Generally, a RMD is calculated for each account by dividing the prior December 31 balance of that IRA or retirement plan account by a life expectancy factor that the IRS publishes in Tables in Publication 590-B, Distributions from Individual Retirement Arrangements (IRAs).
There are a plethora of websites that have online RMD calculators you can use to calculate your RMD. This list includes Charles Schwab, Fidelity, E*Trade, New York Life, Investor.gov