Yet another lottery jackpot is growing to record size this year, just months after two record-breaking Mega Millions and Powerball jackpots were hit.

If you haven’t been following the lottery craze over the last few months, allow us to bring you up to speed.

In July, a Mega Millions ticket worth nearly $1.4 billion was sold in Illinois. It was the third-largest lottery prize in U.S. history and wasn’t claimed for nearly two months.

By the time that ticket was claimed, another jackpot was growing, this time with Powerball. Without a winner for roughly three months, the jackpot was able to reach $2.04 billion – the largest prize in Powerball and U.S. lottery history – before a ticket sold in California matched the winning numbers.

As we again wait for a winner to come forward (the lucky Californian still has over 320 days to come forward), yet another large lottery jackpot is brewing.

After no ticket matched all six numbers in Tuesday’s Mega Millions drawing, the jackpot has reached an estimated $510 million.

It’s almost one of the largest grand prizes in the game’s history, ranking 11th behind a $516 million jackpot hit last year, according to Mega Millions. But, as we saw with the Powerball prize just two months ago, that jackpot could grow quickly before the next drawing on Friday.

But how is the Mega Millions jackpot calculated?

There are a couple of factors that the jackpot’s estimated value is based on.

As Mega Millions explains, members of the Mega Millions group meet Tuesdays and Fridays (the days drawings are held) to determine the estimated jackpots for the next two drawings.

An obvious factor that’s considered is ticket sales. State-level ticket sales estimates are used to determine the estimated cash value of the Mega Millions jackpots. Officials also review that day’s 30-year U.S. Treasuries rate. This is an important figure because of the annuity option winners can claim which divides their prize into 29 annual payments that are 5% larger than that previously awarded.

Jackpots also often end up being larger after the actual drawing than estimates predicted. Part of this is because 70% of sales happen the day of the drawing, and sales can be impacted by external factors like weather (which could impact Friday’s drawing), which makes estimating exact numbers in advance challenging. Changes to the U.S. Treasuries rate can also impact the jackpot prize.

So how much will the prize grow before Friday night’s drawing? It’s difficult to say.

Between the drawing on Friday, December 9 and Tuesday, December 13, the jackpot grew by roughly $29 million to an estimated $429 million. Now, a week later, the jackpot has grown by about $81 million to $510 million.

Based on those changes, it’s likely the Mega Millions will increase by a few million dollars before Friday night’s drawing. It’s possible the jackpot will break into the top 10 largest in the game’s history – it’s just $6 million behind the 10th largest.

The other important ingredient to a growing jackpot – you can’t have frequent jackpot winners. Once someone lands the grand prize, it resets to a lower value. However, lottery games like Mega Millions are designed to go through multiple drawings without a jackpot winner. The odds of winning the game’s top prize are 1 in 302.6 million.

If you’re ready to try your luck for the half-billion jackpot, Mega Millions is played in 45 states and the District of Columbia. Tickets are $2 and there are a total of nine ways to win a prize.