diff --git a/src/app/infrastructure/storage-estimate.ts b/src/app/infrastructure/storage-estimate.ts
new file mode 100644
index 0000000..741d4f7
--- /dev/null
+++ b/src/app/infrastructure/storage-estimate.ts
@@ -0,0 +1,39 @@
+/**
+ * Storage estimate adapter (spec §02 → Storage Monitor; docs/architecture/02 §6).
+ *
+ * This is the ONLY module that touches the browser's Storage Manager API
+ * (`navigator.storage.estimate()`). It feature-detects the API, then defers all
+ * presentation math to the portable core (`src/core/storage-estimate.ts`). When
+ * the API is unavailable — older browsers, insecure contexts — it returns an
+ * unavailable summary rather than throwing, so callers get a total function.
+ */
+
+import { summarizeStorage, type StorageSummary } from '../../core/storage-estimate';
+
+/** The unavailable summary, returned when the Storage Manager API is absent. */
+const UNAVAILABLE: StorageSummary = {
+  usedBytes: 0,
+  quotaBytes: 0,
+  fraction: 0,
+  level: 'ok',
+  available: false,
+};
+
+/**
+ * Read the browser's storage estimate and summarize it. Returns an unavailable
+ * summary when `navigator.storage.estimate` is missing or the call rejects;
+ * never throws. All math lives in `summarizeStorage`.
+ */
+export async function readStorageEstimate(): Promise<StorageSummary> {
+  const storage = typeof navigator !== 'undefined' ? navigator.storage : undefined;
+  if (!storage || typeof storage.estimate !== 'function') {
+    return UNAVAILABLE;
+  }
+
+  try {
+    const { usage, quota } = await storage.estimate();
+    return summarizeStorage({ usage, quota });
+  } catch {
+    return UNAVAILABLE;
+  }
+}
diff --git a/src/core/storage-estimate.test.ts b/src/core/storage-estimate.test.ts
new file mode 100644
index 0000000..b2540d9
--- /dev/null
+++ b/src/core/storage-estimate.test.ts
@@ -0,0 +1,126 @@
+import { describe, expect, test } from 'vitest';
+import {
+  CRITICAL_THRESHOLD,
+  WARNING_THRESHOLD,
+  humanizeBytes,
+  summarizeStorage,
+} from './storage-estimate';
+
+describe('summarizeStorage', () => {
+  test('computes the fraction and passes usage/quota through', () => {
+    const s = summarizeStorage({ usage: 250, quota: 1000 });
+    expect(s.available).toBe(true);
+    expect(s.usedBytes).toBe(250);
+    expect(s.quotaBytes).toBe(1000);
+    expect(s.fraction).toBeCloseTo(0.25);
+    expect(s.level).toBe('ok');
+  });
+
+  test('clamps fraction to 1 when usage exceeds quota', () => {
+    const s = summarizeStorage({ usage: 1500, quota: 1000 });
+    expect(s.fraction).toBe(1);
+    expect(s.level).toBe('critical');
+  });
+
+  test('guards divide-by-zero: quota of 0 is unusable', () => {
+    const s = summarizeStorage({ usage: 10, quota: 0 });
+    expect(s.available).toBe(false);
+    expect(s.fraction).toBe(0);
+    expect(s.usedBytes).toBe(0);
+    expect(s.quotaBytes).toBe(0);
+    expect(s.level).toBe('ok');
+  });
+
+  describe('level boundaries', () => {
+    test('just below warning (0.79) is ok', () => {
+      expect(summarizeStorage({ usage: 79, quota: 100 }).level).toBe('ok');
+    });
+
+    test('exactly at warning (0.80) is warning', () => {
+      const s = summarizeStorage({ usage: 80, quota: 100 });
+      expect(s.fraction).toBeCloseTo(WARNING_THRESHOLD);
+      expect(s.level).toBe('warning');
+    });
+
+    test('just below critical (0.94) is warning', () => {
+      expect(summarizeStorage({ usage: 94, quota: 100 }).level).toBe('warning');
+    });
+
+    test('exactly at critical (0.95) is critical', () => {
