[ f = \frac12 \cdot R \cdot C \cdot \ln\left(\fracV_T+V_T-\right) ]
The 74HC14 thresholds are proportional to Vcc. At 3.3V, the hysteresis shifts, changing the natural log constants. Your calculator needs a Vcc input. 74hc14 oscillator calculator full
time constant and the natural logarithm of the voltage threshold ratios. While many calculators use a simplified approximation, the accurate full-feature formula is: [ f = \frac12 \cdot R \cdot C