src/quartermaster/month_service.py

@@ -42,6 +42,12 @@ class MonthSectionView:
     total_applied: Decimal
 
 
+@dataclass(frozen=True)
+class ZeroAmounts:
+    planned: Decimal
+    applied: Decimal
+
+
 def valid_year_month(year_month: str) -> bool:
     return bool(YEAR_MONTH_RE.match(year_month))
 
@@ -124,6 +130,24 @@ def _rows(entries: list[MonthEntry]) -> list[MonthRow]:
     return [MonthRow(entry=e, state=deviation_state(e)) for e in entries]
 
 
+def month_zero(month: Month) -> ZeroAmounts:
+    income_planned = Decimal("0")
+    income_applied = Decimal("0")
+    other_planned = Decimal("0")
+    other_applied = Decimal("0")
+    for entry in month.entries:
+        if entry.section == Section.income:
+            income_planned += entry.planned
+            income_applied += entry.applied
+        else:
+            other_planned += entry.planned
+            other_applied += entry.applied
+    return ZeroAmounts(
+        planned=(income_planned - other_planned).quantize(Decimal("0.01")),
+        applied=(income_applied - other_applied).quantize(Decimal("0.01")),
+    )
+
+
 def section_view(month: Month, section: Section, label: str) -> MonthSectionView:
     entries = [e for e in month.entries if e.section == section]
     entries.sort(key=lambda e: e.id)

src/quartermaster/routes.py

@@ -57,12 +57,27 @@ def _render_target(request: Request, db: Session) -> HTMLResponse:
     )
 
 
+def _render_zero(request: Request, db: Session) -> HTMLResponse:
+    zero = service.budget_zero(db)
+    return templates.TemplateResponse(
+        request,
+        "partials/budget_zero.html",
+        {"zero": zero, "tone": service.zero_tone(zero)},
+    )
+
+
+def _append_oob(base: HTMLResponse, *extras: HTMLResponse) -> HTMLResponse:
+    body = base.body.decode() + "".join(e.body.decode() for e in extras)
+    return HTMLResponse(body)
+
+
 @router.get("/", response_class=HTMLResponse)
 def index(request: Request, db: Session = Depends(get_session)) -> HTMLResponse:
     sections = [_section_view(db, s) for s in Section]
     target = service.get_debt_target(db)
     debt_minimums = service.list_entries(db, Section.debt_minimum)
     current_ym = month_service.current_year_month()
+    zero = service.budget_zero(db)
     return templates.TemplateResponse(
         request,
         "index.html",
@@ -72,6 +87,8 @@ def index(request: Request, db: Session = Depends(get_session)) -> HTMLResponse:
             "debt_minimums": debt_minimums,
             "current_year_month": current_ym,
             "all_months": month_service.list_months(db),
+            "zero": zero,
+            "tone": service.zero_tone(zero),
         },
     )
 
@@ -90,10 +107,10 @@ def create_entry(
     parsed = _parse_amount(amount)
     service.add_entry(db, section, clean_name, parsed)
     response = _render_section(request, db, section)
+    extras: list[HTMLResponse] = [_render_zero(request, db)]
     if section == Section.debt_minimum:
-        target_html = _render_target(request, db).body.decode()
-        response = HTMLResponse(response.body.decode() + target_html)
-    return response
+        extras.append(_render_target(request, db))
+    return _append_oob(response, *extras)
 
 
 @router.delete("/entries/{entry_id}", response_class=HTMLResponse)
@@ -106,10 +123,10 @@ def remove_entry(
     if entry is None:
         raise HTTPException(status_code=404, detail="entry not found")
     response = _render_section(request, db, entry.section)
+    extras: list[HTMLResponse] = [_render_zero(request, db)]
     if entry.section == Section.debt_minimum:
-        target_html = _render_target(request, db).body.decode()
-        response = HTMLResponse(response.body.decode() + target_html)
-    return response
+        extras.append(_render_target(request, db))
+    return _append_oob(response, *extras)
 
 
 @router.post("/debt-target", response_class=HTMLResponse)

src/quartermaster/routes_month.py

@@ -70,6 +70,24 @@ def _render_target(request: Request, db: Session, month: Month) -> HTMLResponse:
     )
 
