The Cardiff-based company, whose brands include Elephant and Bell, said its vehicle count remained the same as a year ago at just over three million, while rate cuts of 7% on new business also depressed turnover.
But with Admiral focusing on margins rather than market share, it grew profits to £181.4 million in the six months to June 30, helped by a better claims environment and a 20% rise in profits at comparison site Confusion.
Chief executive Henry Engelhardt said: "I'm really pleased with our results in the first half of 2013. Any time you can increase profits by 6% when competitors are cutting prices you've got to be happy.
"We didn't grow the UK insurance business, which was great news because of the competitive environment."
Admiral said that after two years of significant rate increases in 2010 and 2011, the UK car insurance market was now in its second year of being more price competitive.
This resulted in a 10% reduction in UK car insurance turnover to £924.5 million but profits still increased by 5% to £192.7 million.
The half-year performance means more than 6,600 staff will be eligible to receive £1,500 of shares each via the company's share scheme.