diff --git a/src/hisp/bin.py b/src/hisp/bin.py
index 478cefe..c09f9d0 100644
--- a/src/hisp/bin.py
+++ b/src/hisp/bin.py
@@ -9,11 +9,10 @@ class SubBin:
     mode: str
     dfw: bool
     parent_bin_index: int
-    low_wetted_area: float  # m^2
-    high_wetted_area: float  # m^2
-    total_area: float  # m^2
-    f: float  # fraction of heat values in low_wetted_area from SMITER
-    # (f = H_low * low_wetted_area /(H_low * low_wetted_area + H_high * high_wetted_area))
+    low_wetted_area: float
+    high_wetted_area: float
+    total_area: float
+    f: float
 
     def __init__(
         self,
@@ -21,6 +20,27 @@ def __init__(
         thickness: float = None,
         material: str = None,
     ):
+        """
+        A SubBin object represents a sub-bin of a FW bin in a reactor.
+
+
+        Args:
+            thickness: The thickness of the subbin (in m).
+            material: The material of the subbin.
+            mode: The mode of the subbin (shadowed, wetted, low_wetted, high_wetted).
+
+        Attributes:
+            thickness: The thickness of the subbin (in m).
+            material: The material of the subbin.
+            mode: The mode of the subbin (shadowed, wetted, low_wetted, high_wetted).
+            dfw: A boolean indicating if the subbin is a Divertor First Wall (DFW) subbin.
+            parent_bin_index: The index of the parent bin.
+            low_wetted_area: The low wetted area of the parent bin (in m^2).
+            high_wetted_area: The high wetted area of the parent bin (in m^2).
+            total_area: The total area of the parent bin (in m^2).
+            f: The fraction of heat values in the low wetted area. Calculated from SMITER as:
+                f = H_low * low_wetted_area / (H_low * low_wetted_area + H_high * high_wetted_area)
+        """
         self.thickness = thickness
         self.material = material
         self.mode = mode
@@ -48,6 +68,32 @@ def wetted_frac(self):
         elif self.mode == "high_wetted":
             return (1 - self.f) * self.total_area / self.high_wetted_area
 
+    @property
+    def surface_area(self) -> float:
+        """Calculates the surface area of the subbin (in m^2).
+
+        Returns:
+            The surface area of the subbin (in m^2).
+        """
+        if self.shadowed:
+            low_wetted_area = self.low_wetted_area
+            high_wetted_area = self.high_wetted_area
+            if (
+                isinstance(self.low_wetted_area, type(np.nan))
+                or self.low_wetted_area is None
+            ):
+                low_wetted_area = 0
+            if (
+                isinstance(self.high_wetted_area, type(np.nan))
+                or self.high_wetted_area is None
+            ):
+                high_wetted_area = 0
+            return self.total_area - low_wetted_area - high_wetted_area
+        elif self.mode in ["wetted", "low_wetted"]:
+            return self.low_wetted_area
+        elif self.mode == "high_wetted":
+            return self.high_wetted_area
+
 
 class FWBin:
     index: int
diff --git a/test/test_bin.py b/test/test_bin.py
index f1d32a4..0d92077 100644
--- a/test/test_bin.py
+++ b/test/test_bin.py
@@ -1,7 +1,15 @@
 import numpy as np
 import pytest
 import pandas as pd
-from hisp.bin import FWBin3Subs, FWBin2Subs, DivBin, FWBin, BinCollection, Reactor
+from hisp.bin import (
+    FWBin3Subs,
+    FWBin2Subs,
+    DivBin,
+    FWBin,
+    BinCollection,
+    Reactor,
+    SubBin,
+)
 
 # create Reactor
 fw_bins = [FWBin3Subs() for _ in range(18)]
@@ -143,3 +151,56 @@ def test_arc_length():
     bins = BinCollection([bin_1, bin_2, bin_3])
     assert np.allclose(bins.arc_length(), [1, 3, 4])
     assert np.allclose(bins.arc_length(middle=True), [0.5, 2, 3.5])
+
+
+def test_sub_bin_surface_area_3_subbins():
+
+    # BUILD
+    my_bin = FWBin()
+
+    subbin_shadowed = SubBin(mode="shadowed")
+    subbin_low_wetted = SubBin(mode="low_wetted")
+    subbin_high_wetted = SubBin(mode="high_wetted")
+    my_bin.sub_bins = [subbin_shadowed, subbin_low_wetted, subbin_high_wetted]
+
+    bin_low_wetted_area = 5
+    bin_high_wetted_area = 3
+    bin_shadowed_area = 2
+
+    bin_total_area = bin_low_wetted_area + bin_high_wetted_area + bin_shadowed_area
+
+    for subbin in my_bin.sub_bins:
+        subbin.low_wetted_area = bin_low_wetted_area
+        subbin.high_wetted_area = bin_high_wetted_area
+        subbin.total_area = bin_total_area
+
+    # TEST
+    assert (
+        subbin_shadowed.surface_area
+        == bin_total_area - bin_low_wetted_area - bin_high_wetted_area
+    )
+    assert subbin_low_wetted.surface_area == bin_low_wetted_area
+    assert subbin_high_wetted.surface_area == bin_high_wetted_area
+
+
+def test_sub_bin_surface_area_2_subbins():
+
+    # BUILD
+    my_bin = FWBin()
+
+    subbin_shadowed = SubBin(mode="shadowed")
+    subbin_wetted = SubBin(mode="wetted")
+    my_bin.sub_bins = [subbin_shadowed, subbin_wetted]
+
+    bin_wetted_area = 5
+    bin_shadowed_area = 2
+
+    bin_total_area = bin_wetted_area + bin_shadowed_area
+
+    for subbin in my_bin.sub_bins:
+        subbin.low_wetted_area = bin_wetted_area
+        subbin.total_area = bin_total_area
+
+    # TEST
+    assert subbin_shadowed.surface_area == bin_total_area - bin_wetted_area
+    assert subbin_wetted.surface_area == bin_wetted_area