diff --git a/docs/api/tools.md b/docs/api/tools.md
new file mode 100644
index 0000000..5906413
--- /dev/null
+++ b/docs/api/tools.md
@@ -0,0 +1,7 @@
+---
+title: Model tools
+---
+
+## Extracting model information
+
+::: klax.count_parameters
\ No newline at end of file
diff --git a/klax/__init__.py b/klax/__init__.py
index c0974a9..77457e2 100644
--- a/klax/__init__.py
+++ b/klax/__init__.py
@@ -35,6 +35,7 @@
 from ._serialization import (
     text_serialize_filter_spec as text_serialize_filter_spec,
 )
+from ._tools import count_parameters
 from ._training import fit as fit
 from ._wrappers import Constraint as Constraint
 from ._wrappers import NonNegative as NonNegative
diff --git a/klax/_tools.py b/klax/_tools.py
new file mode 100644
index 0000000..e6e6957
--- /dev/null
+++ b/klax/_tools.py
@@ -0,0 +1,41 @@
+import operator
+
+import equinox as eqx
+import jax
+from jaxtyping import PyTree
+
+from ._wrappers import NonTrainable
+
+
+def count_parameters(model: PyTree) -> int:
+    """Count the number of trainable parameters in a model.
+
+    Under the hood this just counts the number of inexact
+    JAX/NumPy array elements in the pytree that are not
+    wrapped by [`klax.NonTrainable`][].
+
+    Warning:
+        If you use `jax.lax.stop_gradient` or any other method
+        besides [`klax.NonTrainable`][] to make arrays not receive
+        gradient updates, then this function will overestimate
+        the number of trainable parameters!
+        Consider using [`klax.NonTrainable`][] or counting the trainable
+        parameters manually.
+
+    Args:
+        model: Arbitrary pytree.
+
+    Returns:
+        Integer count of inexact inexact JAX/NumPy array elements.
+
+    """
+    return jax.tree.reduce(
+        operator.add,
+        jax.tree.map(
+            lambda x: x.size
+            if eqx.is_inexact_array(x) and not isinstance(x, NonTrainable)
+            else None,
+            model,
+            is_leaf=lambda x: isinstance(x, NonTrainable),
+        ),
+    )
diff --git a/mkdocs.yml b/mkdocs.yml
index 4b355a3..e1c0a6a 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -104,6 +104,7 @@ nav:
       - api/losses.md
       - api/callbacks.md
       - api/serialization.md
+      - api/tools.md
       - api/initialization.md
       - Neural Networks:
           - api/nn/linear.md
diff --git a/tests/test_tools.py b/tests/test_tools.py
new file mode 100644
index 0000000..b0d5702
--- /dev/null
+++ b/tests/test_tools.py
@@ -0,0 +1,32 @@
+import equinox as eqx
+import numpy as np
+from jax import numpy as jnp
+from jaxtyping import Array
+
+from klax import NonTrainable, count_parameters
+
+
+class TestParameterCount:
+    def test_pytree_with_jax_leaves(self):
+        tree = (jnp.ones((3, 3)), jnp.zeros((2, 3)))
+        assert count_parameters(tree) == 15
+
+    def test_pytree_with_numpy_leaves(self):
+        tree = (np.ones((3, 3)), np.zeros((2, 3)))
+        assert count_parameters(tree) == 15
+
+    def test_on_equinox_module(self):
+        class DummyModel(eqx.Module):
+            no_parameter: int
+            weight: tuple[Array, Array]
+            non_trainable: NonTrainable
+
+            def __init__(
+                self,
+            ):
+                self.no_parameter = 1
+                self.weight = (jnp.ones((3, 3)), jnp.zeros((3, 3)))
+                self.non_trainable = NonTrainable(jnp.ones((2, 2)))
+
+        dummy_model = DummyModel()
+        assert count_parameters(dummy_model) == 18