PT100 / PT1000 RTD Calculator
Calculate platinum RTD resistance or temperature using the IEC 60751 Callendar–Van Dusen equation, with tolerance range.
About platinum RTDs
T ≥ 0 °C: R = R₀ × (1 + A·T + B·T²)
T < 0 °C: R = R₀ × (1 + A·T + B·T² + C·(T − 100)·T³)
PT100 (R₀ = 100 Ω) and PT1000 (R₀ = 1000 Ω) are platinum resistance temperature detectors. This uses the IEC 60751 Callendar–Van Dusen coefficients (with a 3920 ppm/°C option). Switch the mode to solve resistance from a temperature, or temperature from a measured resistance; the tolerance gives the resulting min/max band.
How it works
Platinum's resistance climbs in a smooth, well-characterised way as it heats up, which is what makes it the reference sensor for accurate temperature measurement. The Callendar–Van Dusen equation captures that curve: the A term is the near-linear slope, the B term the slight downward bend at high temperatures, and the C term a small correction that only applies below 0 °C. The "3850 ppm/°C" figure is the standard temperature coefficient — it means a PT100 rises about 0.385 Ω for every degree, so 100 Ω at 0 °C becomes roughly 138.5 Ω at 100 °C.
Worked example
Measure 110 Ω on a PT100 and switch the mode to solve temperature: the calculator inverts the equation to about 25.7 °C. Because the curve is nearly straight, a quick sanity check is (110 − 100) / 0.385 ≈ 26 °C — close, with the small difference coming from the B-term curvature.
Class tolerance and self-heating
The tolerance band comes from the sensor's accuracy class — Class A is tighter than Class B, and both widen as you move away from 0 °C, which is why the min/max values here spread out at temperature extremes. In a real circuit, watch the excitation current too: pushing too much current through the RTD warms it by its own I²R dissipation (self-heating) and biases the reading high. Keep the sense current small, typically under 1 mA for a PT100.
Self-heating and lead resistance
Two error sources sit outside the Callendar–Van Dusen equation. The excitation current you use to measure the sensor also warms it, and that self-heating adds directly to the reading — which is why measurement currents are kept small and why a sensor in still air reads higher than one in moving water. And in a two-wire connection the resistance of the leads is indistinguishable from the sensor's own, so a long cable reads high; three- and four-wire arrangements exist precisely to cancel that. The conversion here is exact for the standard; the wiring is where accuracy is usually lost.