# init(capacity:) Returns an initialized mutable data object capable of holding the specified number of bytes. ``` init?(capacity: Int) ``` ## Parameters `capacity` The number of bytes the data object can initially contain. ## Return Value An initialized `NSMutableData` object capable of holding `capacity` bytes. The returned object has the same memory alignment guarantees as `malloc(_:)`. ## Discussion This method doesn’t necessarily allocate the requested memory right away. Mutable data objects allocate additional memory as needed, so `capacity` simply establishes the object’s initial capacity. When it does allocate the initial memory, though, it allocates the specified amount. This method sets the length of the data object to `0`. If the capacity specified in `capacity` is greater than four memory pages in size, this method may round the amount of requested memory up to the nearest full page. ## See Also [`+ dataWithCapacity:`](/documentation/Foundation/NSMutableData/dataWithCapacity:) Creates and returns a mutable data object capable of holding the specified number of bytes. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)