diff --git a/README.md b/README.md
index ec2aab6..d418bad 100644
--- a/README.md
+++ b/README.md
@@ -214,6 +214,14 @@ declare const removeVertex: (graph: Graph, vertex: string) => Graph;
 
 Removes a vertex from a graph.
 
+### size
+
+```ts
+declare const size: (graph: Graph) => number;
+```
+
+Returns the number of edges in the graph.
+
 ### toD3
 
 ```ts
diff --git a/index.ts b/index.ts
index 1a8b5a7..eb032d2 100644
--- a/index.ts
+++ b/index.ts
@@ -180,6 +180,20 @@ const removeVertex = (graph: Graph, vertex: string): Graph => {
   return result;
 };
 
+const size = (graph: Graph): number => {
+  let result = 0;
+
+  for (let u in graph) {
+    for (let v in graph[u]) {
+      if (graph[u][v] > 0) {
+        result += 1;
+      }
+    }
+  }
+
+  return result;
+};
+
 const toD3 = (graph: Graph): D3Graph => {
   const nodes = [];
   const links = [];
@@ -258,6 +272,7 @@ export {
   outdegrees,
   removeEdge,
   removeVertex,
+  size,
   toD3,
   topologicalSort,
   transpose,
diff --git a/test.ts b/test.ts
index d5e14e2..3a81cde 100644
--- a/test.ts
+++ b/test.ts
@@ -11,6 +11,7 @@ import {
   outdegrees,
   removeEdge,
   removeVertex,
+  size,
   toD3,
   topologicalSort,
   transpose,
@@ -301,6 +302,29 @@ test("removeVertex", (t) => {
   );
 });
 
+test("size", (t) => {
+  t.plan(3);
+
+  t.equal(size({}), 0);
+
+  t.equal(
+    size({
+      a: { a: 1, b: 0 },
+      b: { a: 0, b: 0 },
+    }),
+    1,
+  );
+
+  t.equal(
+    size({
+      a: { a: 3, b: 0, c: 0 },
+      b: { a: 0, b: 0, c: 0 },
+      c: { a: 1, b: 0, c: 0 },
+    }),
+    2,
+  );
+});
+
 test("toD3", (t) => {
   t.plan(1);
   t.deepEqual(