diff --git a/app/routes/api.py b/app/routes/api.py
index d415d4a..f8e83c3 100644
--- a/app/routes/api.py
+++ b/app/routes/api.py
@@ -1,7 +1,7 @@
-from flask import Blueprint, jsonify
+from flask import Blueprint, jsonify, session
 from app import SYDNEY_TZ
 from app.auth import login_required
-from app.db import query
+from app.db import query, query_one
 from datetime import timezone
 
 bp = Blueprint("api", __name__, url_prefix="/api")
@@ -11,6 +11,12 @@ bp = Blueprint("api", __name__, url_prefix="/api")
 @login_required
 def chart_data(user_id):
     """Return weight & BMI over time for Chart.js."""
+    # Privacy guard: don't expose private user data to others
+    if user_id != session.get("user_id"):
+        target = query_one("SELECT is_private FROM users WHERE id = %s", (user_id,))
+        if target and target["is_private"]:
+            return jsonify({"labels": [], "weights": [], "bmis": []})
+
     checkins = query(
         """SELECT weight_kg, bmi, checked_in_at
            FROM checkins WHERE user_id = %s
@@ -43,6 +49,7 @@ def comparison():
             (SELECT weight_kg FROM checkins WHERE user_id = u.id ORDER BY checked_in_at DESC LIMIT 1) as current_weight
         FROM users u
         WHERE (SELECT COUNT(*) FROM checkins WHERE user_id = u.id) > 0
+          AND u.is_private = FALSE
         ORDER BY u.display_name
     """)
 
@@ -74,6 +81,12 @@ def comparison():
 @login_required
 def weekly_change(user_id):
     """Return weekly weight changes for bar chart."""
+    # Privacy guard: don't expose private user data to others
+    if user_id != session.get("user_id"):
+        target = query_one("SELECT is_private FROM users WHERE id = %s", (user_id,))
+        if target and target["is_private"]:
+            return jsonify({"labels": [], "changes": []})
+
     checkins = query(
         """SELECT weight_kg, checked_in_at
            FROM checkins WHERE user_id = %s
diff --git a/app/routes/auth.py b/app/routes/auth.py
index 6a93d92..b22267b 100644
--- a/app/routes/auth.py
+++ b/app/routes/auth.py
@@ -16,6 +16,7 @@ def signup():
         gender = request.form.get("gender") or None
         goal_weight_kg = request.form.get("goal_weight_kg") or None
         starting_weight_kg = request.form.get("starting_weight_kg") or None
+        is_private = request.form.get("is_private") == "on"
 
         # Validation
         if not username or not password:
@@ -35,9 +36,9 @@ def signup():
         # Create user
         password_hash = generate_password_hash(password)
         user = execute_returning(
-            """INSERT INTO users (username, password_hash, display_name, height_cm, age, gender, goal_weight_kg, starting_weight_kg)
-               VALUES (%s, %s, %s, %s, %s, %s, %s, %s) RETURNING id""",
-            (username, password_hash, display_name or username, height_cm, age, gender, goal_weight_kg, starting_weight_kg),
+            """INSERT INTO users (username, password_hash, display_name, height_cm, age, gender, goal_weight_kg, starting_weight_kg, is_private)
+               VALUES (%s, %s, %s, %s, %s, %s, %s, %s, %s) RETURNING id""",
+            (username, password_hash, display_name or username, height_cm, age, gender, goal_weight_kg, starting_weight_kg, is_private),
         )
 
         session["user_id"] = user["id"]
diff --git a/app/routes/dashboard.py b/app/routes/dashboard.py
index 291fd1a..944c603 100644
--- a/app/routes/dashboard.py
+++ b/app/routes/dashboard.py
@@ -48,9 +48,10 @@ def index():
         SELECT c.*, u.display_name, u.username
         FROM checkins c
         JOIN users u ON c.user_id = u.id
+        WHERE u.is_private = FALSE OR u.id = %s
         ORDER BY c.checked_in_at DESC
         LIMIT 10
-    """)
+    """, (user["id"],))
 
