diff --git a/app.py b/app.py
index ab00382..1416e8b 100644
--- a/app.py
+++ b/app.py
@@ -172,6 +172,12 @@ class H(BaseHTTPRequestHandler):
                 req = json.loads(body or b"{}")
                 with LOCK: out = L.set_budget(req["month"], req["budgets"])
                 return self._send(200, {"message": f"Saved the {req['month']} budget ({out['rows']} envelopes). Backup: {out['backup']}."})
+            if u.path == "/api/add_category":
+                try:
+                    with LOCK: out = L.add_category(json.loads(body or b"{}").get("name"))
+                except ValueError as e: return self._send(400, {"error": str(e)})
+                note = " It will show on the web pages; the Excel summary tabs only have 20 category slots." if out["beyond_excel"] else ""
+                return self._send(200, {"message": f"Added category '{out['name']}'.{note} Backup: {out['backup']}.", "name": out["name"]})
             if u.path == "/api/set_categories":
                 with LOCK: out = L.set_categories(json.loads(body or b"[]"))
                 return self._send(200, {"message": f"Updated {out['updated']} payee(s). Backup: {out['backup']}."})
diff --git a/app_budget.html b/app_budget.html
index 68cacb2..248d91e 100644
--- a/app_budget.html
+++ b/app_budget.html
@@ -36,6 +36,7 @@ input.amt{width:110px;text-align:right}
     <div class="spacer"></div>
     <button id="copy" title="Use last month's amounts">Copy last month</button>
     <button id="suggest" title="Average of the last 3 months of spending">Suggest from spending</button>
+    <button id="addenv" title="Add a new category / envelope">Add envelope</button>
     <button id="movebtn">Move money</button>
     <button class="primary" id="save" disabled>Save budget</button>
   </div>
@@ -105,6 +106,11 @@ $("#showall").onchange=render;
 $("#copy").onclick=async()=>{const p=await (await fetch("/api/budget?month="+add(month,-1))).json();p.envelopes.forEach(e=>{const r=rows.find(x=>x.envelope==e.envelope);if(r)r.edit=e.budgeted});dirty=true;render()};
 $("#suggest").onclick=()=>{rows.forEach(r=>{if(r.suggest)r.edit=r.suggest});dirty=true;render()};
 $("#movebtn").onclick=()=>{const o=rows.map(r=>`<option>${esc(r.envelope)}</option>`).join("");$("#mfrom").innerHTML=o;$("#mto").innerHTML=o;$("#movepanel").style.display="block"};
+$("#addenv").onclick=async()=>{
+  const name=(prompt("Name for the new envelope (category):")||"").trim();if(!name)return;
+  try{const r=await fetch("/api/add_category",{method:"POST",headers:{"X-Token":TOKEN},body:JSON.stringify({name})});const j=await r.json();if(!r.ok)throw new Error(j.error);
+    rows.push({envelope:j.name,budgeted:0,spent:0,prior:0,available:0,suggest:0,edit:0});$("#showall").checked=true;render();$("#msg").innerHTML=`<div class="alert">${esc(j.message)}</div>`}
+  catch(e){$("#msg").innerHTML=`<div class="alert bad">${esc(e.message)}</div>`}};
 $("#mcancel").onclick=()=>$("#movepanel").style.display="none";
 $("#mgo").onclick=()=>{const f=rows.find(r=>r.envelope==$("#mfrom").value),t=rows.find(r=>r.envelope==$("#mto").value),a=+$("#mamt").value;
   if(!f||!t||f===t||!(a>0))return;f.edit=Math.max(0,(+f.edit||0)-a);t.edit=(+t.edit||0)+a;dirty=true;$("#movepanel").style.display="none";$("#mamt").value="";$("#showall").checked||(0);render()};
diff --git a/app_page.html b/app_page.html
index c6fb3ca..6d8ab6a 100644
--- a/app_page.html
+++ b/app_page.html
@@ -55,7 +55,7 @@ th{font-size:12px;color:var(--mute);font-weight:600}td.num{text-align:right;font
   </div>
   <div class="card" id="catcard">
     <div class="row"><h2 style="margin:0">4 · Categorize payees</h2><div class="spacer"></div>
