Mercedes have been left in the wake of Ferrari and Red Bull during testing in Barcelona.
But they are using the eighth and final day to check out their single lap pace to fully analyse the situation.
Hamilton thinks there is a substantial gap between his vehicle and title-rival Sebastian Vettel.
And he reckons the Silver Arrows may not know the full details of the deficit until the Spanish Grand Prix in May.
“I think the gap [to Ferrari] is potentially half a second,” Hamilton said.
“But we don’t know because everyone has different engine modes and fuel loads.
“It will be four races before we really know where we stand.”
Hamilton has been happy with how his Mercedes has performed during testing.
But he feels this year’s World Championship battle could be one of the toughest he has faced.
“The last couple of days have been particularly good, we have been improving every day,” he added.
“Testing is testing. There’s no reward for being quick in testing.
“Ferrari are the fastest.
“This is going to be the toughest battle yet.
“Ferrari’s pace is very, very good at the moment. The challenge is going to be harder than ever.”
The 2019 F1 season gets underway in Australian on March 17.