+      const s = summarizeStorage({ usage: 95, quota: 100 });
+      expect(s.fraction).toBeCloseTo(CRITICAL_THRESHOLD);
+      expect(s.level).toBe('critical');
+    });
+  });
+
+  describe('availability', () => {
+    test('missing usage -> unavailable', () => {
+      const s = summarizeStorage({ quota: 1000 });
+      expect(s.available).toBe(false);
+      expect(s.fraction).toBe(0);
+    });
+
+    test('missing quota -> unavailable', () => {
+      const s = summarizeStorage({ usage: 1000 });
+      expect(s.available).toBe(false);
+      expect(s.fraction).toBe(0);
+    });
+
+    test('both missing (empty input) -> unavailable', () => {
+      const s = summarizeStorage({});
+      expect(s.available).toBe(false);
+    });
+
+    test('non-finite or negative values -> unavailable', () => {
+      expect(summarizeStorage({ usage: NaN, quota: 1000 }).available).toBe(false);
+      expect(summarizeStorage({ usage: Infinity, quota: 1000 }).available).toBe(false);
+      expect(summarizeStorage({ usage: -1, quota: 1000 }).available).toBe(false);
+      expect(summarizeStorage({ usage: 10, quota: -5 }).available).toBe(false);
+    });
+
+    test('zero usage with a real quota is available and ok', () => {
+      const s = summarizeStorage({ usage: 0, quota: 1000 });
+      expect(s.available).toBe(true);
+      expect(s.fraction).toBe(0);
+      expect(s.level).toBe('ok');
+    });
+  });
+});
+
+describe('humanizeBytes', () => {
+  test('zero renders as whole bytes', () => {
+    expect(humanizeBytes(0)).toBe('0 B');
+  });
+
+  test('small values stay in whole bytes', () => {
+    expect(humanizeBytes(1)).toBe('1 B');
+    expect(humanizeBytes(512)).toBe('512 B');
+    expect(humanizeBytes(1023)).toBe('1023 B');
+  });
+
+  test('KB boundary (1024) and within KB', () => {
+    expect(humanizeBytes(1024)).toBe('1.0 KB');
+    expect(humanizeBytes(1536)).toBe('1.5 KB');
+  });
+
+  test('MB boundary and within MB', () => {
+    expect(humanizeBytes(1024 * 1024)).toBe('1.0 MB');
+    expect(humanizeBytes(5 * 1024 * 1024)).toBe('5.0 MB');
+    expect(humanizeBytes(2.5 * 1024 * 1024)).toBe('2.5 MB');
+  });
+
+  test('GB boundary', () => {
+    expect(humanizeBytes(1024 * 1024 * 1024)).toBe('1.0 GB');
+  });
+
+  test('caps at the largest unit (TB)', () => {
+    expect(humanizeBytes(1024 ** 4)).toBe('1.0 TB');
+    expect(humanizeBytes(1024 ** 5)).toBe('1024.0 TB');
+  });
+
+  test('negative and non-finite inputs degrade to 0 B', () => {
+    expect(humanizeBytes(-1)).toBe('0 B');
+    expect(humanizeBytes(NaN)).toBe('0 B');
+    expect(humanizeBytes(Infinity)).toBe('0 B');
+  });
+});
diff --git a/src/core/storage-estimate.ts b/src/core/storage-estimate.ts
new file mode 100644
index 0000000..b72461e
--- /dev/null
+++ b/src/core/storage-estimate.ts
@@ -0,0 +1,107 @@
+/**
+ * Storage estimate summarization (spec §02 → Storage Monitor; spec §10 → "Warn
+ * before storage failure"; docs/architecture/02 §6).
+ *
+ * Portable core: no browser APIs, no React. The browser's Storage Manager API
+ * (`navigator.storage.estimate()`) yields a raw `{ usage, quota }` pair in
+ * bytes — both optional, since a browser may omit them. This module turns that
+ * raw pair into presentation-ready facts: a clamped fraction, an escalating
+ * level, and an availability flag. The infrastructure adapter
+ * (`src/app/infrastructure/storage-estimate.ts`) feature-detects the API and
+ * feeds its output through here; all the math lives in this pure function so it
+ * is trivially testable.
