unit of volume in imperial (28.413 mL) and US customary (29.5735 mL) systems.
• Foot-pound (ft·lb) – a unit of work or torque.
• Fermi – another name for the femtometer.

Common Mistakes and Misconceptions

A frequent error is confusing the farad (F) with the Fahrenheit degree (°F) – they are unrelated. Another is mixing the US fluid ounce with the imperial fluid ounce; they differ by about 4%. When converting temperatures, forgetting to subtract 32 before multiplying by 5/9 is a classic mistake. Also, note that the foot is defined as exactly 0.3048 m, but the US survey foot (obsolete) was slightly different. For most everyday purposes, the standard foot is used.

How to Choose the Right Unit

Choosing the correct unit depends on the context:

• For temperature in daily life in the US, use °F; in science and most other countries, use °C or K.
• For electrical capacitance, use farads (or submultiples like µF, nF, pF).
• For subatomic distances, use femtometers.
• For human height or room dimensions, use feet and inches in the US; elsewhere, meters.
• For volume of liquids, use fluid ounces (US or imperial, depending on the country).

Limits and Practical Notes

The formulas for temperature conversion are exact. However, the farad is rarely used in its full size; practical capacitors are labeled in microfarads or smaller. The femtometer is not used in everyday life. When using the foot, always specify whether you mean international foot or US survey foot if precision matters. For fluid ounces, always confirm which system is being used, as the difference can affect recipes and medication doses.

Frequently Asked Questions

What is the most common unit starting with F? The foot (ft) is likely the most common in everyday use, especially in the US, for measuring length. The Fahrenheit degree is also very common in weather reports and cooking.

Is a farad a large unit? Yes, a farad is extremely large. A typical electrolytic capacitor might be 1000 µF (0.001 F), and most capacitors are in the picofarad to microfarad range. A one-farad capacitor is a special high-capacitance device.

How many femtometers in a meter? There are 10¹� femtometers in one meter. That is one quadrillion femtometers.

What is the difference between a US fluid ounce and an imperial fluid ounce? A US fluid ounce is approximately 29.5735 mL, while an imperial fluid ounce is exactly 28.4130625 mL. The imperial ounce is about 4% smaller.

Why is Fahrenheit still used? Fahrenheit is used in the US for everyday temperature because of historical adoption and cultural inertia. It offers a finer scale for weather temperatures, with 0°F very cold and 100°F very hot, which some find intuitive.

What is a foot-pound? A foot-pound is a unit of work or energy equal to the work done when a force of one pound is applied over a distance of one foot. It is also used as a unit of torque, though in that context it is often written as pound-foot.

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in t | Temperature (°F) | °F | 98.6 | -459.67
out c | Celsius (°C) | °C | (t - 32) * 5 / 9 | 2
out k | Kelvin (K) | K | (t - 459.67) * 5 / 9 + 273.15 | 2