The calculator includes the standard deduction $6,350 for singles and  $12,700 for filing jointly. 
68.5% of Americans use the standard deduction.  

It also has input fields for the Earned Income Credit.

Not Everyone qualifies for the Earned Income Credit, If you don't have an eligible child your income many not exceed $14,880 ($20,430 if filing jointly);

Even if you qualify for the EIC, that does not mean your taxes are zero.