     # Milestones
     milestones = query(
diff --git a/app/routes/leaderboard.py b/app/routes/leaderboard.py
index 3577b14..2fce8bb 100644
--- a/app/routes/leaderboard.py
+++ b/app/routes/leaderboard.py
@@ -21,6 +21,7 @@ def index():
             (SELECT COUNT(*) FROM checkins WHERE user_id = u.id) as total_checkins,
             (SELECT checked_in_at FROM checkins WHERE user_id = u.id ORDER BY checked_in_at DESC LIMIT 1) as last_checkin
         FROM users u
+        WHERE u.is_private = FALSE
         ORDER BY u.created_at
     """)
 
diff --git a/app/routes/profile.py b/app/routes/profile.py
index 44ddeb8..c00a493 100644
--- a/app/routes/profile.py
+++ b/app/routes/profile.py
@@ -22,14 +22,15 @@ def update():
     gender = request.form.get("gender") or None
     goal_weight_kg = request.form.get("goal_weight_kg") or None
     starting_weight_kg = request.form.get("starting_weight_kg") or None
+    is_private = request.form.get("is_private") == "on"
 
     execute(
         """UPDATE users
            SET display_name = %s, height_cm = %s, age = %s, gender = %s,
-               goal_weight_kg = %s, starting_weight_kg = %s
+               goal_weight_kg = %s, starting_weight_kg = %s, is_private = %s
            WHERE id = %s""",
         (display_name or user["username"], height_cm, age, gender,
-         goal_weight_kg, starting_weight_kg, user["id"]),
+         goal_weight_kg, starting_weight_kg, is_private, user["id"]),
     )
 
     if request.headers.get("HX-Request"):
diff --git a/app/static/css/style.css b/app/static/css/style.css
index 76b007d..9a73c20 100644
--- a/app/static/css/style.css
+++ b/app/static/css/style.css
@@ -43,7 +43,9 @@
     --shadow-glow: 0 0 20px rgba(59, 130, 246, 0.15);
 }
 
-*, *::before, *::after {
+*,
+*::before,
+*::after {
     margin: 0;
     padding: 0;
     box-sizing: border-box;
@@ -88,7 +90,9 @@ body {
     font-size: 1.15rem;
 }
 
-.nav-icon { font-size: 1.4rem; }
+.nav-icon {
+    font-size: 1.4rem;
+}
 
 .nav-links {
     display: flex;
@@ -118,7 +122,9 @@ body {
     background: var(--accent-glow);
 }
 
-.nav-link-icon { font-size: 1.1rem; }
+.nav-link-icon {
+    font-size: 1.1rem;
+}
 
 .nav-toggle {
     display: none;
@@ -139,7 +145,10 @@ body {
     transition: all 0.2s;
 }
 
-.nav-actions { display: flex; align-items: center; }
+.nav-actions {
+    display: flex;
+    align-items: center;
+}
 
 /* ========== CONTAINER ========== */
 .container {
@@ -149,7 +158,9 @@ body {
 }
 
 /* ========== FLASH MESSAGES ========== */
-.flash-messages { margin-bottom: 1rem; }
+.flash-messages {
+    margin-bottom: 1rem;
+}
 
 .flash {
     padding: 0.75rem 1rem;
@@ -162,9 +173,23 @@ body {
     animation: slideIn 0.3s ease;
 }
 
-.flash-success { background: var(--success-bg); color: var(--success); border: 1px solid rgba(16, 185, 129, 0.2); }
-.flash-error { background: var(--danger-bg); color: var(--danger); border: 1px solid rgba(239, 68, 68, 0.2); }
-.flash-info { background: rgba(6, 182, 212, 0.1); color: var(--info); border: 1px solid rgba(6, 182, 212, 0.2); }
+.flash-success {
+    background: var(--success-bg);
+    color: var(--success);
+    border: 1px solid rgba(16, 185, 129, 0.2);
+}
+
+.flash-error {
+    background: var(--danger-bg);
+    color: var(--danger);
+    border: 1px solid rgba(239, 68, 68, 0.2);
+}
+
+.flash-info {
+    background: rgba(6, 182, 212, 0.1);
+    color: var(--info);
+    border: 1px solid rgba(6, 182, 212, 0.2);
+}
 
