http://www.cra-arc.gc.ca/benefits/cctb/faq_payments-e.html#q9
9. How are the Canada Child Tax Benefit and the National Child Benefit Supplement calculated?
You can use our new online service to get an estimate of your benefits.
Your CCTB payments for the 12 - month period of July 2007 to June 2008 are calculated using the following information:
The number of children you have;
Your province or territory of residence;
Your 2006 family net income and;
Your child's eligibility for the Child Disability Benefit.
Basic benefit:
The basic benefit is $1,283 ($106.91 a month) for each child under age 18 (the basic benefit is different for residents of Alberta, see the note below).
There is a supplement of $90 ($7.50 a month) for your third and each additional child.
We subtract a benefit reduction from this amount if your family net income is more than $37,178. For a one-child family, the reduction is 2% of the amount of your family net income that is more than $37,178. For families with two or more children, the reduction is 4%.
Note
The Alberta provincial government has chosen to vary the amount of the basic benefit that its residents receive. The basic benefit is:
$1,174 ($97.83 a month) for children under 7;
$1,253 ($104.41 a month) for children 7 to 11;
$1,402 ($116.83 a month) for children 12 to 15; and
$1,486 ($123.83 a month) for children 16 or 17.
National Child Benefit Supplement (NCBS):
One-child family: $1,988 a year ($165.66 a month). This amount is reduced by 12.2% of the amount of family net income that is more than $20,883.
Two-child family: $1,758 a year ($146.50 a month). This amount is reduced by 23% of the amount of family net income that is more than $20,883.
Three-or-more-child family: $1,673 a year ($139.41 a month). The total will be reduced by 33.3% of the amount of family net income that is more than $20,883.
Note
If you are on social assistance, the NCBS may affect the amount of your social assistance payments. Many provinces and territories will consider the NCBS you get as income and will adjust your basic social assistance by this amount.