diff --git a/opacus/privacy_engine.py b/opacus/privacy_engine.py
index 9d8947c7..1436ac0d 100644
--- a/opacus/privacy_engine.py
+++ b/opacus/privacy_engine.py
@@ -410,6 +410,17 @@ def make_private(
         if poisson_sampling:
             module.forbid_grad_accumulation()
 
+        if hasattr(data_loader.dataset, '__len__'):
+            true_dataset_size = len(data_loader.dataset)S
+        else:
+            raise ValueError(
+                "Dataset must have __len__ for privacy accounting. "
+                "IterableDataset is not supported."
+    )
+        original_batch_size = data_loader.batch_size
+        if original_batch_size is None:
+            raise ValueError("DataLoader must have a batch_size")
+
         data_loader = self._prepare_data_loader(
             data_loader,
             distributed=distributed,
@@ -418,8 +429,8 @@ def make_private(
             rand_on_empty=rand_on_empty,
         )
 
-        sample_rate = 1 / len(data_loader)
-        expected_batch_size = int(len(data_loader.dataset) * sample_rate)
+        sample_rate = original_batch_size / true_dataset_size
+        expected_batch_size = int(true_dataset_size * sample_rate)
 
         # expected_batch_size is the *per worker* batch size
         if distributed:
diff --git a/opacus/tests/test_weighted_sampler.py b/opacus/tests/test_weighted_sampler.py
new file mode 100644
index 00000000..d58b0be9
--- /dev/null
+++ b/opacus/tests/test_weighted_sampler.py
@@ -0,0 +1,46 @@
+import torch
+from torch.utils.data import TensorDataset, DataLoader, WeightedRandomSampler
+from opacus import PrivacyEngine
+
+
+def test_weighted_sampler_privacy_accounting():
+    """
+    Test that WeightedRandomSampler doesn't break privacy accounting.
+    
+    Regression test for issue where sample_rate was computed from
+    sampler.num_samples instead of the true dataset size, causing
+    privacy budget to burn 100x-1000x faster than expected.
+    """
+    # Dataset with 100,000 samples
+    X = torch.randn(100_000, 10)
+    y = torch.randint(0, 2, (100_000,))
+    dataset = TensorDataset(X, y)
+    
+    # WeightedRandomSampler with only 128 samples per epoch
+    weights = torch.ones(100_000)
+    sampler = WeightedRandomSampler(weights, num_samples=128, replacement=True)
+    loader = DataLoader(dataset, batch_size=16, sampler=sampler)
+    
+    model = torch.nn.Linear(10, 2)
+    optimizer = torch.optim.SGD(model.parameters(), lr=0.01)
+    
+    privacy_engine = PrivacyEngine()
+    model, optimizer, loader = privacy_engine.make_private_with_epsilon(
+        module=model,
+        optimizer=optimizer,
+        data_loader=loader,
+        epochs=1,
+        target_epsilon=8.0,
+        target_delta=1e-5,
+        max_grad_norm=1.0,
+    )
+    
+    # Verify privacy accounting uses true dataset size
+    expected_sample_rate = 16 / 100_000  # 0.00016
+    actual_sample_rate = optimizer.expected_batch_size / len(loader.dataset)
+    
+    assert optimizer.expected_batch_size == 16, \
+        f"expected_batch_size should be 16, got {optimizer.expected_batch_size}"
+    
+    assert abs(actual_sample_rate - expected_sample_rate) < 1e-6, \
+        f"sample_rate should be {expected_sample_rate}, got {actual_sample_rate}"
\ No newline at end of file