 .flash-close {
     background: none;
@@ -175,7 +200,9 @@ body {
     opacity: 0.7;
 }
 
-.flash-close:hover { opacity: 1; }
+.flash-close:hover {
+    opacity: 1;
+}
 
 /* ========== PAGE HEADERS ========== */
 .page-header {
@@ -263,7 +290,9 @@ body {
     transition: opacity 0.2s;
 }
 
-.stat-card:hover::before { opacity: 1; }
+.stat-card:hover::before {
+    opacity: 1;
+}
 
 .stat-label {
     font-size: 0.75rem;
@@ -287,8 +316,13 @@ body {
     margin-top: 0.25rem;
 }
 
-.stat-change.positive { color: var(--success); }
-.stat-change.negative { color: var(--danger); }
+.stat-change.positive {
+    color: var(--success);
+}
+
+.stat-change.negative {
+    color: var(--danger);
+}
 
 /* ========== FORMS ========== */
 .form-group {
@@ -321,7 +355,9 @@ body {
     box-shadow: 0 0 0 3px var(--accent-glow);
 }
 
-.form-input::placeholder { color: var(--text-muted); }
+.form-input::placeholder {
+    color: var(--text-muted);
+}
 
 select.form-input {
     appearance: none;
@@ -343,7 +379,10 @@ select.form-input {
     align-items: flex-end;
 }
 
-.form-inline .form-group { flex: 1; margin-bottom: 0; }
+.form-inline .form-group {
+    flex: 1;
+    margin-bottom: 0;
+}
 
 /* ========== BUTTONS ========== */
 .btn {
@@ -396,9 +435,19 @@ select.form-input {
     background: var(--bg-card);
 }
 
-.btn-sm { padding: 0.4rem 0.8rem; font-size: 0.8rem; }
-.btn-lg { padding: 0.85rem 1.75rem; font-size: 1rem; }
-.btn-block { width: 100%; }
+.btn-sm {
+    padding: 0.4rem 0.8rem;
+    font-size: 0.8rem;
+}
+
+.btn-lg {
+    padding: 0.85rem 1.75rem;
+    font-size: 1rem;
+}
+
+.btn-block {
+    width: 100%;
+}
 
 /* ========== TABLES ========== */
 .table-wrap {
@@ -431,9 +480,13 @@ td {
     color: var(--text-secondary);
 }
 
-tr:last-child td { border-bottom: none; }
+tr:last-child td {
+    border-bottom: none;
+}
 
-tr:hover td { background: var(--bg-card-hover); }
+tr:hover td {
+    background: var(--bg-card-hover);
+}
 
 /* ========== LEADERBOARD ========== */
 .rank-badge {
@@ -447,10 +500,25 @@ tr:hover td { background: var(--bg-card-hover); }
     font-size: 0.8rem;
 }
 
-.rank-1 { background: linear-gradient(135deg, #fbbf24, #f59e0b); color: #1a1a1a; }
-.rank-2 { background: linear-gradient(135deg, #cbd5e1, #94a3b8); color: #1a1a1a; }
-.rank-3 { background: linear-gradient(135deg, #d97706, #b45309); color: white; }
-.rank-other { background: var(--bg-secondary); color: var(--text-muted); }
+.rank-1 {
+    background: linear-gradient(135deg, #fbbf24, #f59e0b);
+    color: #1a1a1a;
+}
+
+.rank-2 {
+    background: linear-gradient(135deg, #cbd5e1, #94a3b8);
+    color: #1a1a1a;
+}
+
+.rank-3 {
+    background: linear-gradient(135deg, #d97706, #b45309);
+    color: white;
+}
+
+.rank-other {
+    background: var(--bg-secondary);
+    color: var(--text-muted);
+}
 
 .progress-bar-track {
     width: 100%;
@@ -476,7 +544,9 @@ tr:hover td { background: var(--bg-card-hover); }
     border-bottom: 1px solid var(--border);
 }
 
