Pt100 Resistance & Temperature Calculator
Instrument technicians and calibration engineers use this to convert a Pt100 RTD resistance to temperature, or a temperature to the expected resistance, when bench-testing a sensor or building a calibration table. It uses the IEC 60751 reference function for a standard 100 ohm platinum RTD.
A Pt100 reads 100.00 ohm at 0 C and follows a defined resistance-temperature curve. This calculator implements the IEC 60751 Callendar-Van Dusen equation in both directions across the sensor's rated range of -200 to 850 C. Enter a temperature to get the ideal resistance, or a resistance to get the temperature.
The Calculator
Result
The temperature-from-resistance solution uses the positive branch, valid for t ≥ 0 C (R ≥ 100 ohm).
The Formula
IEC 60751 defines the resistance with reference resistance R0 = 100 ohm and coefficients A = 3.9083 × 10-3, B = -5.775 × 10-7, and C = -4.183 × 10-12. For temperatures at or above 0 C:
Below 0 C a third term is added:
For t ≥ 0 C the first equation is a quadratic in t and inverts in closed form:
Worked example. At t = 100 C, R = 100 × (1 + 3.9083e-3 × 100 + (-5.775e-7) × 100²) = 100 × (1 + 0.39083 - 0.005775) = 138.51 ohm. Inverting 138.51 ohm through the quadratic returns 100.0 C.
Assumptions and Limits
- This is the ideal IEC 60751 curve for a nominal 100 ohm platinum sensor. A real RTD has a tolerance class (for example Class A or Class B) that adds an allowed error at each temperature; do not treat the computed value as the true reading.
- The closed-form temperature-from-resistance solution here is valid only for t ≥ 0 C (R ≥ 100 ohm). Below 0 C the full equation requires an iterative solution, which this tool does not perform.
- Lead-wire resistance in a 2-wire hookup adds directly to the reading. Use a 3- or 4-wire connection, or subtract the lead resistance, before comparing to this table.
- Results are engineering estimates. Verify against your calibrator and the sensor's certificate.
FAQ
Why is my sensor not exactly 100.00 ohm at 0 C?
Tolerance. A Class B Pt100 is allowed roughly ±0.3 C at 0 C, and lead wire or a loose terminal can add more. The 100.00 ohm figure is the ideal reference; the real sensor is within its class tolerance of it.
Does this work for a Pt1000?
The shape of the curve is the same, but a Pt1000 has R0 = 1000 ohm, so every resistance is ten times these values. This tool is fixed to Pt100; divide a Pt1000 reading by 10 before using it, then multiply the resistance output by 10.
Can I use it below 0 C?
The resistance-from-temperature direction is valid across the full -200 to 850 C range. The temperature-from-resistance direction only solves the t ≥ 0 C branch here; for sub-zero resistances, use a calibrator or an iterative solver.
Source
The coefficients and reference function are defined by IEC 60751, published by the International Electrotechnical Commission (iec.ch).
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