Once the upgrade takes effect on Friday, Gmail users will be able to store up to two gigabytes of email and attachments for each account. Even more capacity will be made available after that as it becomes feasible, the company said.
"Our goal is to make sure storage is no longer an issue for web mail users," said Marissa Mayer, Google's director of consumer web products.
The announcement was made on the first anniversary of the internet search engine's launch of Gmail, which surprised many users with its generous storage allotment and prompted rivals Yahoo Inc and Microsoft to boost the capacity of their own free web mail services.
Just last week, Yahoo Inc said it would offer one gigabyte of storage to users of its free service. When Google introduced Gmail, Yahoo! was providing just four megabytes of storage.
Microsoft's Hotmail now offers 250 megabytes, up from two megabytes at Gmail's launch.
Google, Microsoft and Yahoo! are in a tight race to get their services on the world's computer desktops.
In recent months, each has launched utilities to help people find information stored on a PC, block popup ads, conduct internet searches and provide other features.
Mayer said Mountain View, California-based Google can offer so much capacity because of the way the company has set up its computer systems. She declined to provide any more detail.
But even after a year, Gmail remains available only to those who have been invited by existing users. In recent months, Google has made more invitations available, but it is still officially a beta - or test - offering. Mayer declined to say when that status might change. The search company intends to always offer a free version, though she said fee-based options are a possibility.