The cost of posting letters and parcels will depend on their size and shape as well as weight from September 2006.
Large, lightweight items are more costly to deal with, Royal Mail says
Royal Mail's plan for higher charges for light but large items, and lower costs for some heavier items, has been approved by regulator Postcomm.
Prices on around 30% of post are expected to be affected but overall revenue to Royal Mail will not change.
The move would allow Royal Mail "to bring its prices much more closely into line with its costs", Postcomm said.
Royal Mail had argued that lightweight mail that was large or bulky cost more to transport, sort, handle and deliver than regular size items.
Lorna Clarkson, director of commercial policy and pricing, said: "This new system will be fairer for customers, as well as more accurately reflecting the costs to our business."
Post offices will have templates so that customers can measure their mail to find out the cost.
Royal Mail estimates that 20% of residential mail and 30% of business mail will be affected, with about half costing more and half costing less.
From September next year, the first-class price of a large letter weighing 750g and less than 25mm thick will drop from £2.71 to £1.23.
But a large greetings card weighing less than 60 grams, which currently requires a 30p stamp to send first class, will cost 42p to post.
"This is a major change to the way people pay for their post," said Postcomm chairman Nigel Stapleton.
"It will promote the development of a successful and growing postal market by allowing Royal Mail to bring its prices much more closely into line with its costs.
"This is particularly important given the full liberalisation of the postal market in January."
The Royal Mail's 350-year letter delivery monopoly comes to an end on 1 January next year.
The overhaul of charges had been proposed for April 2006, but was put back to allow customers at least 12 months to prepare.
Similar pricing regimes already operate in countries including Australia, Canada, Japan and the US.