 
+def _render_zero(request: Request, month: Month) -> HTMLResponse:
+    zero = month_service.month_zero(month)
+    return templates.TemplateResponse(
+        request,
+        "partials/month_zero.html",
+        {
+            "zero": zero,
+            "planned_tone": service.zero_tone(zero.planned),
+            "applied_tone": service.zero_tone(zero.applied),
+        },
+    )
+
+
+def _append_oob(base: HTMLResponse, *extras: HTMLResponse) -> HTMLResponse:
+    body = base.body.decode() + "".join(e.body.decode() for e in extras)
+    return HTMLResponse(body)
+
+
 @router.get("", response_class=HTMLResponse)
 def view_month(
     year_month: str,
@@ -92,6 +110,7 @@ def view_month(
                 "all_months": all_months,
             },
         )
+    zero = month_service.month_zero(month)
     return templates.TemplateResponse(
         request,
         "month.html",
@@ -107,6 +126,9 @@ def view_month(
                 (e for e in month.entries if e.section == Section.debt_minimum),
                 key=lambda e: e.id,
             ),
+            "zero": zero,
+            "planned_tone": service.zero_tone(zero.planned),
+            "applied_tone": service.zero_tone(zero.applied),
         },
     )
 
@@ -144,7 +166,10 @@ def add_month_entry(
         db, month, section, clean_name, _parse_amount(planned)
     )
     db.refresh(month)
-    return _render_section(request, month, section)
+    return _append_oob(
+        _render_section(request, month, section),
+        _render_zero(request, month),
+    )
 
 
 @router.delete("/entries/{entry_id}", response_class=HTMLResponse)
@@ -159,11 +184,10 @@ def delete_month_entry(
     if section is None:
         raise HTTPException(status_code=404, detail="entry not found")
     db.refresh(month)
-    response = _render_section(request, month, section)
+    extras: list[HTMLResponse] = [_render_zero(request, month)]
     if section == Section.debt_minimum:
-        target_html = _render_target(request, db, month).body.decode()
-        response = HTMLResponse(response.body.decode() + target_html)
-    return response
+        extras.append(_render_target(request, db, month))
+    return _append_oob(_render_section(request, month, section), *extras)
 
 
 @router.post("/entries/{entry_id}", response_class=HTMLResponse)
@@ -195,7 +219,10 @@ def update_month_entry(
     if updated is None:
         raise HTTPException(status_code=404, detail="entry not found")
     db.refresh(month)
-    return _render_section(request, month, updated.section)
+    return _append_oob(
+        _render_section(request, month, updated.section),
+        _render_zero(request, month),
+    )
 
 
 @router.post("/target", response_class=HTMLResponse)

src/quartermaster/service.py

@@ -9,6 +9,12 @@ from sqlalchemy.orm import Session
 from quartermaster.models import DebtTarget, Entry, Section
 
 
+def zero_tone(value: Decimal) -> str:
+    if value == 0:
+        return "zero"
+    return "positive" if value > 0 else "negative"
+
+
 @dataclass(frozen=True)
 class SectionView:
     section: Section
@@ -26,6 +32,18 @@ def section_total(entries: list[Entry]) -> Decimal:
     return sum((e.amount for e in entries), Decimal("0"))
 
 
+def budget_zero(db: Session) -> Decimal:
+    stmt = select(Entry)
+    total_income = Decimal("0")
+    total_non_income = Decimal("0")
+    for entry in db.scalars(stmt):
+        if entry.section == Section.income:
+            total_income += entry.amount
+        else:
+            total_non_income += entry.amount
+    return (total_income - total_non_income).quantize(Decimal("0.01"))
+
+
 def add_entry(db: Session, section: Section, name: str, amount: Decimal) -> Entry:
     entry = Entry(section=section, name=name.strip(), amount=amount)
     db.add(entry)

