diff --git a/massive_star/convert_large_to_small.py b/massive_star/convert_large_to_small.py
new file mode 100644
index 0000000..1e66b95
--- /dev/null
+++ b/massive_star/convert_large_to_small.py
@@ -0,0 +1,91 @@
+import numpy as np
+from pynucastro.networks.rate_collection import Composition
+import argparse
+
+implemented_networks = {
+    "aprox19": ['h1', 'he3', 'he4', 'c12', 'n14', 'o16', 'ne20', 'mg24', 
+                  'si28', 's32', 'ar36', 'ca40', 'ti44', 'cr48', 'fe52', 
+                  'fe54', 'ni56', 'n'],
+}
+# the aprox19 nuclei list excludes p_nse. We'll deal with that split by later setting 'p' to 0
+
+
+def main(file_name, new_file_name, network):
+
+    # READ MESA FILE
+    # this code adapted from py_mesa_reader. 
+    bulk_data = np.genfromtxt(file_name, skip_header=5, names=True, ndmin=1, dtype=None)
+    bulk_names = bulk_data.dtype.names
+    with open(file_name) as f:
+        for i, line in enumerate(f):
+            if i == 1:
+                header_names = line.split()
+            elif i == 2:
+                header_data = [eval(datum) for datum in line.split()]
+            elif i > 2:
+                break
+    header_data = dict(zip(header_names, header_data))
+
+
+    # SHRINK WITH PYNUCASTRO
+    try:
+        start, end = bulk_names.index('neut'), bulk_names.index('opacity')-1
+        large_network = bulk_names[start:end+1]
+    except ValueError as e:
+        print("The given start or end species is not listed in the MESA file.")
+        raise e
+
+    small_network = implemented_networks[network]
+
+    network_col_names = []
+    network_col_bulk = []
+    c = Composition(large_network)
+    for row in bulk_data:
+        c.set_array(list(row)[start:end+1])
+        new_c = c.bin_as(small_network)
+        dictionary = new_c.data
+        dictionary['p'] = 0
+
+        if not network_col_names:
+            network_col_names = dictionary.keys()
+
+        network_col_bulk.append(list(dictionary.values()))
+        
+    network_col_bulk = np.array(network_col_bulk)
+
+
+    # OUTPUT TO FILE
+    with open(new_file_name, 'w') as f:
+        def writeline(array):
+            f.write(''.join([str(x).rjust(40) for x in array])+'\n')
+        def replace_cols(data, replacement, start, end):
+            return list(data)[:start]+list(replacement)+list(data)[end+1:]
+        
+        writeline(range(1,len(header_data)+1))
+        writeline(header_data.keys())
+        writeline(header_data.values())
+        f.write('\n')
+
+        col_names = replace_cols(bulk_names, network_col_names, start, end)
+        writeline(range(1,len(col_names)+1))
+        writeline(col_names)
+        for i in range(bulk_data.size):
+            writeline(replace_cols(bulk_data[i], network_col_bulk[i], start, end))
+        f.write('\n')
+
+
+    print(f'Binned network (size={end-start+1}) into network {network} and saved file in {new_file_name}.')
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser(description="Bin a large network into a smaller network specified by user. Input and output files use the MESA file format.")
+    parser.add_argument('network', choices=implemented_networks.keys(), help="The name of the smaller network to bin into.")
+    parser.add_argument('file_name')
+    parser.add_argument('new_file_name', nargs='?')
+    args = parser.parse_args()
+    if args.new_file_name is None:
+        file_stem, file_ending = args.file_name.rsplit('.',1)
+        new_file_name = f"{file_stem}.{args.network}.{file_ending}"
+    else:
+        new_file_name = args.new_file_name
+    main(args.file_name, new_file_name, args.network)