diff --git a/tests/__main__.py b/tests/__main__.py
index 43cdfb2..053099f 100644
--- a/tests/__main__.py
+++ b/tests/__main__.py
@@ -1,7 +1,7 @@
 # Licensed under the EUPL-1.2 or later.
 # You may obtain a copy of the licence in all the official languages of the
 # European Union at https://joinup.ec.europa.eu/collection/eupl/eupl-text-eupl-12
-# pylint: disable=too-many-lines
+# pylint: disable=too-many-lines, unidiomatic-typecheck
 
 """The tests for the Stream."""
 
@@ -14,6 +14,8 @@
 import traceback
 from collections import Counter
 from collections.abc import Callable
+from decimal import Decimal
+from fractions import Fraction
 from functools import partial
 from numbers import Number, Real
 from operator import add
@@ -128,6 +130,7 @@ def assert_raises(exc: type[BaseException], fun: Callable[[], object]) -> None:
 assert_raises(AssertionError, lambda: assert_raises(Exception, lambda: None))
 assert_raises(TypeError, lambda: hash(Stream(...)))
 assert_raises(TypeError, lambda: hash(Stream([0, 1])))
+assert_raises(StreamEmptyError, Stream([]).avg)
 
 assert_raises(ValueError, lambda: sliding_window([], -1))
 assert_raises(ValueError, lambda: sliding_window((), 0))
@@ -225,6 +228,23 @@ def raise_exceptions(number: int) -> int:
     == sum(range(20)) - 1 - 3 - 5
 )
 
+average = sum(data_to_average := range(10, 100, 7)) / len(data_to_average)
+average_float: float = Stream(data_to_average).avg()
+assert type(average_float) is float
+assert average_float == average
+average_float = Stream(data_to_average).map(int).avg()
+assert type(average_float) is float
+assert average_float == average
+average_float = Stream(data_to_average).map(float).avg()
+assert type(average_float) is float
+assert average_float == average
+average_decimal: Decimal = Stream(data_to_average).map(Decimal).avg()
+assert type(average_decimal) is Decimal
+assert average_decimal == average
+average_fraction: Fraction = Stream(data_to_average).map(Fraction).avg()
+assert type(average_fraction) is Fraction
+assert average_fraction == average
+
 
 errors: list[ValueError] = []
 assert (
diff --git a/typed_stream/_impl/_types.py b/typed_stream/_impl/_types.py
index 4209909..85411d4 100644
--- a/typed_stream/_impl/_types.py
+++ b/typed_stream/_impl/_types.py
@@ -12,7 +12,7 @@
 from os import PathLike
 from typing import Generic, Protocol, TypeAlias, TypeGuard, TypeVar
 
-from ._typing import override
+from ._typing import Self, override
 
 __all__ = (
     "ClassWithCleanUp",
@@ -22,6 +22,7 @@
     "PrettyRepr",
     "StarCallable",
     "SupportsAdd",
+    "SupportsAverage",
     "SupportsComparison",
     "SupportsGreaterThan",
     "SupportsLessThan",
@@ -85,6 +86,16 @@ def __add__(self: T, other: T) -> T:
         """Add another instance of the same type to self."""
 
 
+class SupportsAverage(Protocol[T_co]):
+    """A type that supports calculating an average if in an Iterable."""
+
+    def __add__(self, other: Self) -> Self:
+        """Add another instance of the same type to self."""
+
+    def __truediv__(self, other: int) -> T_co:
+        """Divide by an int."""
+
+
 class Closeable(abc.ABC):
     """Class that can be closed."""
 
diff --git a/typed_stream/_impl/stream.py b/typed_stream/_impl/stream.py
index 239cdc0..6b42f53 100644
--- a/typed_stream/_impl/stream.py
+++ b/typed_stream/_impl/stream.py
@@ -30,6 +30,7 @@
 from ._types import (
     StarCallable,
     SupportsAdd,
+    SupportsAverage,
     SupportsComparison,
     TypeGuardingCallable,
 )
@@ -315,6 +316,30 @@ def all(self) -> bool:
         """
         return self._finish(all(self._data), close_source=True)
 
+    def avg(self: Stream[SupportsAverage[K]]) -> K:
+        """Calculate the average of the elements in self.
+
+        Raises StreamEmptyError if the stream is empty.
+
+        >>> Stream(data := range(1, 5)).avg()
+        2.5
+        >>> Stream(data).map(int).avg()
+        2.5
+        >>> Stream(data).map(float).avg()
+        2.5
+        >>> Stream(data).map(__import__("decimal").Decimal).avg()
+        Decimal('2.5')
+        >>> Stream(data).map(__import__("fractions").Fraction).avg()
+        Fraction(5, 2)
+        >>> Stream([]).avg()
+        Traceback (most recent call last):
+        ...
+        typed_stream.exceptions.StreamEmptyError
+        """
+        counter = itertools.count()
+
+        return self.peek(lambda _: next(counter)).sum() / next(counter)
+
     @overload
     def catch(
         self,