src/quartermaster/static/app.css

@@ -157,6 +157,59 @@ button[type=submit] {
   border-bottom-style: dashed;
 }
 
+/* Zero Amount widget ----------------------------------------------------- */
+
+:root {
+  --tone-zero: #2f6b4f;
+  --tone-positive: #b77b00;
+  --tone-negative: #a03030;
+}
+
+.zero-widget {
+  margin-top: 1rem;
+  padding: 1rem 1.25rem;
+  background: #fff;
+  border: 1px solid var(--rule);
+  border-radius: 6px;
+  text-align: center;
+}
+
+.zero-widget .zero-label {
+  font-size: 0.75rem;
+  text-transform: uppercase;
+  letter-spacing: 0.15em;
+  color: var(--muted);
+  margin-bottom: 0.25rem;
+}
+
+.zero-widget .zero-value {
+  font-size: 2.4rem;
+  font-weight: 700;
+  font-variant-numeric: tabular-nums;
+  line-height: 1.1;
+}
+
+.zero-widget .zero-sublabel {
+  font-size: 0.7rem;
+  text-transform: uppercase;
+  letter-spacing: 0.1em;
+  color: var(--muted);
+}
+
+.zero-widget-pair .zero-values {
+  display: grid;
+  grid-template-columns: 1fr 1fr;
+  gap: 1.5rem;
+}
+
+.zero-widget-pair .zero-value {
+  font-size: 2rem;
+}
+
+.tone-zero { color: var(--tone-zero); }
+.tone-positive { color: var(--tone-positive); }
+.tone-negative { color: var(--tone-negative); }
+
 /* Monthly view ----------------------------------------------------------- */
 
 .month-nav {

src/quartermaster/templates/index.html

@@ -18,6 +18,7 @@
       </select>
     {% endif %}
   </nav>
+  {% include "partials/budget_zero.html" %}
   {% for section in sections %}
     {% if section.section.value == 'debt_minimum' %}
       {% include "partials/section.html" %}

src/quartermaster/templates/month.html

@@ -2,6 +2,7 @@
 {% block content %}
 <div class="budget">
   {% include "partials/month_nav.html" %}
+  {% include "partials/month_zero.html" %}
   {% for section in sections %}
     {% if section.section.value == 'debt_minimum' %}
       {% include "partials/month_section.html" %}

src/quartermaster/templates/partials/budget_zero.html

@@ -0,0 +1,4 @@
+<section class="zero-widget" id="zero-widget" hx-swap-oob="outerHTML">
+  <div class="zero-label">Zero Amount</div>
+  <div class="zero-value tone-{{ tone }}">${{ '%.2f' | format(zero) }}</div>
+</section>

src/quartermaster/templates/partials/month_zero.html

@@ -0,0 +1,13 @@
+<section class="zero-widget zero-widget-pair" id="zero-widget" hx-swap-oob="outerHTML">
+  <div class="zero-label">Zero Amount</div>
+  <div class="zero-values">
+    <div class="zero-cell">
+      <div class="zero-sublabel">Planned</div>
+      <div class="zero-value tone-{{ planned_tone }}">${{ '%.2f' | format(zero.planned) }}</div>
+    </div>
+    <div class="zero-cell">
+      <div class="zero-sublabel">Applied</div>
+      <div class="zero-value tone-{{ applied_tone }}">${{ '%.2f' | format(zero.applied) }}</div>
+    </div>
+  </div>
+</section>

