diff --git a/frontend/src/__tests__/money-format.test.ts b/frontend/src/__tests__/money-format.test.ts
index fc281aa43..12b2d2054 100644
--- a/frontend/src/__tests__/money-format.test.ts
+++ b/frontend/src/__tests__/money-format.test.ts
@@ -17,7 +17,7 @@ describe('money formatting (issue #739)', () => {
   it('groups thousands in every cost period', () => {
     expect(formatCostForPeriod(10288, 'yearly')).toBe('$123,456');
     expect(formatCostForPeriod(100000, 'daily')).toBe('$3,333.33');
-    expect(formatCostForPeriod(2000000, 'hourly')).toBe('$2,777.7778');
+    expect(formatCostForPeriod(2000000, 'hourly')).toBe('$2,739.7260');
     expect(formatCostForPeriod(1234.4, 'monthly')).toBe('$1,234');
   });
 });
diff --git a/frontend/src/__tests__/recommendations.test.ts b/frontend/src/__tests__/recommendations.test.ts
index a5dbbc56b..dd7560f3f 100644
--- a/frontend/src/__tests__/recommendations.test.ts
+++ b/frontend/src/__tests__/recommendations.test.ts
@@ -3,6 +3,7 @@
  */
 import { loadRecommendations, openPurchaseModal, getPurchaseModalRecommendations, clearPurchaseModalRecommendations, refreshRecommendations, setupRecommendationsHandlers, pickBestVariantPerCell, seedGlobalDefaults, effectiveMonthlySavings, effectiveSavingsPct, displaySavingsPct, onDemandMonthly, groupRecsByCell, cellSummary, pageLevelRange, resetExpandedCells, resetAutoRefreshInFlight, scaleCost, formatCostForPeriod, periodSuffix, loadColumnVisibility, saveColumnVisibility, resetColumnVisibilityState, TOGGLEABLE_COLUMNS, COLUMN_DEFS, isHomogeneousSelection, renderUsageSparkline, loadColumnFilters, saveColumnFilters, resetColumnFiltersState, spGroupKey, groupSpCellKeys, formatCapacity, scaleRecForCapacity, weightedDisplaySavingsPct } from '../recommendations';
 import type { CostPeriod } from '../state';
+import { HOURS_PER_MONTH } from '../lib/hours-per-month';
 
 // Mock the api module
 jest.mock('../api', () => ({
@@ -5813,8 +5814,11 @@ describe('Issue #319: scaleCost', () => {
     expect(scaleCost(100, 'monthly')).toBe(100);
   });
 
-  test('hourly factor is 1/720', () => {
-    expect(scaleCost(720, 'hourly')).toBeCloseTo(1.0);
+  // Issues #337/#394: hourly uses the backend's 730-hour month, not 720.
+  test('hourly factor is 1/730, the shared hours-per-month', () => {
+    expect(HOURS_PER_MONTH).toBe(730);
+    expect(scaleCost(730, 'hourly')).toBeCloseTo(1.0, 10);
+    expect(scaleCost(720, 'hourly')).not.toBeCloseTo(1.0, 4);
   });
 
   test('daily factor is 1/30', () => {
@@ -5851,7 +5855,7 @@ describe('Issue #319: formatCostForPeriod', () => {
   });
 
   test('hourly uses 4 decimal places', () => {
-    expect(formatCostForPeriod(720, 'hourly')).toMatch(/\$1\.0000/);
+    expect(formatCostForPeriod(730, 'hourly')).toMatch(/\$1\.0000/);
   });
 
   test('daily uses 2 decimal places', () => {
@@ -6003,13 +6007,13 @@ describe('Issue #319: table cells scale with period', () => {
     (state.getRecommendationsColumnFilters as jest.Mock).mockReturnValue({});
   });
 
-  test('hourly: savings cell shows $1.0000', async () => {
+  test('hourly: savings cell shows $0.9863 (720 / 730 hours)', async () => {
     (state.getCostPeriod as jest.Mock).mockReturnValue('hourly');
     await loadRecommendations();
     const rows = document.querySelectorAll('tr.recommendation-row');
     expect(rows.length).toBeGreaterThan(0);
     const savingsCell = rows[0]!.querySelector('.savings');
-    expect(savingsCell?.textContent).toMatch(/\$1\.0000/);
+    expect(savingsCell?.textContent).toMatch(/\$0\.9863/);
   });
 
   test('daily: savings cell shows $24.00', async () => {
@@ -6047,7 +6051,7 @@ describe('Issue #319: table cells scale with period', () => {
     (state.getRecommendations as jest.Mock).mockReturnValue([recWithUpfront]);
     await loadRecommendations();
     const rows = document.querySelectorAll('tr.recommendation-row');
-    // We just verify "1000" appears somewhere but "0.0014" (1000/720) does not
+    // We just verify "1000" appears somewhere but "0.0014" (1000/730) does not
     const rowText = rows[0]!.textContent ?? '';
     expect(rowText).toContain('1000');
     expect(rowText).not.toMatch(/0\.0014/);
@@ -6099,8 +6103,8 @@ describe('Issue #319: table cells scale with period', () => {
 
