According to her will which was made public, Princess Diana left a net estate of £12,966,022 - the London Telegraph reports - after inheritance tax of £8,502,330 was deducted following her death in 1997.
The amount is likely to have grown to around £20 million since then because of investment, to be split between Prince Harry and Prince William equally - in Australia, that's over $15 million each.
Princess Diana specified in her will that her estate - which includes stocks and shares, jewellery, cash from her £17 million divorce settlement, dresses and other personal belongings - should be shared equally between her two sons.
It also stated that the brothers would be entitled to their respective shares when they reach the age of 25 - which was changed by executors to age 30 after Diana's death.
Royal contributor Victoria Arbiter from US ABC News says even though Prince William can now receive his inheritance, it's unlikely it will change his low-key lifestyle:
"That’s a big chunk of money, but William leads a very normal life. He lives off his RAF salary, and we’re not going to see anything grand or over the top," she says.
At present, William is said to earn around £40,000 per year as a pilot, and it's believed he can also access about £2 million left in trust by his great-grandmother Queen Elizabeth The Queen Mother, the Telegraph reports.