diff --git a/api/controller.js b/api/controller.js
index 3e03a4b..31bf8b8 100644
--- a/api/controller.js
+++ b/api/controller.js
@@ -26,6 +26,10 @@ exports.calculate = function (req, res) {
     power: function (a, b) {
       return Math.pow(a, b);
     },
+    // natural logarithm (ln) - uses only operand1
+    ln: function (a) {
+      return Math.log(Number(a));
+    },
   };
 
   if (!req.query.operation) {
@@ -46,12 +50,15 @@ exports.calculate = function (req, res) {
     throw new Error("Invalid operand1: " + req.query.operand1);
   }
 
-  if (
-    !req.query.operand2 ||
-    !req.query.operand2.match(/^(-)?[0-9\.]+(e(-)?[0-9]+)?$/) ||
-    req.query.operand2.replace(/[-0-9e]/g, "").length > 1
-  ) {
-    throw new Error("Invalid operand2: " + req.query.operand2);
+  // For unary ops like ln we don't require operand2; for others we do
+  if (req.query.operation !== "ln") {
+    if (
+      !req.query.operand2 ||
+      !req.query.operand2.match(/^(-)?[0-9\.]+(e(-)?[0-9]+)?$/) ||
+      req.query.operand2.replace(/[-0-9e]/g, "").length > 1
+    ) {
+      throw new Error("Invalid operand2: " + req.query.operand2);
+    }
   }
 
   res.json({ result: operation(req.query.operand1, req.query.operand2) });
diff --git a/public/client.js b/public/client.js
index 9df4075..fc4db6a 100644
--- a/public/client.js
+++ b/public/client.js
@@ -35,6 +35,9 @@ function calculate(operand1, operand2, operation) {
     case "^":
       uri += "?operation=power";
       break;
+    case "ln":
+      uri += "?operation=ln";
+      break;
     default:
       setError();
       return;
@@ -113,8 +116,17 @@ function signPressed() {
 }
 
 function operationPressed(op) {
+  // store operand1
   operand1 = getValue();
   operation = op;
+
+  // ln is a unary operator (natural log) — compute immediately using operand1
+  if (op === "ln") {
+    state = states.complete;
+    calculate(operand1, 0, operation);
+    return;
+  }
+
   state = states.operator;
 }
 
diff --git a/public/index.html b/public/index.html
index 8fcfca5..5725dbb 100644
--- a/public/index.html
+++ b/public/index.html
@@ -43,6 +43,10 @@
             <button class="btn" onClick="numberPressed(0)">0</button>
             <button class="btn" onClick="decimalPressed()">.</button>
             <button class="btn" onClick="operationPressed('+')">+</button>
+
+            <!-- New natural log (ln) unary operator button -->
+            <button class="btn" onClick="operationPressed('ln')">ln</button>
+
             <button class="btn" onClick="operationPressed('^')">^</button>
 
             <button class="btn span-4" onClick="equalPressed()">=</button>