How to use the NTC thermistor calculator
- Pick your NTC type. Most Arduino temperature modules and loose thermistors are 10 kΩ, B3950; 3D-printer hot-end thermistors are usually 100 kΩ, B3950. Choose Custom to type R25 and Beta from your datasheet.
- Choose a mode:
- R → °C — you measured the thermistor's resistance with a multimeter.
- °C → R — you want to know what resistance to expect at a temperature.
- Divider + ADC — the thermistor is wired to a microcontroller pin with a series resistor, and you have the
analogRead()value.
- Read the temperature, resistance and sensitivity. The curve shows where you are, and the table lists resistance (and expected ADC counts in divider mode) from −20 °C to 120 °C.
What an NTC thermistor is
NTC stands for Negative Temperature Coefficient: the resistance goes down as the temperature goes up. A 10 kΩ NTC is 10 kΩ at 25 °C, about 33.6 kΩ at 0 °C and only about 700 Ω at 100 °C (for B3950). This large change makes it cheap and sensitive — but the curve is not a straight line, so you need a formula.
Formula
The Beta equation is the standard simple model. Temperatures are in kelvin (K = °C + 273.15):
- R0 — resistance at the reference temperature T0 (R25 at 25 °C = 298.15 K)
- B — the Beta value from the datasheet, in kelvin
For a divider read by an ADC with n bits:
| Temperature | 10 kΩ B3950 | 10 kΩ B3435 | 100 kΩ B3950 |
|---|---|---|---|
| −20 °C | 105.4 kΩ | 77.5 kΩ | 1.054 MΩ |
| 0 °C | 33.62 kΩ | 28.70 kΩ | 336.2 kΩ |
| 25 °C | 10.00 kΩ | 10.00 kΩ | 100.0 kΩ |
| 50 °C | 3.588 kΩ | 4.101 kΩ | 35.88 kΩ |
| 100 °C | 698 Ω | 987 Ω | 6.975 kΩ |
Choosing the series resistor
The divider is most sensitive at the temperature where the series resistor equals the NTC's resistance. For a 10 kΩ NTC, a 10 kΩ resistor is best around 25 °C — good for rooms, aquariums and weather stations. For a 3D-printer hot end at 200 °C+, a 100 kΩ NTC with a 4.7 kΩ series resistor is common because the thermistor's resistance is low when hot. The calculator tells you the matching temperature for your resistor.
Common mistakes
- Using °C inside the formula. The Beta equation needs kelvin. Add 273.15 first.
- Swapping the divider formula. If the NTC is on the Vcc side, the voltage rises as it gets hotter. Pick the matching wiring option or your readings will move the wrong way.
- Mixing up B values. "B25/50" and "B25/85" are measured between different temperature pairs. Use the one your datasheet gives for your range.
- Self-heating. Current through the NTC warms it. Keep divider current small, under about 0.5 mA for a 10 kΩ part.
- Trusting the raw ESP32 ADC. The ESP32 ADC is not linear near its ends; use
analogReadMilliVolts().
Need the plain voltage from an ADC count? Use the ADC voltage calculator or the voltage divider calculator. Comparing boards for a weather station? Read Arduino vs ESP32 vs Raspberry Pi Pico, or find sensors in our products.
