Browser storage quota refers to the maximum amount of data a website can store in the user's browser. This limit exists to prevent websites from consuming excessive disk space and to protect user privacy. Different storage types (LocalStorage, IndexedDB, Cache API) have different quota rules. Understanding these limits is crucial for web developers building offline-capable apps, PWAs, or applications that handle large datasets client-side.

Chrome & Edge: LocalStorage up to 10 MB per origin; IndexedDB can use up to 60% of free disk space (shared with Cache API).
Firefox: LocalStorage up to 10 MB; IndexedDB can use up to 50% of free disk space, with a global limit of 2 GB per origin for "best-effort" storage.
Safari (Desktop): LocalStorage up to 5-10 MB; IndexedDB quota is around 1 GB per origin, but data may be evicted after 7 days of inactivity.
Safari (iOS): LocalStorage capped at 5 MB; IndexedDB limited and may be cleared under storage pressure.
Note: Quotas are not guaranteed — browsers may evict data when disk space runs low.

LocalStorage has a fixed, small quota (typically 5-10 MB per origin) that is enforced synchronously — attempting to exceed it throws a QuotaExceededError. IndexedDB, on the other hand, has a dynamic quota that scales with available disk space and is shared with the Cache API. IndexedDB is designed for larger datasets and supports asynchronous operations, making it the preferred choice for significant client-side data storage.

Use the navigator.storage.estimate() API to query storage usage and quota. This returns a Promise with {quota, usage} in bytes, covering IndexedDB and Cache API storage. Example:

const {quota, usage} = await navigator.storage.estimate();
console.log(`Used: ${(usage/1e6).toFixed(1)} MB of ${(quota/1e6).toFixed(1)} MB`);

For LocalStorage, you must manually calculate usage by iterating over keys and measuring string sizes using new Blob([value]).size.

When you exceed the quota:
LocalStorage / SessionStorage: A DOMException: QuotaExceededError is thrown immediately. The write operation fails and no data is stored.
IndexedDB: The transaction fails with an AbortError or QuotaExceededError. Some browsers may trigger a permission prompt asking the user to grant more storage.
Cache API: The cache.put() operation rejects with a quota-related error. Older cache entries may be evicted automatically to make room.
Always wrap storage operations in try-catch blocks to handle these errors gracefully.

Yes. Mobile browsers, especially iOS Safari, enforce stricter storage limits due to limited device storage. iOS Safari limits LocalStorage to 5 MB and may aggressively evict IndexedDB data when the device is low on space. Android Chrome is more generous but still subject to disk space constraints. PWAs installed on the home screen may request persistent storage via navigator.storage.persist() to reduce the risk of eviction.

You can request persistent storage by calling navigator.storage.persist(). If granted, the browser will prioritize keeping your data and may increase the quota. This is especially useful for PWAs. Note that persistent storage requires user interaction and is not guaranteed. Some browsers (like Chrome) may also trigger a quota increase prompt when you approach the limit during an IndexedDB write operation.