-.activity-item:last-child { border-bottom: none; }
+.activity-item:last-child {
+    border-bottom: none;
+}
 
 .activity-avatar {
     width: 32px;
@@ -492,7 +562,9 @@ tr:hover td { background: var(--bg-card-hover); }
     flex-shrink: 0;
 }
 
-.activity-content { flex: 1; }
+.activity-content {
+    flex: 1;
+}
 
 .activity-name {
     font-weight: 600;
@@ -594,7 +666,9 @@ tr:hover td { background: var(--bg-card-hover); }
     font-weight: 500;
 }
 
-.auth-card .auth-footer a:hover { text-decoration: underline; }
+.auth-card .auth-footer a:hover {
+    text-decoration: underline;
+}
 
 /* ========== GRID LAYOUTS ========== */
 .grid-2 {
@@ -611,17 +685,34 @@ tr:hover td { background: var(--bg-card-hover); }
 
 /* ========== ANIMATIONS ========== */
 @keyframes slideIn {
-    from { transform: translateY(-8px); opacity: 0; }
-    to { transform: translateY(0); opacity: 1; }
+    from {
+        transform: translateY(-8px);
+        opacity: 0;
+    }
+
+    to {
+        transform: translateY(0);
+        opacity: 1;
+    }
 }
 
 @keyframes fadeIn {
-    from { opacity: 0; }
-    to { opacity: 1; }
+    from {
+        opacity: 0;
+    }
+
+    to {
+        opacity: 1;
+    }
 }
 
-.htmx-added { animation: slideIn 0.3s ease; }
-.htmx-settling { animation: fadeIn 0.2s ease; }
+.htmx-added {
+    animation: slideIn 0.3s ease;
+}
+
+.htmx-settling {
+    animation: fadeIn 0.2s ease;
+}
 
 /* ========== DELETE BUTTON ========== */
 .btn-icon {
@@ -671,6 +762,58 @@ tr:hover td { background: var(--bg-card-hover); }
     margin: 0 auto 1rem;
 }
 
+/* ========== TOGGLE SWITCH ========== */
+.toggle-switch {
+    position: relative;
+    display: inline-block;
+    width: 44px;
+    height: 24px;
+    flex-shrink: 0;
+}
+
+.toggle-switch input {
+    opacity: 0;
+    width: 0;
+    height: 0;
+}
+
+.toggle-slider {
+    position: absolute;
+    inset: 0;
+    background: var(--bg-secondary);
+    border: 1px solid var(--border);
+    border-radius: 24px;
+    cursor: pointer;
+    transition: all 0.3s ease;
+}
+
+.toggle-slider::before {
+    content: '';
+    position: absolute;
+    left: 3px;
+    top: 50%;
+    transform: translateY(-50%);
+    width: 16px;
+    height: 16px;
+    background: var(--text-muted);
+    border-radius: 50%;
+    transition: all 0.3s ease;
+}
+
+.toggle-switch input:checked+.toggle-slider {
+    background: var(--accent);
+    border-color: var(--accent);
+}
+
+.toggle-switch input:checked+.toggle-slider::before {
+    left: 23px;
+    background: white;
+}
+
+.toggle-switch input:focus+.toggle-slider {
+    box-shadow: 0 0 0 3px var(--accent-glow);
+}
+
 /* ========== RESPONSIVE ========== */
 @media (max-width: 768px) {
     .navbar {
@@ -689,13 +832,21 @@ tr:hover td { background: var(--bg-card-hover); }
         padding: 0.5rem;
     }
 
-    .nav-links.open { display: flex; }
+    .nav-links.open {
+        display: flex;
+    }
 
-    .nav-toggle { display: flex; }
+    .nav-toggle {
+        display: flex;
+    }
 
-    .nav-actions { display: none; }
+    .nav-actions {
+        display: none;
+    }
 
-    .container { padding: 1rem; }
+    .container {
+        padding: 1rem;
+    }
 
     .stats-grid {
         grid-template-columns: repeat(2, 1fr);
@@ -705,7 +856,8 @@ tr:hover td { background: var(--bg-card-hover); }
         grid-template-columns: 1fr;
     }
 
