Social Security Disability Insurance is one of those government programs that more or less gives back what you pay in. If you become disabled before retirement age and can't work, SSDI kicks in if you're qualified – meaning you meet the Social Security Administration's criteria for disability and you worked for some period of time. The more you paid in while working, the more you'll receive.
If you want to do it the hard way, you can figure out what your SSDI payments will be by determining your Average Indexed Monthly Earnings or "AIME" and your Primary Insurance Amount or "PIA." You would then incorporate these figures in a complicated equation that tells you how much you can expect to receive if you go out on disability. Or you can simply go to the SSA's website and make use of the benefit calculator offered there. The average monthly SSDI payment in April 2014 was about $1,145, according to the SSA. Benefits are tweaked periodically to keep pace with inflation.
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If you're covered by disability insurance from some other source, this might offset some portion of your SSDI payments, and the SSA online calculator will take this into consideration. If you receive workers' compensation, this will probably reduce your benefits, but veterans' benefits and private insurance generally do not. Contact the SSA or an attorney if you're receiving some other form of insurance and you're unsure how the SSA will treat it.