diff --git a/pyndulum/pendulum_equations.py b/pyndulum/pendulum_equations.py
index 40654b9..552faa7 100644
--- a/pyndulum/pendulum_equations.py
+++ b/pyndulum/pendulum_equations.py
@@ -93,6 +93,18 @@ def bpm(len: float) -> float:
     """
     return 60.0 / get_period(len)
 
+def length_from_period(p: float) -> float:
+    """
+    Calculate the length of a pendulum based on period.
+
+    Parameters
+    ----------
+    p : float
+        period [s] for a swing of the pendulum
+        length of the pendulum [m]
+    """
+    return ((p / 2.0 * np.pi) ** 2) * 9.81
+
 def energy(m: float, len: float, theta: float) -> float:
     """
     Calculate the energy of a pendulum.
@@ -111,4 +123,4 @@ def energy(m: float, len: float, theta: float) -> float:
     float
         energy [kg . m2 /s2] of the pendulum
     """
-    return m * 9.81 * max_height(len, theta)
+    return m * 9.81 * max_height(len, theta)
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