-    .grid-2, .grid-3 {
+    .grid-2,
+    .grid-3 {
         grid-template-columns: 1fr;
     }
 
@@ -714,23 +866,35 @@ tr:hover td { background: var(--bg-card-hover); }
         align-items: stretch;
     }
 
-    .form-inline .form-group { margin-bottom: 0.75rem; }
+    .form-inline .form-group {
+        margin-bottom: 0.75rem;
+    }
 
-    .page-header h1 { font-size: 1.4rem; }
+    .page-header h1 {
+        font-size: 1.4rem;
+    }
 
-    .stat-value { font-size: 1.4rem; }
+    .stat-value {
+        font-size: 1.4rem;
+    }
 
-    .auth-card { margin: 1rem; padding: 1.5rem; }
+    .auth-card {
+        margin: 1rem;
+        padding: 1.5rem;
+    }
 
     .table-wrap {
         font-size: 0.8rem;
     }
 
-    th, td { padding: 0.5rem 0.65rem; }
+    th,
+    td {
+        padding: 0.5rem 0.65rem;
+    }
 }
 
 @media (max-width: 480px) {
     .stats-grid {
         grid-template-columns: 1fr;
     }
-}
+}
\ No newline at end of file
diff --git a/app/templates/profile.html b/app/templates/profile.html
index 5c002e0..5bbc214 100644
--- a/app/templates/profile.html
+++ b/app/templates/profile.html
@@ -50,6 +50,21 @@
             </div>
         </div>
 
+        <div class="form-group" style="margin-top: 0.75rem; padding-top: 0.75rem; border-top: 1px solid var(--border);">
+            <label class="toggle-label" for="is_private"
+                style="display: flex; align-items: center; gap: 0.75rem; cursor: pointer;">
+                <span class="toggle-switch">
+                    <input type="checkbox" id="is_private" name="is_private" {{ 'checked' if user.is_private }}>
+                    <span class="toggle-slider"></span>
+                </span>
+                <span>
+                    <span style="font-weight: 600; color: var(--text-primary);">🔒 Private Account</span>
+                    <span style="display: block; font-size: 0.8rem; color: var(--text-muted); margin-top: 0.15rem;">Only
+                        you can see your check-ins</span>
+                </span>
+            </label>
+        </div>
+
         <button type="submit" class="btn btn-primary" style="margin-top: 0.5rem;">Save Changes</button>
     </form>
 </div>
diff --git a/app/templates/signup.html b/app/templates/signup.html
index d41e562..1877aef 100644
--- a/app/templates/signup.html
+++ b/app/templates/signup.html
@@ -61,6 +61,22 @@
                     step="0.1">
             </div>
 
+            <div class="form-group" style="margin-top: 0.5rem;">
+                <label class="toggle-label" for="is_private"
+                    style="display: flex; align-items: center; gap: 0.75rem; cursor: pointer;">
+                    <span class="toggle-switch">
+                        <input type="checkbox" id="is_private" name="is_private">
+                        <span class="toggle-slider"></span>
+                    </span>
+                    <span>
+                        <span style="font-weight: 600; color: var(--text-primary);">🔒 Private Account</span>
+                        <span
+                            style="display: block; font-size: 0.8rem; color: var(--text-muted); margin-top: 0.15rem;">Only
+                            you can see your check-ins</span>
+                    </span>
+                </label>
+            </div>
+
             <button type="submit" class="btn btn-primary btn-block btn-lg" style="margin-top: 0.5rem;">Create
                 Account</button>
         </form>
diff --git a/migrations/002_add_private_flag.sql b/migrations/002_add_private_flag.sql
new file mode 100644
index 0000000..3d868cc
--- /dev/null
+++ b/migrations/002_add_private_flag.sql
@@ -0,0 +1,4 @@
+-- Migration 002: Add private account flag
+-- Allows users to hide their check-ins from other users
+
+ALTER TABLE users ADD COLUMN IF NOT EXISTS is_private BOOLEAN DEFAULT FALSE;