To calculate your maximum heart rate, a commonly used formula is to subtract your age from 220.
For a 13-year-old:
\[ \text{Maximum Heart Rate} = 220 - \text{Age} \] \[ \text{Maximum Heart Rate} = 220 - 13 = 207 \text{ beats per minute (bpm)} \]
So, your estimated maximum heart rate would be about 207 bpm.
It's important to note that a resting heart rate of 26.25 bpm is unusually low and could indicate a health issue, especially in someone your age. If this is an accurate measurement, you should consult a healthcare professional for further evaluation.