The additional child tax credit is calculated by adding 15% of the taxpayer's taxable earned income to any tax-free combat pay. The total amount that is in excess of $11,750 (subject to annual adjustments for inflation) is refundable.
Even taxpayers with income below this threshold are eligible if they have at least three qualifying dependents and have paid Social Security tax in excess of the amount of their earned-income credit for the year. This credit is claimed on form 8812 and is also subject to the same phaseout limitations as the child tax credit.