diff --git a/c_api/IndexBinary_c_ex.cpp b/c_api/IndexBinary_c_ex.cpp
index 2e00ee5e53..583cb325c8 100644
--- a/c_api/IndexBinary_c_ex.cpp
+++ b/c_api/IndexBinary_c_ex.cpp
@@ -10,6 +10,7 @@
 
 #include "IndexBinary_c_ex.h"
 #include <faiss/IndexBinary.h>
+#include <faiss/IndexBinaryIVF.h>
 #include "macros_impl.h"
 
 extern "C" {
@@ -33,9 +34,23 @@ int faiss_IndexBinary_search_with_params(
     CATCH_AND_HANDLE
 }
 
-size_t faiss_IndexBinary_size(FaissIndexBinary* index) {
-    auto xIndex = reinterpret_cast<faiss::IndexBinary*>(index);
-    size_t rv = sizeof(xIndex);
-    return rv;
+int faiss_IndexBinary_size(const FaissIndexBinary* index, size_t* p_size) {
+    try {
+        const faiss::IndexBinary* idx = reinterpret_cast<const faiss::IndexBinary*>(index);
+        // Base: raw binary codes (d / 8 bytes per vector).
+        size_t size = (size_t)idx->ntotal * idx->code_size;
+        // IVF-specific overhead not captured by code_size:
+        //   centroids: quantizer->ntotal * quantizer->sa_code_size()
+        //   stored IDs: ntotal * sizeof(idx_t)  (per-vector ID in each inverted list)
+        if (auto ivf = dynamic_cast<const faiss::IndexBinaryIVF*>(idx)) {
+            auto ivfQuantizer = ivf->quantizer;
+            if (ivfQuantizer != nullptr) {
+                size += (size_t)ivfQuantizer->ntotal * ivfQuantizer->sa_code_size();
+            }
+            size += (size_t)ivf->ntotal * sizeof(faiss::idx_t);
+        }
+        *p_size = size;
+    }
+    CATCH_AND_HANDLE
 }
 }
diff --git a/c_api/IndexBinary_c_ex.h b/c_api/IndexBinary_c_ex.h
index d43a3c6318..bd71d0cef7 100644
--- a/c_api/IndexBinary_c_ex.h
+++ b/c_api/IndexBinary_c_ex.h
@@ -38,11 +38,18 @@ int faiss_IndexBinary_search_with_params(
         int32_t* distances,
         idx_t* labels);
 
-/** return the size of the binary index
- * 
+/** return an approximate size estimate in bytes for the binary index.
+ * The estimate accounts for stored codes and, for IVF-based binary indices,
+ * includes centroid and stored ID overhead.
+ *
+ * This is not a complete in-memory footprint: it does not attempt to include
+ * all internal allocations such as inverted-list container overhead,
+ * direct_map, or quantizer internals beyond centroid storage.
+ *
  * @param index     opaque pointer to index object
+ * @param p_size    output approximate size in bytes
  */
-size_t faiss_IndexBinary_size(FaissIndexBinary* index);
+int faiss_IndexBinary_size(const FaissIndexBinary* index, size_t* p_size);
 
 #ifdef __cplusplus
 }
diff --git a/c_api/Index_c_ex.cpp b/c_api/Index_c_ex.cpp
index c5c1045142..6d299530d9 100644
--- a/c_api/Index_c_ex.cpp
+++ b/c_api/Index_c_ex.cpp
@@ -13,6 +13,7 @@
 #include "macros_impl.h"
 #include <faiss/IndexFlat.h>
 #include <faiss/IndexScalarQuantizer.h>
+#include <faiss/IndexIVF.h>
 
 extern "C" {
 
@@ -39,10 +40,24 @@ int faiss_Index_merge_from(
     CATCH_AND_HANDLE
 }
 
-size_t faiss_Index_size(FaissIndex* index) {
-    auto xIndex = reinterpret_cast<faiss::Index*>(index);
-    size_t rv = sizeof(xIndex);
-    return rv;
+int faiss_Index_size(const FaissIndex* index, size_t* p_size) {
+    try {
+        const faiss::Index* idx = reinterpret_cast<const faiss::Index*>(index);
+        // Base: raw vector codes (works for Flat, SQ, and all other types).
+        size_t size = (size_t)idx->ntotal * idx->sa_code_size();
+        // IVF-specific overhead not captured by sa_code_size():
+        //   centroids: quantizer->ntotal * quantizer->sa_code_size()
+        //   stored IDs: ntotal * sizeof(idx_t)  (per-vector ID in each inverted list)
+        if (auto ivf = dynamic_cast<const faiss::IndexIVF*>(idx)) {
+            auto ivfQuantizer = ivf->quantizer;
+            if (ivfQuantizer != nullptr) {
+                size += (size_t)ivfQuantizer->ntotal * ivfQuantizer->sa_code_size();
+            }
+            size += (size_t)ivf->ntotal * sizeof(faiss::idx_t);
+        }
+        *p_size = size;
+    }
+    CATCH_AND_HANDLE
 }
 
 int faiss_Index_dist_compute(
diff --git a/c_api/Index_c_ex.h b/c_api/Index_c_ex.h
index f8189f0844..5bc4f86e2f 100644
--- a/c_api/Index_c_ex.h
+++ b/c_api/Index_c_ex.h
@@ -28,7 +28,18 @@ int faiss_Index_reconstruct_batch(
 
 int faiss_Index_merge_from(FaissIndex* index, FaissIndex* other, idx_t add_id);
 
-size_t faiss_Index_size(FaissIndex* index);
+/** Estimate the size of the index in bytes.
+ *
+ * The returned value is an approximation based on the stored vector
+ * codes and any additional known overhead (for example centroids and
+ * stored IDs for IVF indices). It does not imply an exact total memory
+ * footprint, and may not be available for index types that do not
+ * support this estimate.
+ *
+ * @param index       opaque pointer to index object
+ * @param p_size      pointer to size_t to store the estimated size
+ */
+int faiss_Index_size(const FaissIndex* index, size_t* p_size);
 
 /** Compute distances between a query vector and a set of vectors 
  *