The distinction between real value and nominal value occurs in many fields. From a philosophical viewpoint, nominal value represents an accepted condition, which is a goal or an approximation, as opposed to the real value, which is always present. Often a nominal value is de facto rather than an exact, typical, or average measurement.
In measurement, a nominal value is often a value existing in name only; it is assigned as a convenient designation rather than calculated by data analysis or following usual rounding methods. The use of nominal values can be based on de facto standards or some technical standards.
One way to consider this is that the real value often has the characteristics of an irrational number. In real-world measuring situations, improving the measurement technique will eventually begin yielding unpredictable least significant digits. For example, a 1-inch long gauge block will measure to be exactly 1 inch long until the measuring techniques reach a certain degree of precision. As techniques improve beyond this threshold, it will become clear that 1 inch is not the real value of the gauge block length, but some other number approximates it.
In various subfields of engineering, a nominal value is one for which the "name" for the value is close to, but not the same as, the actual value. Some examples:
- For dimensional lumber in North America, a "two by four" is not 2 inches by 4 inches, but rather between 1⅜ and 1⅝ inches thick and between 3⅜ and 3⅝ inches wide, sized to the equivalent rough hewn piece of softwood for nominal structural design purposes. The ⅛-inch tolerances absorb the natural variation.
- Mains electricity is nominally 230 V in the European Union, but is allowed to vary ±10%. In North America, the nominal voltage is 120 V, with variance allowed from 114 V to 126 V (±5%). Voltage is also supplied at 208 V, 240 V and 480 V with similar tolerances. In general, electrical devices are designed to work with one nominal voltage, which represents a band of possible actual voltages, power factor and AC waveform shapes.
- Traction power networks routinely operate well above the nominal voltage, but still within the tolerance. For example, a streetcar traction power could be rated 600 ±10% volts nominal, but the actual overhead line voltage would normally be close to 660 volts, only dropping near the nominal value in exceptional conditions.
- A machine is designed to operate at some particular condition, often stated on the device's nameplate. For example, a pump is designed to deliver its nominal pressure and flow while operating at its nominal speed and power. Actual operating conditions may vary.
- NiMH and NiCd rechargeable batteries have a nominal voltage of 1.2 V, but will actually supply real voltages ranging from about 1.45 V to 1.0 V during discharge.
- A solar panel designed to charge a "12 V lead–acid battery" will often be called a "12-volt panel", even though the actual voltage while charging (of both the panel and the battery) is around 14 V, and the open-circuit voltage of the solar panel is around 17 V.
Other cases involve diameter, speed, and volume.
Sometimes the word "nominal" is misused in engineering contexts as a synonym for "normal" or "expected"; for example, The rotor resistances on all the other operating wheels are nominal.