A figure of merit is a number chosen to describe or compare how well something performs at a particular job. There is no single formula behind the term. Each field defines its own, with its own units and its own direction of “better.” So the first question to ask about any figure of merit is: which field, and which exact metric?
What a figure of merit is
A figure of merit (often abbreviated FOM) boils performance down to a quantity you can rank. A good one captures the properties that matter for the application, often several competing ones, in a single value. The formula, the units, whether higher or lower is better, and the conditions under which values can fairly be compared are all set by the application.
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That is why the phrase on its own is ambiguous. Two well-documented examples show how different the meaning can be:
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware match| Thermoelectric materials: zT | Receiver electronics: noise figure | |
|---|---|---|
| What it measures | How promising a material is for converting heat to electricity or for solid-state cooling | How much a receiver degrades the signal-to-noise ratio |
| Formula | zT = S²σT/κ | NF = 10 log10(F), where F is the noise factor |
| Units | Dimensionless | Decibels (dB) |
| Better direction | Higher | Lower (less noise added) |
| Reference condition | Absolute temperature T of the material | IEEE’s definition specifies a 290 K reference source temperature |
Sources: Northwestern University’s thermoelectrics educational resource (zT) and IEEE Technology Navigator’s noise-figure overview.
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Worked example: thermoelectric zT
Northwestern’s thermoelectrics resource gives the thermoelectric materials figure of merit as:
zT = (S²σ / κ) T, or equivalently zT = (α² / (ρκ)) T
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- S (or α): Seebeck coefficient, the voltage produced per unit temperature difference.
- σ: electrical conductivity. ρ is electrical resistivity; since σ = 1/ρ, the two forms are the same equation.
- κ: thermal conductivity.
- T: absolute temperature (in kelvin, not °C).
The metric is dimensionless. It rewards a strong Seebeck response and high electrical conductivity while penalizing thermal conductivity, because heat that leaks through the material is not converted. These properties tend to pull against each other, which is what makes good thermoelectric materials hard to find. The Northwestern page puts it this way: “The efficiency of a thermoelectric material depends primarily on the thermoelectric materials figure-of-merit, known as zT.”
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Pitfalls when reading or comparing figures of merit
Material figure of merit is not device efficiency
A high material zT describes potential. A finished generator or cooler also depends on heat input, thermal conduction, Joule heating, contacts and interconnects, and the temperature range it actually runs over. Northwestern’s device discussion treats device ZT as analogous to material zT only under approximations, with calculations that account for temperature-dependent properties. Keep “material zT,” “device ZT,” and measured end-to-end efficiency distinct. In segmented devices, different materials can serve different temperature zones for this reason.
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A partial proxy is not the whole metric
Thermoelectric power factor, S²σ, is a common shortcut. Northwestern explains that it can peak at a different carrier concentration than zT because it leaves out thermal conductivity. A material with a higher power factor therefore does not necessarily have a higher zT.
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Before comparing two values, check that they share:
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- Field and formula: the same named metric with the same variable definitions.
- Evaluation level: material property, device model, or measured system performance.
- Operating conditions: the same temperature range, or the same reference condition. For noise figure, that is the 290 K reference temperature IEEE specifies so results are consistent across devices and setups.
- Included effects: note any omitted losses or proxy quantities.
How to use the term correctly
Don’t write “the figure of merit is zT” without adding “for thermoelectric materials.” Other disciplines define their own figures of merit, and the two examples here are illustrations, not a complete catalog. When you meet the phrase in a datasheet, paper, or spec, look for the explicit definition, the units, which direction is better, and the conditions under which the number was obtained.
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