diff --git a/locator/core.py b/locator/core.py
index f376768..fd22951 100644
--- a/locator/core.py
+++ b/locator/core.py
@@ -165,7 +165,7 @@ def __init__(self, config=None):  # noqa: C901
         - **width** (*int*): Width of neural network layers.
         - **nlayers** (*int*): Number of neural network layers.
         - **dropout_prop** (*float*): Dropout proportion.
-        - **pca_components** (*int*): If set, prepend a PCA-initialized linear projection of this width as the first layer and fine-tune it. Recommended when n_SNPs >> n_samples. Default None (disabled).
+        - **pca_components** (*int or "auto"*): If set, prepend a PCA-initialized linear projection of this width as the first layer and fine-tune it. Use ``"auto"`` to pick the width from the genotype-PCA scree elbow. Recommended when n_SNPs >> n_samples. Default None (disabled).
         - **pca_finetune** (*bool*): Whether to unfreeze the PCA projection for a low-learning-rate fine-tuning phase. Default True. False keeps the projection frozen at its PCA initialization.
         - **pca_finetune_lr** (*float*): Learning rate for the PCA fine-tuning phase. Default 1e-4.
         - **keras_verbose** (*int*): Verbosity level for Keras training.
diff --git a/locator/pca.py b/locator/pca.py
index 42550de..5412baa 100644
--- a/locator/pca.py
+++ b/locator/pca.py
@@ -80,3 +80,40 @@ def compute_pca_projection_gram(genotype_matrix, n_components):
     W = tf.linalg.matmul(Xc, evecs, transpose_a=True) / scale
     bias = tf.reshape(-tf.linalg.matmul(mean, W), [-1])
     return W, bias
+
+
+def scree_elbow(genotype_matrix):
+    """Pick a PCA rank from the genotype-PCA scree elbow.
+
+    Computes the explained-variance spectrum (Gram-matrix eigenvalues over the
+    samples) and returns the rank at the chord-distance elbow: the point of the
+    explained-variance curve farthest below the straight line joining its first
+    and last points. For genotype data the spectrum drops steeply then flattens,
+    so this is a stable, data-driven projection width.
+
+    Parameters
+    ----------
+    genotype_matrix : tf.Tensor or np.ndarray
+        Genotype matrix of shape ``(n_samples, n_snps)``; training samples only.
+
+    Returns
+    -------
+    int
+        The elbow rank, at least 1.
+    """
+    X = tf.cast(genotype_matrix, tf.float32)
+    mean = tf.reduce_mean(X, axis=0, keepdims=True)
+    Xc = X - mean
+    gram = tf.linalg.matmul(Xc, Xc, transpose_b=True)
+    evals = np.clip(tf.linalg.eigvalsh(gram).numpy()[::-1], 0.0, None)
+    total = evals.sum()
+    if total <= 0.0:
+        return 1
+    # Keep the meaningful components; a centred n-sample matrix has rank n-1.
+    evr = evals[evals > total * 1e-9] / total
+    if len(evr) < 3 or evr[0] == evr[-1]:
+        return max(1, len(evr))
+    x = np.arange(len(evr), dtype=np.float64) / (len(evr) - 1)
+    y = (evr - evr.min()) / (evr.max() - evr.min())
+    chord = y[0] + x * (y[-1] - y[0])
+    return int(np.argmax(chord - y)) + 1
diff --git a/locator/training.py b/locator/training.py
index 22e7f78..a57950e 100644
--- a/locator/training.py
+++ b/locator/training.py
@@ -31,7 +31,7 @@
     loss_with_range_penalty,
     rasterize_species_range,
 )
-from .pca import compute_pca_projection_gram
+from .pca import compute_pca_projection_gram, scree_elbow
 from .sample_weights import weight_samples
 
 
@@ -643,7 +643,7 @@ def loss_fn(y_true, y_pred):  # noqa: F811
 
         self._loss_fn = loss_fn
 
-        pca_components = self.config.get("pca_components")
+        pca_components = self._resolve_pca_components()
         inner = create_network(
             input_shape=input_shape,
             width=self.config.get("width", 256),
@@ -704,6 +704,37 @@ def _get_genotype_table(self):
             self._genotype_table_src = self.filtered_genotypes
         return self._genotype_table
 
+    def _resolve_pca_components(self):
+        """Resolve the pca_components config value to a concrete width.
+
+        Returns None (no projection) or an int. The string ``"auto"`` is
+        resolved to the genotype-PCA scree elbow of the training split and
+        written back to the config, so every fold and the saved metadata of a
+        run share one rank.
+        """
+        pca_components = self.config.get("pca_components")
+        if pca_components is None or isinstance(pca_components, int):
+            return pca_components
+        if pca_components != "auto":
+            raise ValueError(
+                f"pca_components must be None, an int, or 'auto'; got {pca_components!r}"
+            )
+        index_set = getattr(self, "index_set", None)
+        if index_set is None or getattr(self, "filtered_genotypes", None) is None:
+            raise ValueError(
+                "pca_components='auto' needs training data; pass an explicit "
+                "integer when building an architecture to load weights into"
+            )
+        train_geno = tf.gather(
+            self._get_genotype_table(),
+            np.asarray(index_set.train, dtype=np.int32),
+            axis=0,
+        )
+        rank = scree_elbow(train_geno)
+        self.config["pca_components"] = rank
+        print(f"pca_components='auto': using scree-elbow rank {rank}")
+        return rank
+
     def _inject_pca_weights(self, model, pca_components):
         """Initialize the pca_projection layer with PCA loadings, gate closed.
 
diff --git a/tests/test_pca_init.py b/tests/test_pca_init.py
index 35259ad..7b3eb29 100644
--- a/tests/test_pca_init.py
+++ b/tests/test_pca_init.py
@@ -50,6 +50,17 @@ def test_train_builds_pca_projection(genotype_data, basic_config):
     assert layer.units == 8
 
 
+def test_pca_components_auto_resolves_to_elbow(genotype_data, basic_config):
+    """pca_components='auto' picks a concrete projection width from the scree."""
+    genotypes, samples, _, _, _ = genotype_data
+    loc = Locator(_pca_config(basic_config, pca_components="auto", max_epochs=2))
+    loc.train(genotypes=genotypes, samples=samples)
+
+    resolved = loc.config["pca_components"]
+    assert isinstance(resolved, int) and resolved >= 1
+    assert loc.model.get_layer(PCA_LAYER_NAME).units == resolved
+
+
 def test_frozen_projection_keeps_pca_loadings(genotype_data, basic_config):
     """With pca_finetune=False the projection stays at its PCA initialization."""
     genotypes, samples, _, _, _ = genotype_data
@@ -151,3 +162,20 @@ def test_gradient_gate_controls_gradient_flow():
         assert np.allclose(y.numpy(), x.numpy())
         # Gradient reaching the input is scaled by the gate.
         assert np.allclose(grad.numpy(), expected_grad)
+
+
+def test_scree_elbow_finds_low_rank_structure():
+    """scree_elbow returns a small rank for data with a few dominant axes.
+
+    Defined last: it runs TensorFlow ops (see the note above).
+    """
+    from locator.pca import scree_elbow
+
+    rng = np.random.default_rng(0)
+    n, p, k = 60, 2000, 3
+    factors = rng.standard_normal((n, k))
+    loadings = rng.standard_normal((k, p))
+    X = factors @ loadings * 10.0 + rng.standard_normal((n, p))
+
+    rank = scree_elbow(X)
+    assert 2 <= rank <= 8