STOCKHOLM, July 21 (Reuters) - Sweden's Evolution AB reported a 115% jump in second-quarter core earnings, beating market estimates, as the pandemic boosted demand for online casino games, particularly in the United States.

The company, which makes online slot machines and roulette games, has seen more customers turning to online games and the United States becoming a growth market as more states consider making online gambling legal to boost tax receipts.

Evolution, which has more than 500 customers including 888 Casino and Betsson, bought Swedish rival NetEnt for $2.12 billion last year, handing it popular slot games such as Starburst and Piggy Riches.

Apart from buying companies, the company has also been launching new studios.

Its studio in Michigan - the third in the United States - is approved and ready for launch, and the company plans to add two new delivery hubs in Europe this year.

However, the pandemic has slowed down its expansion plans.

"The overall situation is improving but it is still very hard and the pandemic is not over," CEO Martin Carlesund said in a statement.

Evolution's quarterly EBITDA rose to 174.7 million euros ($205.69 million) beating the mean forecast of 172.3 million euros, according to Refinitiv estimates, and its corresponding margin rose to 68%.

"Our EBITDA margin reached a new high level during the first quarter this year, a level that we maintained this quarter and I expect to maintain also during the rest of the year," Carlesund said.

Shares of the company has nearly doubled in the last 12 months.

($1 = 0.8493 euros) (Reporting by Supantha Mukherjee in Stockholm; editing by Niklas Pollard)