diff --git a/bbn_wave_freq_m5atomS3/TrochoidalWave.h b/bbn_wave_freq_m5atomS3/TrochoidalWave.h
new file mode 100644
index 0000000..f94cbd3
--- /dev/null
+++ b/bbn_wave_freq_m5atomS3/TrochoidalWave.h
@@ -0,0 +1,19 @@
+#ifndef TrochoidalWave_h
+#define TrochoidalWave_h
+
+/*
+
+ See https://bareboat-necessities.github.io/my-bareboat/bareboat-math.html
+ 
+ */
+
+double trochoid_wave_length(double periodSec);
+
+const double g_std = 9.80665; // standard gravity acceleration m/s2
+
+double trochoid_wave_length(double periodSec) {
+  double lengthMeters = g_std * periodSec * periodSec / (2 * PI);
+  return lengthMeters;
+}
+
+#endif