   test('hourly numeric filter compares the scaled (per-hour) savings, not the raw monthly value', async () => {
     // Both recs would pass a ">0.75" filter against raw monthly_cost (360
-    // and 720 are both > 0.75). After hourly scaling (÷720), only r-high
-    // (1.0) passes; r-low (0.5) is filtered out. If numericCellValue were
+    // and 720 are both > 0.75). After hourly scaling (÷730), only r-high
+    // (0.986) passes; r-low (0.493) is filtered out. If numericCellValue were
     // to drop the scaleCost call, this test would surface the regression.
     const recLow  = { ...mockRec, id: 'r-low',  resource_type: 'low',  savings: 360, monthly_cost: 360 };
     const recHigh = { ...mockRec, id: 'r-high', resource_type: 'high', savings: 720, monthly_cost: 720 };
@@ -7609,7 +7613,7 @@ describe('Issue #484: numeric filter matches the displayed rounded value', () =>
   // display rounds to PERIOD_DECIMALS.hourly (4) decimals.
   //
   // raw monthly = 123.456789 → daily = monthly/30 = 4.1152263; hourly =
-  // monthly/720 = 0.171467... Use a monthly that's clean enough to keep
+  // monthly/730 = 0.169119... Use a monthly that's clean enough to keep
   // the assertions readable while still distinguishing exact rounding.
   const recs = [
     { id: 'r-exact',   provider: 'aws', cloud_account_id: 'a1', service: 'ec2', resource_type: 't3.medium', region: 'us-east-1', count: 1, term: 1, savings: 123.456789, upfront_cost: 500 },
diff --git a/frontend/src/lib/hours-per-month.ts b/frontend/src/lib/hours-per-month.ts
new file mode 100644
index 000000000..3f438b7e5
--- /dev/null
+++ b/frontend/src/lib/hours-per-month.ts
@@ -0,0 +1,9 @@
+/**
+ * Hours in a month for every hourly <-> monthly cost conversion in the UI.
+ *
+ * 730 is AWS's canonical convention: 8760 hours in a 365-day year divided by
+ * 12 months. The backend uses the same figure (ricost.HoursPerMonth in
+ * internal/ricost/monthly_cost.go), so an hourly figure shown here matches the
+ * monthly figures the API derives. Keep the two in sync (issues #337, #394).
+ */
+export const HOURS_PER_MONTH = 730;
diff --git a/frontend/src/modules/savings-history.ts b/frontend/src/modules/savings-history.ts
index df00cef5a..ebb12fd61 100644
--- a/frontend/src/modules/savings-history.ts
+++ b/frontend/src/modules/savings-history.ts
@@ -9,6 +9,7 @@ import { formatTrendAxisTick, upsertChart } from './chart-utils';
 import { formatCompactCurrency, formatCurrency } from '../utils';
 import { setChartSummary, renderChartTable, removeChartTable } from '../lib/chart-a11y';
 import { applyChartDefaults, chartColor, withAlpha } from '../lib/chart-theme';
+import { HOURS_PER_MONTH } from '../lib/hours-per-month';
 
 // Register Chart.js components
 Chart.register(...registerables);
@@ -26,8 +27,8 @@ const API_UNIT = 'monthly' as const;
 
 export type SavingsUnit = 'hourly' | 'monthly' | 'yearly';
 
-// Conversion factors relative to monthly as the canonical unit.
-const HOURS_PER_MONTH = 730;   // 365.25 * 24 / 12
+// Conversion factors relative to monthly as the canonical unit. Hourly uses
+// the shared 730-hour month (365 * 24 / 12), the same as the backend.
 const MONTHS_PER_YEAR = 12;
 
 /**
diff --git a/frontend/src/recommendations.ts b/frontend/src/recommendations.ts
index 1fea0942d..1a3daa4c8 100644
--- a/frontend/src/recommendations.ts
+++ b/frontend/src/recommendations.ts
@@ -40,6 +40,7 @@ function canActOnRecommendations(): boolean {
 import { parseNumericFilter, applyColumnFilters as applyColumnFiltersLib } from './lib/column-filters';
 import { fetchOverridesForAccounts } from './lib/overrides';
 import { ownValue } from './lib/own-property';
+import { HOURS_PER_MONTH } from './lib/hours-per-month';
 // Re-export the shared primitives so existing consumers that import from
 // recommendations.ts keep working without import-path churn (issue #166).
 export { parseNumericFilter } from './lib/column-filters';
@@ -1726,9 +1727,12 @@ export function onDemandMonthly(r: LocalRecommendation): number | null {
 // Cost-period scaling (issue #319)
 //
 
-/** Conversion factors relative to a monthly base. */
+/**
+ * Conversion factors relative to a monthly base. Hourly uses the 730-hour
+ * month the backend uses (issues #337, #394); daily stays a 30-day month.
+ */
 const PERIOD_FACTOR: Record<CostPeriod, number> = {
-  hourly:  1 / 720,   // 24 × 30 hrs/mo
+  hourly:  1 / HOURS_PER_MONTH,
   daily:   1 / 30,
   monthly: 1,
   yearly:  12,
diff --git a/frontend/src/riexchange-confirm.ts b/frontend/src/riexchange-confirm.ts
index 41c52b8d9..fcfb1a5d6 100644
--- a/frontend/src/riexchange-confirm.ts
+++ b/frontend/src/riexchange-confirm.ts
@@ -8,9 +8,7 @@
  */
 
 import type { ExchangeQuoteSummary, RIExchangeHistoryRecord } from './api';
-
-// Hours in an average month; only used for the approximate monthly delta.
-const HOURS_PER_MONTH = 730;
+import { HOURS_PER_MONTH } from './lib/hours-per-month';
 
 export interface ExchangeConfirmParts {
   quote: ExchangeQuoteSummary;