Converts between temperature and thermoelectric voltage for type K and J thermocouples with the NIST ITS-90 reference functions — with or without cold junction compensation. For service, commissioning or checking a temperature input. Free, no sign-up; the calculation runs only in your browser.
Reference junction: 0 °C
The number of decimals shown is not the accuracy of a thermocouple or a measurement.
Related tools: Pt100 / Pt1000 → · 4–20 mA calculator → · All tools →
The calculator checks the conversion. If the measurement still doesn't match, the cause is often polarity, the wrong cable, the type configured on the input or cold junction compensation — that is instrumentation, commissioning and troubleshooting work.
A thermocouple does not generate a "temperature" but a thermoelectric voltage E in mV. It depends on the temperature difference between the measuring junction and the reference (cold) junction — where the thermocouple wires connect to the terminals.
The NIST ITS-90 reference functions give the voltage against a reference junction at 0 °C. Check points: 0 °C = 0 mV; type K at 100 °C ≈ 4.096 mV; type J at 100 °C ≈ 5.269 mV.
In practice the terminals of a measuring module are at, say, 25 °C, not 0 °C. The module therefore has to know their temperature and work out what the thermocouple would read with the reference junction at 0 °C.
Modern transmitters and PLC inputs often do this automatically. The calculator shows the step explicitly:
°C → mV: E_measured = E(T_hot) − E(T_cold) mV → °C: E_hot = E_measured + E(T_cold); T_hot = E⁻¹(E_hot)
Without CJC the cold junction is taken as 0 °C. Ignoring a cold junction at 25 °C makes a reading around room temperature about 25 °C too low.
The voltage comes from the NIST ITS-90 reference functions per interval: type K −270 … 0 °C and 0 … 1372 °C (the positive branch adds an exponential term to the polynomial), type J −210 … 760 °C and 760 … 1200 °C.
Voltage to temperature solves the reference function numerically within the type's range (Newton's method safeguarded by bisection, bounded iterations) — no extrapolation beyond the range.
For type K near −270 °C the sensitivity is tiny (microvolts per °C), so a small voltage change there means a large temperature change.
Each type has its own reference function and range:
Positive voltage corresponds to the standard NIST polarity of the type: the measuring junction is hotter than the reference junction. Wire colour codes differ between standards and are not covered here.
The calculation uses the NIST ITS-90 reference functions for the ideal standard thermocouple characteristic. The result does not replace calibration, a tolerance check of the specific sensor, a polarity check, a check of the extension or compensating cable, a check of the CJC sensor or an assessment of the whole measuring chain.