The European Commission says 3 million migrants are expected to arrive in Europe by the end of 2016.
The EU's executive arm said the influx will raise the EU population by 0.4 percent once unsuccessful asylum applications were taken into account.
It is also expected to raise the EU's economic output by 0.2-0.3 percent, it added.
Meanwhile, the UN's refugee agency, the UNHCR, said migrants are likely to keep coming to Europe at a rate of up to 5,000 per day via Turkey this winter, meaning that more than 1 million people will have fled to the continent this year.
About 760,000 people have already crossed the Mediterranean by sea this year, after fleeing conflicts in Syria, Afghanistan, Iraq, and Africa.
Many of them are heading for Germany, hoping to get asylum there.