diff --git a/src/index.ts b/src/index.ts
index 58dd2d0..e556a00 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -4,6 +4,7 @@ import { FunctionDef, formatFunctionDefinitions } from "./functions";
 
 type Message = OpenAI.Chat.CreateChatCompletionRequestMessage;
 type Function = OpenAI.Chat.CompletionCreateParams.Function;
+type FunctionCall = OpenAI.Chat.CompletionCreateParams.FunctionCallOption;
 
 let encoder: Tiktoken | undefined;
 
@@ -17,9 +18,11 @@ let encoder: Tiktoken | undefined;
 export function promptTokensEstimate({
   messages,
   functions,
+  function_call,
 }: {
   messages: Message[];
   functions?: Function[];
+  function_call?: 'none' | 'auto' | FunctionCall;
 }): number {
   // It appears that if functions are present, the first system message is padded with a trailing newline. This
   // was inferred by trying lots of combinations of messages and functions and seeing what the token counts were.
@@ -49,6 +52,13 @@ export function promptTokensEstimate({
     tokens -= 4;
   }
 
+  // If function_call is 'none', add one token.
+  // If it's a FunctionCall object, add 4 + the number of tokens in the function name.
+  // If it's undefined or 'auto', don't add anything.
+  if (function_call && function_call !== 'auto') {
+    tokens += function_call === 'none' ? 1 : stringTokens(function_call.name) + 4;
+  }
+
   return tokens;
 }
 
diff --git a/tests/token-counts.test.ts b/tests/token-counts.test.ts
index 2d9ec03..3bc51d0 100644
--- a/tests/token-counts.test.ts
+++ b/tests/token-counts.test.ts
@@ -3,9 +3,11 @@ import { promptTokensEstimate } from "../src";
 
 type Message = OpenAI.Chat.CreateChatCompletionRequestMessage;
 type Function = OpenAI.Chat.CompletionCreateParams.Function;
+type FunctionCall = OpenAI.Chat.CompletionCreateParams.FunctionCallOption;
 type Example = {
   messages: Message[];
   functions?: Function[];
+  function_call?: "none" | "auto" | FunctionCall;
   tokens: number;
   validate?: boolean;
 };
@@ -109,6 +111,39 @@ const TEST_CASES: Example[] = [
     ],
     tokens: 31,
   },
+  {
+    messages: [{ role: "user", content: "hello" }],
+    functions: [
+      {
+        name: "foo",
+        parameters: { type: "object", properties: {} },
+      },
+    ],
+    function_call: "none",
+    tokens: 32,
+  },
+  {
+    messages: [{ role: "user", content: "hello" }],
+    functions: [
+      {
+        name: "foo",
+        parameters: { type: "object", properties: {} },
+      },
+    ],
+    function_call: "auto",
+    tokens: 31,
+  },
+  {
+    messages: [{ role: "user", content: "hello" }],
+    functions: [
+      {
+        name: "foo",
+        parameters: { type: "object", properties: {} },
+      },
+    ],
+    function_call: { name: "foo" },
+    tokens: 36,
+  },
   {
     messages: [{ role: "user", content: "hello" }],
     functions: [
@@ -263,6 +298,31 @@ const TEST_CASES: Example[] = [
     ],
     tokens: 40,
   },
+  {
+    messages: [
+      { role: "system", content: "Hello:" },
+      { role: "system", content: "Hello" },
+      { role: "user", content: "Hi there" },
+    ],
+    functions: [
+      { name: "do_stuff", parameters: { type: "object", properties: {} } },
+      { name: "do_other_stuff", parameters: { type: "object", properties: {} } },
+    ],
+    tokens: 49,
+  },
+  {
+    messages: [
+      { role: "system", content: "Hello:" },
+      { role: "system", content: "Hello" },
+      { role: "user", content: "Hi there" },
+    ],
+    functions: [
+      { name: "do_stuff", parameters: { type: "object", properties: {} } },
+      { name: "do_other_stuff", parameters: { type: "object", properties: {} } },
+    ],
+    function_call: { name: "do_stuff" },
+    tokens: 55,
+  },
   {
     messages: [{ role: "user", content: "hello" }],
     functions: [
@@ -394,6 +454,7 @@ describe.each(TEST_CASES)("token counts (%j)", (example) => {
         model: "gpt-3.5-turbo",
         messages: example.messages,
         functions: example.functions as any,
+        function_call: example.function_call,
         max_tokens: 10,
       });
       expect(response.usage?.prompt_tokens).toBe(example.tokens);
@@ -406,6 +467,7 @@ describe.each(TEST_CASES)("token counts (%j)", (example) => {
       promptTokensEstimate({
         messages: example.messages,
         functions: example.functions,
+        function_call: example.function_call,
       }),
     ).toBe(example.tokens);
   });