Thermistor Beta Calculator

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Calculate an NTC thermistor's Beta value from two resistance-temperature points, or use a known Beta value to find temperature or resistance.

Beta from the two points
K

This is an interval-based approximation. Check the datasheet before using it far outside the two source temperatures.

Beta interval label

The label names the two Celsius temperatures used. It is not the numeric Beta value.

Temperature from measured resistance
C

This Beta-model estimate depends on the datasheet interval, component tolerance, and thermistor self-heating.

Resistance at the target temperature
kOhm

This is a Beta-model estimate. Compare the target temperature with the interval specified for the datasheet Beta value.

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How to use our Thermistor Beta Calculator

  1. Choose "Beta from two points," "Temperature from resistance," or "Resistance from temperature."
  2. For "Beta from two points," enter each resistance and its matching temperature from the same datasheet row or controlled measurement.
  3. A bare resistance number means kOhm, and a bare temperature means C. You can instead add an ohm, kOhm, MOhm, C, F, or K suffix.
  4. For either conversion mode, enter the datasheet reference resistance R0, its matching reference temperature T0, and the numeric "Beta value (K)."
  5. Enter "Measured resistance" to find temperature, or "Target temperature" to find resistance, then click "Calculate."
  6. For a two-point answer, save both the Beta value and its interval label. Check that resistance falls as temperature rises; otherwise the points do not fit an NTC thermistor.
Example inputs for Thermistor Beta Calculator
Example inputs for Thermistor Beta Calculator

Definitions

NTC thermistor: A temperature-sensitive resistor whose resistance decreases as temperature increases.

Beta value: A positive constant in K that describes an approximate NTC resistance-temperature curve over a stated temperature interval.

Beta interval: The two temperatures used to specify or calculate Beta. B25/85 means 25 C to 85 C.

Reference resistance R0: The known thermistor resistance at reference temperature T0. It is often called R25 when T0 is 25 C.

Kelvin (K): An absolute temperature scale used by the Beta equations. K equals C plus 273.15.

Common mistakes and quick fixes

Mistake: Treating B25/85 as a Beta value of 2585.
Fix: B25/85 names the temperatures used for the specification. Enter the separate numeric Beta value in K from the datasheet.

Mistake: Leaving off units when the two resistance values are not both in kOhm.
Fix: Bare resistance values mean kOhm. Add "ohm," "kOhm," or "MOhm" when needed, such as "10000 ohm" or "0.01 MOhm."

Mistake: Pairing a resistance with the wrong temperature from a datasheet table.
Fix: Copy each resistance and temperature from the same table row before calculating Beta.

Mistake: Entering an ADC code or divider voltage as "Measured resistance."
Fix: First convert the circuit reading to the thermistor's resistance, then enter that resistance.

Mistake: Using one Beta value across temperatures far outside its stated interval.
Fix: Compare your operating temperatures with the datasheet Beta interval. Use the maker's resistance table or a fuller model when the interval does not fit.

Limitations & Key Assumptions / Boundary Conditions

  • This calculator applies only to NTC thermistors. It rejects two-point data where resistance rises as temperature rises.
  • A two-point Beta value approximates the curve only over the interval between those points. One Beta value may not fit the full resistance-temperature curve.
  • Results depend on correct resistance measurements, stable known temperatures, and the correct R0, T0, and Beta values from the thermistor datasheet.
  • Thermistor tolerance, lead resistance, measurement-circuit error, and self-heating can change real measured resistance and temperature.
  • The calculator needs thermistor resistance, not a raw divider voltage or ADC reading. Convert circuit readings to resistance before using a conversion mode.

Methodology

Two-point Beta calculation

The calculator converts each entered temperature to kelvin because the reciprocal-temperature equation uses absolute temperature. For an NTC thermistor, the higher-temperature point must have lower resistance.

T(K) = T(C) + 273.15

β (K) = ln(R1 / R2) / (1 / T1 - 1 / T2)

R1 and R2 are converted to the same unit before the calculation. Their ratio has no unit, so entering ohm, kOhm, or MOhm gives the same answer when the entries describe the same physical resistances.

Known-Beta conversions

In the conversion modes, R0 and T0 are the reference point from the datasheet, and Beta is the separate numeric value in K.

T(K) = 1 / (1 / T0 + ln(R / R0) / β)

R = R0 x exp(β x (1 / T - 1 / T0))

Worked mini-example

If R1 is 2 kOhm at 26.85 C and R2 is 1 kOhm at 126.85 C, the temperatures are 300 K and 400 K. The two-point equation gives Beta = 831.8 K after rounding to one decimal. With R0 = 2 kOhm at 300 K, Beta = 831.7766 K, and measured R = 1 kOhm, the temperature equation returns 400 K, or 126.85 C.

Sources

  1. Calculating Temperature from Resistance | North Star Sensors - Northstarsensors