RTD PT100 and PT1000 calculator

Linear RTD approximations go wrong below zero and nobody accounts for lead-wire error on a 2-wire PT100. Callendar-Van Dusen in both directions per IEC 60751, including the sub-zero cubic term, with lead-wire and self-heating notes.

Open the calculator in EM·LINKBelow is a worked example. The working calculator runs free in EM·LINK.

Worked example

Example inputs

PT100 (R0 = 100 Ω)▾
Resistance to temperature▾
93.16Ω
2-wire▾
0.5Ω

Example result

Temperature−20.00 °C

After removing 1.00 Ω of lead resistance from the 2-wire reading.

StepValue
Measured93.16 Ω
Lead correction (2 × 0.5 Ω)−1.00 Ω
Sensor resistance92.16 Ω
Callendar-Van Dusen−20.00 °C
Linear, α = 0.00385−20.36 °C
Without lead correction−17.46 °C

A PT1000 at −20.00 °C reads 921.60 Ω.

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Run it with your own values

The working calculator is RTD PT100 / PT1000 in EM·LINK, our free browser tool. The button opens it directly.

Open in EM·LINK

How to read the result

Checked against IEC 60751 reference points: 100.0000 ohm at 0 C and 138.5055 ohm at 100 C for a PT100.

On a 2-wire hookup, 0.5 ohm per leg adds 1 ohm, which shifts a PT100 by about 2.5 C. 3-wire cancels it if the legs match; 4-wire is immune.

How this is calculated

Below 0 °C the full Callendar-Van Dusen equation applies, including the cubic C term. The tool solves it for temperature numerically.

T >= 0 C: R = R0 (1 + A T + B T^2) T < 0 C: R = R0 (1 + A T + B T^2 + C T^3 (T - 100)) A = 3.9083e-3, B = -5.775e-7, C = -4.183e-12
Meter93.16 ΩPT100 0.5 Ω lead0.5 Ω lead92.16 Ω at −20.00 °C A 2-wire meter sees the sensor plus both leads. Subtract 2 × 0.5 Ω before solving for temperature.

The full derivation, step by step, is in the EM·LINK documentation.

Calculated per IEC 60751. These tools are an engineering aid, not a certification.

Next step

Logging these sensors across the plant? Embedos Edge takes analog and digital I/O.

Common questions

What is the PT100 resistance at 100 C?

138.5055 ohm per IEC 60751.

How do I convert PT100 resistance below 0 C?

Use the full Callendar-Van Dusen equation with the C term. The tool solves it with Newton's method.

How much error does a 2-wire RTD add?

Twice the one-way lead resistance. At 0.5 ohm per leg that is about 2.5 C on a PT100.