diff --git a/src/equality-map.ts b/src/equality-map.ts
index c91d105..4c962e0 100644
--- a/src/equality-map.ts
+++ b/src/equality-map.ts
@@ -149,6 +149,7 @@ export function createEqualityMap<K, V>(keyComparer?: Equater<K>): EqualityMap<K
 }
 
 export interface AsyncEqualityMap<K, V> extends Iterable<[K, V]> {
+  readonly size: number;
   get(key: K): Promise<V | undefined>;
   set(key: K, value: V): Promise<V | undefined>;
   contains(key: K): Promise<boolean>;
@@ -162,6 +163,10 @@ export interface AsyncEqualityMap<K, V> extends Iterable<[K, V]> {
 class NativeAsyncEqualityMap<K, V> implements AsyncEqualityMap<K, V> {
   readonly #map = new Map<K, V>();
 
+  get size() {
+    return this.#map.size;
+  }
+
   async get(key: K) {
     return this.#map.get(key);
   }
@@ -211,6 +216,10 @@ class CustomAsyncEqualityMap<K, V> implements AsyncEqualityMap<K, V> {
     this.#keyComparer = keyComparer;
   }
 
+  get size() {
+    return this.#list.length;
+  }
+
   async get(key: K) {
     for (const entry of this.#list) {
       if (await this.#keyComparer(key, entry[0])) {