The amount of your monthly benefit for SSDI is calculated just as your full retirement is. SSA takes 31 years of your highest earnings reported, averages them, and then gives you a percentage of that amount. The percentage changes based on your income level and the year you were born.
The average SSDI payment in 2014 was $1,148. The maximum disability benefit in 2014 was $2,642.
To see your entire covered earnings history, you can check your annual Social Security Statement. Social Security sends out printed statements every five years to those not receiving benefits, and every year to those over 60. You can also check your statement online at www.ssa.gov/mystatement/.
The amount of monthly benefits you receive for SSI is calculated differently. In 2015, SSA is giving a 1.7% cost of living increase, which makes the maximum SSI benefit for an individual $733 per month and $1,100 for a couple. Because SSI is a need based disability program, any benefits you get for free, like rent, will be deducted from your monthly check. So if SSA values the value of the room and board you get for free living with family at $200 per month, they will reduce your payments by $200.