The group is expected to be valued at up to £776m, with shares priced at between 234 and 300 pence.
The amount paid out to its 1,500 employees will depend on each person's role and length of service.
Admiral announced that it was seeking a stock market listing back in April after rapid growth at its online business elephant.co.uk and higher premiums at Diamond, its insurer targeted at female drivers.
The company has 1,100 staff in Cardiff and another 400 in Swansea. The flotation will see staff awarded an average of £37,000, dependant on role and length of service.
Senior managers will get the most, but even the most junior workers will be given shares valued at £10,000.
Chairman Alastair Lyons says: "It will enable us to provide a public market for Admiral shares, increasing the profile of the group and enabling employees to see the direct benefit of their hard work."
The decision to float comes five years after Admiral was sold to its management team in a buy-out backed by Barclays Private Equity.
As its largest stakeholder, Barclays Private Equity will sell 15% of the company's shares, while management and employees of Admiral will sell 12%, and Bermuda-based insurer XL Capital will sell 5%.
Launched in 1993, the Cardiff-based group now has more than 900,000 customers across its four brands.
Admiral posted a 30.2% increase in turnover to £269.3m in the six months to 30 June on the back of a 30.5% rise in pre-tax profits to £45.3m.