I came across with following discussion on how to calculate sustainable growth rate of a company. TEDies can add more.

The sustainable growth rate is the highest growth rate a company can sustain without external equity financing, with an unchanging debt-equity ratio. It is closely related to the internal growth rate. The formula to find the sustainable growth rate is:
sustainable growth rate = (ROE X b)/(1 - ROE X b)

The ROE ratio is net income/total equity.
The variable b is the retention or plowback ratio. The plowback ratio is retained earnings/net earnings.

This article demonstrates through example how to calculate the internal growth rate of a firm.

net income ..........$500
total equity ......$2,100
retained income .....$395

1.         

Locate the ROE ratio and the plowback ratio. Plug the problem information into the equations and solve to find the ratios.

ROE ratio = net income/total equity
ROE ratio = $500/$2,100
ROE ratio = 0.24
ROE ratio = 24%

plowback ratio = retained earnings/net earnings
plowback ratio = $395/$500
plowback ratio = 0.79
plowback ratio = 79%

2.         

Take the information and plug it into the sustainable growth rate formula.

sustainable growth rate = (ROE X b)/(1 - ROE X b)
sustainable growth rate = (0.24 X .79)/(1 - 0.24 X 0.79)

3.         

Solve the equation to find the sustainable growth rate. Make sure to follow the correct order of operations.

sustainable growth rate = (0.24 X .79)/(1 - 0.24 X 0.79)
sustainable growth rate = 0.19/(1 - 0.19)
sustainable growth rate = 0.19/0.81
sustainable growth rate = 0.23

How fast a firm can grow without additional external equity financing, the sustainable growth rate, is 23%.