-      <button id="auto">Auto-categorize</button><button class="primary" id="savecats" disabled>Save changes</button></div>
+      <button id="addcat">Add category</button><button id="auto">Auto-categorize</button><button class="primary" id="savecats" disabled>Save changes</button></div>
     <div id="catsum" class="mute" style="margin:10px 0"></div>
     <div class="grid" style="max-height:420px;overflow:auto"><table id="cattbl"></table></div>
   </div>
@@ -118,6 +118,7 @@ async function loadPayees(){
   $("#cattbl").innerHTML=j.payees.length?"<tr><th>Payee</th><th class='num'>Payments</th><th class='num'>Total</th><th>Category</th><th>Type</th></tr>"+j.payees.map((p,i)=>`<tr><td>${esc(p.payee)}</td><td class="num">${p.n}</td><td class="num">${fmt(p.total)}</td><td><select data-i="${i}" data-k="category">${opt(j.categories,p.category)}</select></td><td><select data-i="${i}" data-k="type">${opt(j.types,p.type)}</select></td></tr>`).join(""):"";
   document.querySelectorAll("#cattbl select").forEach(sel=>sel.onchange=()=>{const p=j.payees[+sel.dataset.i];(EDITS[p.payee]=EDITS[p.payee]||{payee:p.payee,category:p.category,type:p.type})[sel.dataset.k]=sel.value;$("#savecats").disabled=false});
 }
+$("#addcat").onclick=async()=>{const name=(prompt("Name for the new category:")||"").trim();if(!name)return;try{const j=await post("/api/add_category",JSON.stringify({name}));await loadPayees();alertBox("good",esc(j.message))}catch(e){alertBox("bad",esc(e.message))}};
 $("#auto").onclick=async()=>{try{const j=await post("/api/autocategorize","");await loadPayees();alertBox("good",esc(j.message))}catch(e){alertBox("bad",esc(e.message))}};
 $("#savecats").onclick=async()=>{try{const j=await post("/api/set_categories",JSON.stringify(Object.values(EDITS)));await loadPayees();alertBox("good",esc(j.message))}catch(e){alertBox("bad",esc(e.message))}};
 refresh();loadPayees();
diff --git a/statement_loader.py b/statement_loader.py
index 55ce06b..32cef4d 100644
--- a/statement_loader.py
+++ b/statement_loader.py
@@ -487,6 +487,25 @@ def set_categories(changes, workbook=WORKBOOK):
     _save_workbook(wb, workbook)
     return dict(updated=n, backup=b.name)
 
+def add_category(name, workbook=WORKBOOK):
+    """Add a category (envelope) to the Lists tab: first free slot in E4:E23, else E25:E40 (the web pages show them all;
+    the Excel summary tabs only have 20 slots)."""
+    from openpyxl import load_workbook
+    from copy import copy
+    name = re.sub(r"\s+", " ", (name or "")).strip()
+    if not name or len(name) > 40: raise ValueError("Give the category a name (40 characters or fewer).")
+    b = _backup(workbook); wb = load_workbook(workbook); ls = wb["Lists"]
+    if name.lower() in {str(c).lower() for c in _categories(wb)}: raise ValueError(f"'{name}' already exists.")
+    row = next((r for r in list(range(4, 24)) + list(range(25, 41)) if ls.cell(r, 5).value is None), None)
+    if row is None: raise ValueError("The Lists tab has no free category slots left.")
+    cell = ls.cell(row, 5); cell.value = name
+    src = ls.cell(4, 5); cell.font, cell.border, cell.alignment, cell.fill = copy(src.font), copy(src.border), copy(src.alignment), copy(src.fill)
+    if row > 23:
+        for dv in ls.data_validations.dataValidation:
+            if dv.formula1 == "$E$4:$E$23": dv.formula1 = "$E$4:$E$40"
+    _save_workbook(wb, workbook)
+    return dict(name=name, backup=b.name, beyond_excel=row > 23)
+
 def autocategorize(workbook=WORKBOOK):
     """Apply the built-in merchant rules to payees still set to Other / Unsorted."""
     from openpyxl import load_workbook