The price range for the shares has been set at 190 pence to 235 pence per Ninety One Share with the float expected in mid-March.
The company said the offer is expected to raise up to £226m.
Investec Asset Management announced earlier this year plans to float on the London and Johannesburg Stock Exchanges with staff set to own 20% of shares. After the IPO the company will be rebranded as Ninety One.
Under the offer approximately 10% of Ninety One shares will be sold by Investec to new or existing institutional and other investors with Ninety One staff owning 20% of shares via an employee share ownership vehicle.
Hendrik du Toit, founder and chief executive, said: "Today marks another important milestone for Ninety One in the process leading up to its demerger and listing on 16 March 2020.
“In spite of the current backdrop of market volatility and uncertainty, we remain committed to the execution of this transaction, because of its long-term benefits. We are encouraged by significant investor interest.
“We are confident in our capital-light business model of organically developed, specialist, active investment offerings. This is supported by an attractive financial profile, carefully-developed culture, employee ownership commitment and a successful track record."
The Ninety One Group was set up in South Africa in 1991 and has £121bn in assets under management. For the six months ended September, the group had net inflows of £3.2bn and operating profit before exceptional items of £97.3m.