PT100 / RTD resistance-temperature calculator
Convert between PT100 resistance and temperature per IEC 60751 (Callendar–Van Dusen): enter resistance (Ω) and read °C, or enter °C and read resistance.
Resistance to temperature
Formula: R(t) = R₀(1 + A·t + B·t²) for t ≥ 0, R(t) = R₀(1 + A·t + B·t² + C·(t − 100)·t³) for t < 0, with R₀ = 100 Ω, A = 3.9083×10⁻³, B = −5.775×10⁻⁷, C = −4.183×10⁻¹² (IEC 60751).
PT100 reference — resistance at common temperatures
Nominal PT100 resistance per IEC 60751 at common temperatures.
PT100 is a platinum resistance thermometer: 100 Ω at 0 °C, roughly 0.385 Ω/°C over 0–100 °C. A 3-wire or 4-wire connection removes lead resistance from the reading. IEC 60751 Class A accuracy is ±(0.15 + 0.002·|t|) °C. AO Ctrl supplies the RTD probes and thermocouples used across industrial automation.
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Frequently asked questions
What is the resistance of a PT100 at 100 °C?
Per the IEC 60751 Callendar–Van Dusen polynomial used by this calculator, a PT100 reads 138.51 Ω at 100 °C. Other reference points from the same table are 119.4 Ω at 50 °C, 157.33 Ω at 150 °C and 212.05 Ω at 300 °C. The formula for t ≥ 0 is R(t) = R0(1 + A·t + B·t²) with R0 = 100 Ω, A = 3.9083×10⁻³ and B = −5.775×10⁻⁷.
Why does my PT100 read below 100 ohms?
Below 100 Ω means the measured temperature is below 0 °C, because 100 Ω is the nominal resistance at 0 °C. The converter then uses the full IEC 60751 polynomial including the C coefficient, R(t) = R0(1 + A·t + B·t² + C·(t − 100)·t³) with C = −4.183×10⁻¹², so sub-zero conversions stay accurate — the quadratic inverse alone would drift about 2.4 °C at −200 °C.
What accuracy class is an IEC 60751 PT100?
IEC 60751 Class A accuracy is ±(0.15 + 0.002·|t|) °C. In practice a PT100 changes roughly 0.385 Ω/°C over 0–100 °C, so a 3-wire or 4-wire connection should be used to remove lead resistance from the reading. The tool converts resistance to temperature per IEC 60751 and is calibrated to that polynomial.
AO Ctrl provides this tool for reference only.