Instrumentation Calculators

RTD Calculator: Converting Measured Resistance to Process Temperature

  • A Resistance Temperature Detector (RTD) is a sensor used to measure temperature by correlating the resistance of the RTD element with temperature. 
  • Resistance Temperature Detectors (RTDs) are widely used in industrial and scientific applications to measure temperature precisely.

Formula for calculation

One common formula for estimating temperature based on RTD resistance is

  T=Rref + {(RT- Rref)/α Rref}

In this formula, 

  • RT represents the resistance of the RTD at temperature T
  • T is the temperature at which you want to calculate with measured resistance.
  • Rref is the reference resistance at the reference temperature Tref
  • α denotes the temperature coefficient factor of the RTD,
  • Tref is the reference temperature. 

This formula assumes a linear relationship between resistance and temperature, making it a simple and useful tool for quick temperature estimation.

Example Calculation

The most common type of RTD is the PT100, which has a resistance of 100 ohms at 0 degrees Celsius.

Let’s consider a practical example using a PT100 RTD with specific values for the formula components. Assume that:

  • Rref = 100 ohms (at Tref = 0°C)
  • α = 0.00385 (a common value for PT100 RTDs)
  • We have a given that measured resistance RT = 119.25 ohms. 
  • Tref = 0°C (reference temperature of the RTD )

Follow these steps to calculate the process temperature:

Understanding the Formula

The formula relates the process temperature (T) to the measured resistance

  • T=Rref + {(RT- Rref)/α Rref}

Substituting Known Values and ​Calculating Temperature

  • Insert the provided values into the formula:

T = 0+(119.25−100)/(0.00385X100)

T = 19.25/0.385 ≈50°C

With the measured resistance of 119.25 ohms, the calculated process temperature is approximately 50°C. This result is consistent with what we would expect from a PT100 RTD.


While the formula provides a practical way to estimate process temperature, keep in mind a few important considerations:

  • Lead wire resistance and other factors may affect the accuracy of the measurement.
  • The formula assumes a linear relationship between resistance and temperature, which might not hold true over a wide temperature range. 
  • The T=Rref + {(RT- Rref)/α Rref} formula provides a way to estimate process temperature based on RTD resistance. 
  • The example demonstrates its application with a PT100 RTD. However, for accurate results, factors like lead wire resistance and non-linearity should be considered, and alternative equations like the Callendar-Van Dusen equation might be more suitable.

RTD Calculator for converting Measured Resistance to Process Temperature

When an RTD is put in a process, the measured resistance to process temperature can be calculated with the help of the calculator that can be found below.

Ref the below link Converting Process Temperature to Measured output Resistance RTD

RTD Calculator: Converting Process Temperature to Measured Output Resistance 

Click here for more Instrumentation Calculators

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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