NTC thermistor calculator
Resistance, divider output, and ADC counts across temperature — a firmware lookup table from four numbers.
| Temp | NTC resistance | Vout / Vcc | ADC counts |
|---|---|---|---|
| -20 °C | 105.38 kΩ | 0.087 | 355 |
| -10 °C | 58.25 kΩ | 0.147 | 600 |
| 0 °C | 33.62 kΩ | 0.229 | 939 |
| 10 °C | 20.17 kΩ | 0.331 | 1357 |
| 20 °C | 12.54 kΩ | 0.444 | 1817 |
| 30 °C | 8.04 kΩ | 0.554 | 2270 |
| 40 °C | 5.3 kΩ | 0.654 | 2676 |
| 50 °C | 3.59 kΩ | 0.736 | 3014 |
| 60 °C | 2.49 kΩ | 0.801 | 3280 |
| 70 °C | 1.76 kΩ | 0.850 | 3482 |
| 80 °C | 1.27 kΩ | 0.887 | 3633 |
| 90 °C | 933.6 Ω | 0.915 | 3745 |
| 100 °C | 697.5 Ω | 0.935 | 3828 |
Wired NTC-on-top (the reading rises with temperature). The B-equation is accurate to ~±1 °C over 0–70 °C; use a Steinhart–Hart fit beyond that.
The circuit this computes
Temperature Sensing (NTC divider) — fully explained →How it works
The NTC's resistance follows R(T) = R25 · e^(B·(1/T − 1/298.15)) with T in kelvin. In a divider with the NTC on top, the output ratio is Rfix / (Rfix + R(T)) — rising with temperature — and the ADC count is just that ratio times full-scale. Powering the divider from the ADC's own reference makes the measurement ratiometric: supply wobble cancels out.
Common questions
A single number describing how steeply the NTC's resistance falls with temperature, fitted between two points (usually 25 °C and 85 °C — hence B25/85). 3950 is the most common for hobby 10k NTCs. It's on the datasheet, and using the wrong B gives readings that are right at 25 °C and drift wrong away from it.
Within about ±1 °C over 0–70 °C, which covers most products. For wider ranges or tighter accuracy, use the three-coefficient Steinhart–Hart fit — same divider, just a better curve in firmware.
Self-heating: the divider current warms the NTC, which dutifully reports its own warmth. Keep the current well under a milliamp (bigger resistors), or pulse the divider from a GPIO and sample right after switching it on.