AGNC Investment (AGNC 1.02%) pays a monster monthly dividend. The real estate investment trust (REIT) currently yields around 14%. That's 10 times higher than the S&P 500's (^GSPC 0.64%) dividend yield.

The mortgage REIT has maintained its massive monthly dividend for the last 60 consecutive months. However, given the size of its payout, dividend sustainability remains a concern. Here's the number investors should keep an eye on to make sure AGNC Investment can maintain its monster dividend.

The important number for AGNC investors

For most dividend stocks, the dividend payout ratio is a key metric to monitor for dividend stability. The problem is that AGNC Investment isn't like the average dividend stock. The mortgage REIT uses leverage (debt and debt-like instruments) to buy more mortgage-backed securities (MBS) to boost its returns. It also uses dynamic risk management strategies to protect its portfolio from risks.

Because of that strategy, earnings-based financial metrics aren't the best measure of dividend stability. CEO Peter Federico stated on the REIT's fourth-quarter earnings conference call that "we don't want investors to look at [earnings measures] as a driver of our dividend policy because it is not." He commented that the company only measures the earnings in the current period, not the long-run earnings potential of its portfolio.

Instead, Federico highlighted a different number that investors should keep an eye on. He stated on the call, "The first thing we look at from a dividend perspective is what is the total cost of capital hurdle rate versus our expected return at current valuation levels of the portfolio." He noted that the REIT needs to earn a return on equity of 16.7% to cover its operating expenses, preferred stock dividend, and monthly common stock dividend.

The CEO remarked on the call that the portfolio's ROE is currently between 17% and 18.5%. Federico commented that this level aligns with its cost of capital and is one of the reasons it has continued to maintain its dividend. So as long as its ROE remains above its cost of capital, the REIT should be able to maintain its monster dividend.