diff --git a/src/observable/map/BaseObservableMap.ts b/src/observable/map/BaseObservableMap.ts
index f81a94f8..c82e531c 100644
--- a/src/observable/map/BaseObservableMap.ts
+++ b/src/observable/map/BaseObservableMap.ts
@@ -18,6 +18,7 @@ import {BaseObservable} from "../BaseObservable";
 import {JoinedMap} from "./index";
 import {MappedMap} from "./index";
 import {FilteredMap} from "./index";
+import {BaseObservableValue, MapSizeObservableValue} from "../value/index";
 import {SortedMapList} from "../list/SortedMapList.js";
 
 
@@ -66,19 +67,23 @@ export abstract class BaseObservableMap<K, V> extends BaseObservable<IMapObserve
 
     join<E extends BaseObservableMap<K, V>>(...otherMaps: Array<E>): JoinedMap<K, V> {
         return new JoinedMap([this as BaseObservableMap<K, V>].concat(otherMaps));
-     }
+    }
 
-     mapValues<MappedV>(mapper: Mapper<V, MappedV>, updater?: Updater<V, MappedV>): MappedMap<K, V, MappedV> {
-         return new MappedMap(this, mapper, updater);
-     }
+    mapValues<MappedV>(mapper: Mapper<V, MappedV>, updater?: Updater<V, MappedV>): MappedMap<K, V, MappedV> {
+        return new MappedMap(this, mapper, updater);
+    }
 
-     sortValues(comparator: Comparator<V>): SortedMapList {
-         return new SortedMapList(this, comparator);
-     }
+    sortValues(comparator: Comparator<V>): SortedMapList {
+        return new SortedMapList(this, comparator);
+    }
 
-     filterValues(filter: Filter<K, V>): FilteredMap<K, V> {
-         return new FilteredMap(this, filter);
-     }
+    filterValues(filter: Filter<K, V>): FilteredMap<K, V> {
+        return new FilteredMap(this, filter);
+    }
+
+    observeSize(): BaseObservableValue<number> {
+        return new MapSizeObservableValue(this);
+    }
 
     abstract [Symbol.iterator](): Iterator<[K, V]>;
     abstract get size(): number;
@@ -94,4 +99,4 @@ export type Updater<V, MappedV> = (params: any, mappedValue?: MappedV, value?: V
 
 export type Comparator<V> = (a: V, b: V) => number;
 
-export type Filter<K, V> = (v: V, k: K) => boolean;
\ No newline at end of file
+export type Filter<K, V> = (v: V, k: K) => boolean;
diff --git a/src/observable/value/MapSizeObservableValue.ts b/src/observable/value/MapSizeObservableValue.ts
new file mode 100644
index 00000000..13bac604
--- /dev/null
+++ b/src/observable/value/MapSizeObservableValue.ts
@@ -0,0 +1,71 @@
+/*
+Copyright 2022 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import {BaseObservableValue} from "./index";
+import {BaseObservableMap} from "../map/index";
+import type {SubscriptionHandle} from "../BaseObservable";
+
+export class MapSizeObservableValue<K, V> extends BaseObservableValue<number> {
+    private subscription?: SubscriptionHandle;
+
+    constructor(private readonly map: BaseObservableMap<K, V>)
+    {
+        super();
+    }
+
+    onSubscribeFirst(): void {
+        this.subscription = this.map.subscribe({
+            onAdd: (key: K, value: V) => {
+                this.emit(this.get());
+            },
+            onRemove: (key: K, value: V) => {
+                this.emit(this.get());
+            },
+            onUpdate: (key: K, value: V) => {},
+            onReset: () => {
+                this.emit(this.get());
+            },
+        });
+    }
+
+    onUnsubscribeLast(): void {
+        this.subscription = this.subscription?.();
+    }
+
+    get(): number {
+        return this.map.size;
+    }
+}
+
+import {ObservableMap} from "../map/index";
+
+export function tests() {
+    return {
+        "emits update on add and remove": assert => {
+            const map = new ObservableMap<string, number>();
+            const size = new MapSizeObservableValue(map);
+            const updates: number[] = [];
+            size.subscribe(size => {
+                updates.push(size);
+            });
+            map.add("hello", 1);
+            map.add("world", 2);
+            map.remove("world");
+            map.remove("hello");
+            assert.deepEqual(updates, [1, 2, 1, 0]);
+        }
+    };
+}
diff --git a/src/observable/value/index.ts b/src/observable/value/index.ts
index ceb53628..1b0f1347 100644
--- a/src/observable/value/index.ts
+++ b/src/observable/value/index.ts
@@ -12,4 +12,5 @@ export {EventObservableValue} from './EventObservableValue';
 export {FlatMapObservableValue} from './FlatMapObservableValue';
 export {PickMapObservableValue} from './PickMapObservableValue';
 export {RetainedObservableValue} from './RetainedObservableValue';
+export {MapSizeObservableValue} from './MapSizeObservableValue';
 export {ObservableValue} from './ObservableValue';