tests/test_zero_amount.py

@@ -0,0 +1,140 @@
+from __future__ import annotations
+
+from decimal import Decimal
+
+from quartermaster import month_service, service
+from quartermaster.models import Section
+
+
+def test_zero_tone_classification():
+    assert service.zero_tone(Decimal("0")) == "zero"
+    assert service.zero_tone(Decimal("0.00")) == "zero"
+    assert service.zero_tone(Decimal("1.00")) == "positive"
+    assert service.zero_tone(Decimal("-0.01")) == "negative"
+
+
+def test_budget_zero_empty_is_zero(db):
+    assert service.budget_zero(db) == Decimal("0.00")
+
+
+def test_budget_zero_positive_when_unassigned(db):
+    service.add_entry(db, Section.income, "Paycheck", Decimal("2500.00"))
+    service.add_entry(db, Section.fixed_bill, "Rent", Decimal("1200.00"))
+    assert service.budget_zero(db) == Decimal("1300.00")
+
+
+def test_budget_zero_negative_when_overbudget(db):
+    service.add_entry(db, Section.income, "Paycheck", Decimal("1000.00"))
+    service.add_entry(db, Section.fixed_bill, "Rent", Decimal("1200.00"))
+    assert service.budget_zero(db) == Decimal("-200.00")
+
+
+def test_budget_zero_exactly_zero(db):
+    service.add_entry(db, Section.income, "Paycheck", Decimal("1500.00"))
+    service.add_entry(db, Section.fixed_bill, "Rent", Decimal("1000.00"))
+    service.add_entry(db, Section.food, "Groceries", Decimal("500.00"))
+    assert service.budget_zero(db) == Decimal("0.00")
+
+
+def test_budget_zero_ignores_debt_target(db):
+    # target is a pointer, not a totalable section; the math does not touch it
+    dm = service.add_entry(db, Section.debt_minimum, "Card A", Decimal("40.00"))
+    service.add_entry(db, Section.income, "Paycheck", Decimal("100.00"))
+    service.set_debt_target(db, dm.id)
+    assert service.budget_zero(db) == Decimal("60.00")
+
+
+def _seed_budget(db):
+    service.add_entry(db, Section.income, "Paycheck", Decimal("2500.00"))
+    service.add_entry(db, Section.fixed_bill, "Rent", Decimal("1200.00"))
+    service.add_entry(db, Section.debt_minimum, "Card A", Decimal("40.00"))
+
+
+def test_month_zero_starts_planned_nonzero_applied_zero(db):
+    _seed_budget(db)
+    month = month_service.create_month(db, "2026-04")
+    z = month_service.month_zero(month)
+    # income 2500 - (1200 + 40) = 1260
+    assert z.planned == Decimal("1260.00")
+    # applied all 0 on fresh snapshot
+    assert z.applied == Decimal("2500.00") * 0 - Decimal("0")
+    assert z.applied == Decimal("0.00")
+
+
+def test_month_zero_reflects_applied_updates(db):
+    _seed_budget(db)
+    month = month_service.create_month(db, "2026-04")
+    income_entry = next(e for e in month.entries if e.origin_name == "Paycheck")
+    rent_entry = next(e for e in month.entries if e.origin_name == "Rent")
+    month_service.update_month_entry(
+        db, month, income_entry.id, applied=Decimal("2500.00")
+    )
+    month_service.update_month_entry(
+        db, month, rent_entry.id, applied=Decimal("1200.00")
+    )
+    db.refresh(month)
+    z = month_service.month_zero(month)
+    # applied income 2500 - applied (1200 + 0) = 1300
+    assert z.applied == Decimal("1300.00")
+
+
+def test_add_entry_returns_zero_widget_oob(client):
+    response = client.post(
+        "/sections/income/entries",
+        data={"name": "Paycheck", "amount": "2500.00"},
+    )
+    assert response.status_code == 200
+    assert 'id="zero-widget"' in response.text
+    assert 'hx-swap-oob="outerHTML"' in response.text
+    assert "tone-positive" in response.text
+
+
+def test_delete_entry_returns_zero_widget_oob(client):
+    client.post(
+        "/sections/income/entries",
+        data={"name": "Paycheck", "amount": "2500.00"},
+    )
+    response = client.delete("/entries/1")
+    assert response.status_code == 200
+    assert 'id="zero-widget"' in response.text
+    assert "tone-zero" in response.text
+
+
+def test_budget_page_renders_zero_widget(client):
+    response = client.get("/")
+    assert response.status_code == 200
+    assert 'id="zero-widget"' in response.text
+    assert "Zero Amount" in response.text
+
+
+def test_month_entry_update_returns_zero_widget_oob(client):
+    client.post(
+        "/sections/income/entries",
+        data={"name": "Paycheck", "amount": "2500.00"},
+    )
+    client.post(
+        "/sections/fixed_bill/entries",
+        data={"name": "Rent", "amount": "1200.00"},
+    )
+    client.post("/month/2026-04/create")
+    # updating rent applied should bring the month zero widget back with new values
+    response = client.post(
+        "/month/2026-04/entries/2", data={"applied": "1200.00"}
+    )
+    assert response.status_code == 200
+    assert 'id="zero-widget"' in response.text
+    # applied: 0 - 1200 = -1200, negative tone
+    assert "tone-negative" in response.text
+
+
+def test_month_page_renders_paired_zero_widget(client):
+    client.post(
+        "/sections/income/entries",
+        data={"name": "Paycheck", "amount": "2500.00"},
+    )
+    client.post("/month/2026-04/create")
+    response = client.get("/month/2026-04")
+    assert response.status_code == 200
+    assert "zero-widget-pair" in response.text
+    assert response.text.count("Planned") >= 1
+    assert response.text.count("Applied") >= 1