What is Modified Adjusted Income (MAGI)
Obamacare subsidies are not calculated using taxable income (line 43 on a Form 1040). According to the IRS and Kaiser Family Foundation Vice President Gary Claxton, they are calculated using something called MAGI: modified adjusted gross income (which is basically line 37, with some minor modifications).
The difference between the two figures is that the income on which you pay a tax has already been reduced by the amount of either Schedule A itemized deductions, or the standard deductions.
For 2014, you can estimate your Modified Adjusted Gross Income, by finding the Adjusted Gross Income listed on your most recent federal tax return and adjusting it based on any changes (additional income or reductions) you expect to see in your income for 2014. Remember, it’s your household’s 2014 (not 2012 or 2013) MAGI that determines your eligibility for a subsidy. A final decision on how much subsidy assistance you may receive in 2014 is made only by the government exchange authority in your state.
To calculate your subsidy I recommend following this link to the State of Colorado income calculator.