How do I convert thermocouple millivolts to temperature?

Add the emf of your reference junction to the measured voltage, then invert the type's reference function. If your meter's terminals are at 25 °C and you read 12.209 mV on a type K, the junction emf is 12.209 + 1.000 = 13.209 mV, which is 324 °C — not 300 °C, and not 300 + 25.

Why can I not just add the cold-junction temperature to the answer?

Because the emf-temperature relationship is not linear. Adding degrees instead of millivolts introduces an error that grows with the difference between the two junctions, and it is a common source of a few degrees of unexplained bias.

What is 0 mV on a thermocouple?

The junction is at the same temperature as the reference junction. With an ice-point reference that means 0 °C; with the meter's terminals as the reference it means the junction is at terminal temperature. A thermocouple never measures an absolute temperature by itself.

Which thermocouple type should I use?

Type T below 0 °C — it is the best behaved of the common types in cold-chain and cryogenic work. Type K for general-purpose wide-range use. Type N where you would use K but need stability at high temperature. Type E when you want the largest output per degree. Type J mainly where existing plant already uses it.

What is the difference between class 1 and class 2?

IEC 60584-1 tolerance bands on the wire. For type K, class 1 is the greater of ±1.5 °C and ±0.004|t|; class 2 is the greater of ±2.5 °C and ±0.0075|t|. At 1000 °C that is ±4 °C against ±7.5 °C. Class 3 covers sub-zero use for the types that define it.

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Thermocouple millivolt to temperature calculator

Convert between millivolts and temperature for type K, T, J, N and E using the NIST ITS-90 reference functions, with proper cold-junction compensation and IEC 60584-1 tolerance classes.

Thermocouple typeNickel-chromium / nickel-alumel. The general-purpose workhorse: wide range, cheap, but the least stable of the common types above about 800 °C.

Direction

Meter readingThe voltage your instrument shows across the thermocouplemVReference (cold) junction temperature0 °C for an ice-point reference; your terminal-block temperature otherwise. This is the input people most often get wrong.°C

A thermocouple measures the difference between its two junctions. The reference-junction emf is added back to your reading before conversion — adding the reference temperature to a converted reading gives a different, wrong answer, because the curve is not linear.

Result

Junction temperature

300.01°C

Meter reading

12.2090mV

Emf vs 0 °C

12.2090mV

reference junction contributes 0.0000 mV

Seebeck coefficient

41.45µV/°C

1 µV ≈ 24.1 m°C

1 °C error in the reference

0.952°C

how much the result moves if your cold junction is 1 °C out

IEC 60584-1 tolerance classes for type K at the computed temperature
Class	Tolerance at 300.0 °C	In millivolts
1	±1.50 °C	±0.0622 mV
2	±2.50 °C	±0.1036 mV
3	not defined here	—

Tolerance is a manufacturing limit on unused wire, not a measurement uncertainty. A calibration replaces it.

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Type K reference points (vs 0 °C)

-270 °C-6.458 mV

-100 °C-3.554 mV

0 °C0.000 mV

25 °C1.000 mV

100 °C4.096 mV

200 °C8.138 mV

400 °C16.397 mV

600 °C24.905 mV

1372 °C54.886 mV

How the calculation works

01Emf against a 0 °C reference junction from the NIST ITS-90 Thermocouple Database (Monograph 175) polynomials for each type, including the exponential term type K adds above 0 °C.
02Cold-junction compensation done in emf, not in degrees: E(t_junction) = E_measured + E(t_reference), then the sum is inverted.
03Inversion is performed on the reference function itself (bisection then Newton), so the reverse direction agrees with the forward one exactly, including at branch boundaries.
04Tolerance classes from IEC 60584-1, stated as the greater of the fixed limit and the proportional term, over each class's defined range.

Limitations

These are the standard reference functions for new, nominal wire. A calibrated thermocouple has its own deviation from them, on its certificate — use that for measurement.
It does not model the errors that dominate thermocouple work in practice: inhomogeneity along the wire, extension-lead mismatch, junction contamination or drift after thermal cycling.
Tolerance is not uncertainty. A class 1 type K is not a ±1.5 °C measurement until the whole chain, reference junction included, has been accounted for.

Frequently asked questions

How do I convert thermocouple millivolts to temperature?: Add the emf of your reference junction to the measured voltage, then invert the type's reference function. If your meter's terminals are at 25 °C and you read 12.209 mV on a type K, the junction emf is 12.209 + 1.000 = 13.209 mV, which is 324 °C — not 300 °C, and not 300 + 25.
Why can I not just add the cold-junction temperature to the answer?: Because the emf-temperature relationship is not linear. Adding degrees instead of millivolts introduces an error that grows with the difference between the two junctions, and it is a common source of a few degrees of unexplained bias.
What is 0 mV on a thermocouple?: The junction is at the same temperature as the reference junction. With an ice-point reference that means 0 °C; with the meter's terminals as the reference it means the junction is at terminal temperature. A thermocouple never measures an absolute temperature by itself.
Which thermocouple type should I use?: Type T below 0 °C — it is the best behaved of the common types in cold-chain and cryogenic work. Type K for general-purpose wide-range use. Type N where you would use K but need stability at high temperature. Type E when you want the largest output per degree. Type J mainly where existing plant already uses it.
What is the difference between class 1 and class 2?: IEC 60584-1 tolerance bands on the wire. For type K, class 1 is the greater of ±1.5 °C and ±0.004|t|; class 2 is the greater of ±2.5 °C and ±0.0075|t|. At 1000 °C that is ±4 °C against ±7.5 °C. Class 3 covers sub-zero use for the types that define it.

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Thermocouple types and tolerance classes (IEC 60584)

Types K, J, T, N and E each have a defined EMF curve and tolerance classes 1, 2 and 3. Choose by range, environment and the tolerance you actually need — then remember the tolerance applies to new wire, not to the thermocouple after a year in service.

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