Quota and persistent storage

How much storage does the browser give you?

Browsers do not give every origin unlimited storage. Instead they allocate a quota — a maximum amount the origin may use — based on available disk space and browser policy. The quota for IndexedDB, Cache API, and OPFS comes from a shared pool called the “default” storage bucket.

The navigator.storage.estimate() API returns a snapshot of how much of that quota your origin has consumed.

const { quota, usage } = await navigator.storage.estimate();
console.log('Quota :', (quota / 1024 / 1024).toFixed(1), 'MB');
console.log('Usage :', (usage / 1024 / 1024).toFixed(2), 'MB');

The returned usage figure covers IndexedDB, Cache API, and OPFS together. It does not include localStorage or cookies, which have separate, smaller limits.

Best-effort vs persistent storage

By default, every origin gets best-effort storage. Under low disk space the browser is free to evict best-effort data — silently, without warning — to reclaim room. The eviction order is typically least-recently-used origin first.

You can ask the browser to upgrade your origin to persistent storage. Persistent data is never evicted without an explicit user action (clearing site data). To request it:

const granted = await navigator.storage.persist();
console.log('Persistent storage granted:', granted); // true or false

To check the current state without requesting:

const isPersisted = await navigator.storage.persisted();
console.log('Already persistent:', isPersisted);

When does the browser grant persistence?

Browsers use engagement heuristics to decide whether to grant the request silently or show a permission prompt. Factors that increase the likelihood of an automatic grant include:

The site is installed as a PWA (added to Home Screen / installed via browser UI)
The user has visited the site frequently
The user has bookmarked the site or granted other permissions (push notifications, geolocation)
Some browsers (Firefox) always ask the user; Chromium-based browsers may grant silently

Two storage durability tiers

flowchart TD
  A[Origin stores data] --> B{Durability tier}
  B -->|Default| C[Best-effort storage]
  B -->|After persist granted| D[Persistent storage]
  C --> E[Browser MAY evict under\ndisk pressure]
  E --> F[Data silently deleted]
  D --> G[Browser will NEVER evict\nwithout user action]
  G --> H[Data safe until user\nclears site data]

Best-effort storage can be evicted silently; persistent storage requires an explicit user action to remove

Runnable: check your quota and persistence state

The snippet below calls both estimate() and persisted() and logs the results. No demo keys are written — the estimate API needs no cleanup.

Browser Storage

async function checkStorage() {
  // Quota and usage
  const estimate = await navigator.storage.estimate();
  const usedMB  = (estimate.usage  / 1024 / 1024).toFixed(3);
  const quotaMB = (estimate.quota  / 1024 / 1024).toFixed(1);
  const pct     = ((estimate.usage / estimate.quota) * 100).toFixed(4);

  console.log('--- Storage estimate ---');
  console.log('Usage : ' + usedMB  + ' MB');
  console.log('Quota : ' + quotaMB + ' MB');
  console.log('Used  : ' + pct + '%');

  // Persistence status
  const persisted = await navigator.storage.persisted();
  console.log('\n--- Persistence ---');
  console.log('Currently persistent:', persisted);

  if (!persisted) {
    console.log('Tip: call navigator.storage.persist() to request durable storage.');
  }
}

checkStorage();

Trade-offs

Option	Benefit	Cost
Best-effort storage (default)	No permission prompt, works immediately	The browser can silently evict data when disk space is low
Persistent storage (`persist()`)	Data survives until the user clears site data themselves	Depends on the browser’s engagement heuristics, which you don’t fully control, and some browsers show a permission prompt the user can deny
Calling `estimate()` before a large write	Know the remaining headroom, avoid a mid-write `QuotaExceededError`	Adds one extra async round-trip before the real operation starts

Common mistakes

Assuming IndexedDB means “stored forever” — by default every origin only gets best-effort storage. You must call navigator.storage.persist() to actually request an upgrade to persistent storage.
Not checking quota before a large write — writing continuously without calling navigator.storage.estimate() first leads to an unexpected QuotaExceededError when disk space runs low.
Confusing the usage from estimate() with localStorage — the returned value only covers IndexedDB, Cache API, and OPFS. It does not include localStorage or cookies, which have separate limits.