How the PT100 calculator works
A PT100 is a platinum resistance thermometer with a resistance of 100 Ω at 0 °C. A PT500 has 500 Ω and a PT1000 has 1000 Ω at the same temperature; all three follow the same curve, scaled by their nominal resistance R₀. The curve is defined in IEC 60751 by the Callendar–Van Dusen equation:
R(t) = R₀ [1 + A·t + B·t² + C·(t − 100)·t³]
with A = 3.9083 × 10⁻³, B = −5.775 × 10⁻⁷ and C = −4.183 × 10⁻¹². The C term only applies below 0 °C; above 0 °C the equation is a simple quadratic. These coefficients give the familiar mean temperature coefficient α = 0.00385 Ω/Ω/°C between 0 and 100 °C, which is why datasheets often call this curve “385” or “European” platinum.
The calculator works in both directions. Type a temperature and it evaluates the equation. Type a resistance and it solves for temperature: exactly with the quadratic formula above 0 °C, and with Newton iteration below 0 °C, where the fourth-order term makes a closed-form solution impractical. Both directions agree to better than a millionth of a degree across the full −200 to 850 °C range, so you can trust the result for calibration work, not just quick checks.
Worked example: PT100 at 100 °C
At 100 °C a PT100 measures 138.506 Ω. The calculator also shows:
- Sensitivity 0.3793 Ω/°C, the slope of the curve at that temperature. At 0 °C it is 0.3908 Ω/°C; it falls slowly as temperature rises. One ohm of error at 100 °C is therefore worth about 2.64 °C.
- Class AA tolerance ±0.27 °C (±0.102 Ω), class A ±0.35 °C (±0.133 Ω) and class B ±0.80 °C (±0.303 Ω). IEC 60751 defines tolerance as a fixed part plus a part proportional to |t|, so tolerance widens at high and low temperatures.
The same element as a PT1000 reads 1385.05 Ω at 100 °C: exactly ten times the PT100 value. Below zero the curve bends more strongly; at −100 °C a PT100 reads 60.256 Ω.
Going the other way, a reading of 110 Ω corresponds to 25.68 °C. That is a common sanity check when a sensor sits at room temperature on the bench.
The resistance table
The table under the converter lists resistance and sensitivity for any range and step you choose, in °C or °F. Click a row to load that temperature into the converter. For printed or downloadable references across the whole range, use the full PT100 table, PT1000 table or PT500 table, each with a CSV file in 1-degree steps.
PT100 or PT1000?
Both follow the same standard, so the choice is about the measuring circuit, not accuracy class.
- Lead resistance matters ten times less with a PT1000. One ohm of cable resistance is about 2.6 °C of error on a PT100 but about 0.26 °C on a PT1000. For 2-wire connections, PT1000 is often the practical choice. See the RTD lead-wire error calculator to quantify this for your cable.
- Self-heating is lower with a PT1000 at the same measuring voltage, because the current is smaller.
- PT100 is the industrial default. Head-mount transmitters, PLC analog cards and calibrators almost always accept PT100 in 3- or 4-wire connection, and most process probes are stocked as PT100.
Common mistakes
Using the wrong curve. A few older American and Japanese elements follow α = 0.003916 or 0.00392 instead of 0.00385. Their resistance at 100 °C is noticeably higher (about 139.2 Ω rather than 138.5 Ω), which reads as roughly 1.7 °C too warm. If the datasheet doesn’t say IEC 60751 or α = 0.00385, check before you use this calculator.
Ignoring lead resistance. Measuring a 2-wire PT100 with a multimeter includes both leads in the reading. With thin or long cable, the error is easily larger than the sensor tolerance.
Treating tolerance class as accuracy of the whole loop. Class A describes the element alone. The transmitter or input card adds its own error, and so do the connection and installation (immersion depth, thermal contact).
Measuring with too much current. Ohmmeters that use several milliamps can heat a small element by tenths of a degree. RTD instruments typically use 1 mA or less for PT100 and 0.1–0.5 mA for PT1000.
Frequently asked questions
What is the resistance of a PT100 at 0 °C?
Exactly 100 Ω by definition. A PT1000 is 1000 Ω and a PT500 is 500 Ω.
Is the Callendar–Van Dusen equation the same as ITS-90?
No. ITS-90 defines reference functions for standard platinum resistance thermometers used in laboratories. Industrial PT100 elements are specified by IEC 60751, which uses the simpler Callendar–Van Dusen equation with fixed coefficients. For calibrated sensors, a certificate may give individual A, B and C values; this calculator uses the standard ones.
Over what range is the equation valid?
IEC 60751 covers −200 to 850 °C. The calculator refuses values outside that range rather than extrapolating.
Which tolerance class applies to my sensor?
The class and its valid temperature range are on the sensor datasheet. Thin-film elements usually carry class A or B over a narrower range than wire-wound elements. Class AA (formerly “1/3 DIN”) is usually limited to roughly 0–150 °C or less.
Can I share a result?
Yes. “Copy link” in the Result heading creates a URL that opens the calculator with your element and temperature or resistance already entered.