diff --git a/datagateway_api/src/search_api/units_conversion.py b/datagateway_api/src/search_api/units_conversion.py
new file mode 100644
index 00000000..16eadf50
--- /dev/null
+++ b/datagateway_api/src/search_api/units_conversion.py
@@ -0,0 +1,150 @@
+from pint import UndefinedUnitError, UnitRegistry
+
+from datagateway_api.src.common.exceptions import FilterError, SearchAPIError
+from datagateway_api.src.datagateway_api.icat.filters import (
+    PythonICATDistinctFieldFilter,
+)
+from datagateway_api.src.datagateway_api.icat.helpers import get_entity_with_filters
+from datagateway_api.src.search_api.session_handler import (
+    client_manager,
+    SessionHandler,
+)
+
+
+class UnitsConversion:
+    unit_reg = UnitRegistry()
+
+    def __init__(self):
+        # TODO - change to self.get_parameter_type_units()
+        self.icat_stored_units = good_isis_units
+        self.unit_groups = self.group_units()
+
+    # TODO - shouldn't need the decorator in API usage
+    @client_manager
+    def get_parameter_type_units(self):
+        # TODO - probably should make this configurable, between units and unitsFullName
+        distinct_name_filter = PythonICATDistinctFieldFilter("units")
+        parameter_type_names = get_entity_with_filters(
+            SessionHandler.client, "ParameterType", [distinct_name_filter],
+        )
+
+        parameter_units = []
+        for name_dict in parameter_type_names:
+            for _, parameter_name in name_dict.items():
+                parameter_units.append(parameter_name)
+
+        return parameter_units
+
+    def group_units(self):
+        unit_groups = {}
+
+        for unit in self.icat_stored_units:
+            pint_unit = UnitsConversion.get_unit(unit)
+            unit_groups.setdefault(str(pint_unit.dimensionality), [])
+            unit_groups[str(pint_unit.dimensionality)].append(unit)
+
+        return unit_groups
+
+    @staticmethod
+    def get_unit(unit):
+        try:
+            return getattr(UnitsConversion.unit_reg, unit)
+        except UndefinedUnitError as e:
+            raise SearchAPIError(e)
+
+    @staticmethod
+    def get_unit_type(unit):
+        return UnitsConversion.get_unit(unit).dimensionality
+
+    def get_alternative_units(self, input_unit):
+        unit_dimensionality = UnitsConversion.get_unit_type(input_unit)
+        other_units = self.unit_groups[str(unit_dimensionality)]
+        alternative_units = [other_unit for other_unit in other_units]
+
+        return alternative_units
+
+    def get_alternative_unit_values(self, input_unit, input_value):
+        unit_dimensionality = UnitsConversion.get_unit_type(input_unit)
+        other_units = self.unit_groups[str(unit_dimensionality)]
+        try:
+            default_unit = input_value * UnitsConversion.get_unit(input_unit)
+
+            alternative_unit_values = {
+                other_unit: default_unit.to(other_unit).magnitude
+                for other_unit in other_units
+            }
+        except SearchAPIError as e:
+            # 400 should be returned because the user's unit cannot be found
+            raise FilterError(e)
+
+        return alternative_unit_values
+
+    def change_units_type(self, input_unit, input_value, request_unit):
+        pint_value = input_value * UnitsConversion.get_unit(input_unit)
+        try:
+            request_pint_value = pint_value.to(request_unit)
+        except UndefinedUnitError as e:
+            raise FilterError(e)
+
+        return request_pint_value.magnitude
+
+
+def group_units():
+    unit_reg = UnitRegistry()
+    unit_groups = {}
+
+    for unit in good_isis_units:
+        pint_unit = getattr(unit_reg, unit)
+        unit_groups.setdefault(str(pint_unit.dimensionality), [])
+        unit_groups[str(pint_unit.dimensionality)].append(unit)
+
+    return unit_groups
+
+
+good_isis_units = [
+    "1",
+    "C",
+    "G",
+    "GPa",
+    "Hz",
+    "K",
+    "MM",
+    "MPa",
+    "MV",
+    "N/A",
+    "T",
+    "W/sr",
+    "cc",
+    "counts",
+    "degree K",
+    "degree c",
+    "degrees",
+    "g",
+    "g in 1 mL water",
+    "g/cm^3",
+    "gauss",
+    "kbar",
+    "kelvin",
+    "l",
+    "m",
+    "mK",
+    "mL",
+    "mg",
+    "mg/mL",
+    "mg/ml",
+    "microseconds",
+    "ml",
+    "ml ",
+    "mm",
+    "msec",
+    "na",
+    "picoseconds",
+    "rad",
+    "sec",
+    "seconds",
+    "uA h",
+    "uAh",
+    "units",
+    "°C",
+    "¿A",
+]