+ *
+ * Thresholds: warning at >= 0.8 is the documented value (arch 02 §6 `WARN_AT`).
+ * Critical at >= 0.95 is not specified by spec/arch; it is this module's default
+ * for the second escalation step ("nearly full") and is the single source of
+ * truth here.
+ */
+
+/** Raw input shape, mirroring the browser's `StorageEstimate` (bytes, both optional). */
+export interface StorageEstimateInput {
+  /** Bytes currently used, or undefined when the browser omits it. */
+  usage?: number;
+  /** Total bytes available (quota), or undefined when the browser omits it. */
+  quota?: number;
+}
+
+/** Escalating fullness level driving the storage monitor's warning copy/colour. */
+export type StorageLevel = 'ok' | 'warning' | 'critical';
+
+/** Presentation-ready summary derived from a raw estimate. */
+export interface StorageSummary {
+  /** Bytes used (0 when unusable). */
+  usedBytes: number;
+  /** Quota in bytes (0 when unusable). */
+  quotaBytes: number;
+  /** usedBytes / quotaBytes, clamped to 0..1; 0 when quota is 0/undefined. */
+  fraction: number;
+  /** Escalating fullness level derived from `fraction`. */
+  level: StorageLevel;
+  /** False when usage/quota are missing or non-finite — the estimate is unusable. */
+  available: boolean;
+}
+
+/** Fraction at which the monitor begins warning (arch 02 §6 `WARN_AT`). */
+export const WARNING_THRESHOLD = 0.8;
+/** Fraction at which the monitor escalates to critical ("nearly full"). */
+export const CRITICAL_THRESHOLD = 0.95;
+
+/** A non-finite or negative number is not a usable byte count. */
+function usableBytes(n: number | undefined): number | null {
+  if (typeof n !== 'number' || !Number.isFinite(n) || n < 0) return null;
+  return n;
+}
+
+/** Map a clamped fraction to its escalation level. */
+function levelFor(fraction: number): StorageLevel {
+  if (fraction >= CRITICAL_THRESHOLD) return 'critical';
+  if (fraction >= WARNING_THRESHOLD) return 'warning';
+  return 'ok';
+}
+
+/**
+ * Turn a raw storage estimate into a presentation-ready summary. Pure: no I/O.
+ * When usage is missing/invalid, or quota is missing/invalid/zero, the estimate
+ * is unusable — `available: false`, everything zeroed, level `ok` (we don't warn
+ * on data we don't have). Otherwise the fraction is `usage / quota` clamped to
+ * 0..1 and the level escalates at the thresholds above.
+ */
+export function summarizeStorage(input: StorageEstimateInput): StorageSummary {
+  const usage = usableBytes(input.usage);
+  const quota = usableBytes(input.quota);
+
+  // Quota of 0 can't yield a meaningful fraction; treat as unusable.
+  if (usage === null || quota === null || quota === 0) {
+    return { usedBytes: 0, quotaBytes: 0, fraction: 0, level: 'ok', available: false };
+  }
+
+  const fraction = Math.min(1, usage / quota);
+  return {
+    usedBytes: usage,
+    quotaBytes: quota,
+    fraction,
+    level: levelFor(fraction),
+    available: true,
+  };
+}
+
+const BYTE_UNITS = ['B', 'KB', 'MB', 'GB', 'TB'] as const;
+
+/**
+ * Render a byte count as a human-readable string using binary (1024) units.
+ * Bytes are shown as a whole number with the `B` unit; KB and up get one
+ * decimal place (e.g. `1.5 MB`). Negative/non-finite inputs degrade to `0 B`.
+ */
+export function humanizeBytes(n: number): string {
+  if (!Number.isFinite(n) || n < 0) return '0 B';
+  if (n < 1024) return `${Math.round(n)} B`;
+
+  let value = n;
+  let unit = 0;
+  while (value >= 1024 && unit < BYTE_UNITS.length - 1) {
+    value /= 1024;
+    unit += 1;
+  }
+  return `${value.toFixed(1)} ${BYTE_UNITS[unit]}`;
+}