diff --git a/jvector-base/src/main/java/io/github/jbellis/jvector/quantization/ImmutablePQVectors.java b/jvector-base/src/main/java/io/github/jbellis/jvector/quantization/ImmutablePQVectors.java
index b1d538b3..0ac4c145 100644
--- a/jvector-base/src/main/java/io/github/jbellis/jvector/quantization/ImmutablePQVectors.java
+++ b/jvector-base/src/main/java/io/github/jbellis/jvector/quantization/ImmutablePQVectors.java
@@ -19,6 +19,8 @@
 import io.github.jbellis.jvector.vector.types.ByteSequence;
 
 public class ImmutablePQVectors extends PQVectors {
+    private final int vectorCount;
+
     /**
      * Construct an immutable PQVectors instance with the given ProductQuantization and compressed data chunks.
      * @param pq the ProductQuantization to use
@@ -37,4 +39,9 @@ public ImmutablePQVectors(ProductQuantization pq, ByteSequence<?>[] compressedDa
     protected int validChunkCount() {
         return compressedDataChunks.length;
     }
+
+    @Override
+    public int count() {
+        return vectorCount;
+    }
 }
diff --git a/jvector-base/src/main/java/io/github/jbellis/jvector/quantization/MutablePQVectors.java b/jvector-base/src/main/java/io/github/jbellis/jvector/quantization/MutablePQVectors.java
index 62e6a522..7627cf62 100644
--- a/jvector-base/src/main/java/io/github/jbellis/jvector/quantization/MutablePQVectors.java
+++ b/jvector-base/src/main/java/io/github/jbellis/jvector/quantization/MutablePQVectors.java
@@ -17,6 +17,7 @@
 package io.github.jbellis.jvector.quantization;
 
 import io.github.jbellis.jvector.vector.VectorizationProvider;
+import java.util.concurrent.atomic.AtomicInteger;
 import io.github.jbellis.jvector.vector.types.ByteSequence;
 import io.github.jbellis.jvector.vector.types.VectorFloat;
 import io.github.jbellis.jvector.vector.types.VectorTypeSupport;
@@ -30,6 +31,8 @@ public class MutablePQVectors extends PQVectors implements MutableCompressedVect
     private static final int INITIAL_CHUNKS = 10;
     private static final float GROWTH_FACTOR = 1.5f;
 
+    protected AtomicInteger vectorCount;
+
     /**
      * Construct a mutable PQVectors instance with the given ProductQuantization.
      * The vectors storage will grow dynamically as needed.
@@ -37,7 +40,7 @@ public class MutablePQVectors extends PQVectors implements MutableCompressedVect
      */
     public MutablePQVectors(ProductQuantization pq) {
         super(pq);
-        this.vectorCount = 0;
+        this.vectorCount = new AtomicInteger(0);
         this.vectorsPerChunk = VECTORS_PER_CHUNK;
         this.compressedDataChunks = new ByteSequence<?>[INITIAL_CHUNKS];
     }
@@ -45,14 +48,14 @@ public MutablePQVectors(ProductQuantization pq) {
     @Override
     public void encodeAndSet(int ordinal, VectorFloat<?> vector) {
         ensureChunkCapacity(ordinal);
-        vectorCount = max(vectorCount, ordinal + 1);
+        vectorCount.updateAndGet(current -> max(current, ordinal + 1));
         pq.encodeTo(vector, get(ordinal));
     }
 
     @Override
     public void setZero(int ordinal) {
         ensureChunkCapacity(ordinal);
-        vectorCount = max(vectorCount, ordinal + 1);
+        vectorCount.updateAndGet(current -> max(current, ordinal + 1));
         get(ordinal).zero();
     }
 
@@ -78,9 +81,14 @@ private void ensureChunkCapacity(int ordinal) {
 
     @Override
     protected int validChunkCount() {
-        if (vectorCount == 0)
+        if (vectorCount.get() == 0)
             return 0;
-        int chunkOrdinal = (vectorCount - 1) / vectorsPerChunk;
+        int chunkOrdinal = (vectorCount.get() - 1) / vectorsPerChunk;
         return chunkOrdinal + 1;
     }
+
+    @Override
+    public int count() {
+        return vectorCount.get();
+    }
 }
diff --git a/jvector-base/src/main/java/io/github/jbellis/jvector/quantization/PQVectors.java b/jvector-base/src/main/java/io/github/jbellis/jvector/quantization/PQVectors.java
index 897bc610..f6a41cef 100644
--- a/jvector-base/src/main/java/io/github/jbellis/jvector/quantization/PQVectors.java
+++ b/jvector-base/src/main/java/io/github/jbellis/jvector/quantization/PQVectors.java
@@ -41,7 +41,6 @@ public abstract class PQVectors implements CompressedVectors {
 
     final ProductQuantization pq;
     protected ByteSequence<?>[] compressedDataChunks;
-    protected int vectorCount;
     protected int vectorsPerChunk;
 
     protected PQVectors(ProductQuantization pq) {
@@ -155,11 +154,6 @@ public static ImmutablePQVectors encodeAndBuild(ProductQuantization pq, int vect
         return new ImmutablePQVectors(pq, chunks, vectorCount, vectorsPerChunk);
     }
 
-    @Override
-    public int count() {
-        return vectorCount;
-    }
-
     @Override
     public void write(DataOutput out, int version) throws IOException
     {
@@ -167,7 +161,7 @@ public void write(DataOutput out, int version) throws IOException
         pq.write(out, version);
 
         // compressed vectors
-        out.writeInt(vectorCount);
+        out.writeInt(count());
         out.writeInt(pq.getSubspaceCount());
         for (int i = 0; i < validChunkCount(); i++) {
             vectorTypeSupport.writeByteSequence(out, compressedDataChunks[i]);
@@ -286,8 +280,8 @@ public ScoreFunction.ApproximateScoreFunction scoreFunctionFor(VectorFloat<?> q,
     }
 
     public ByteSequence<?> get(int ordinal) {
-        if (ordinal < 0 || ordinal >= vectorCount)
-            throw new IndexOutOfBoundsException("Ordinal " + ordinal + " out of bounds for vector count " + vectorCount);
+        if (ordinal < 0 || ordinal >= count())
+            throw new IndexOutOfBoundsException("Ordinal " + ordinal + " out of bounds for vector count " + count());
         return get(compressedDataChunks, ordinal, vectorsPerChunk, pq.getSubspaceCount());
     }
 
@@ -341,7 +335,7 @@ public long ramBytesUsed() {
     public String toString() {
         return "PQVectors{" +
                 "pq=" + pq +
-                ", count=" + vectorCount +
+                ", count=" + count() +
                 '}';
     }
 }