How much tax you need to pay on a taxable income of 40,000

It depends on if you are filing as a single person or married (or jointly), and for which year you are referring to:

2017 RETURNS FILED IN 2018:

Single earning in this tax bracket $37,951 to $91,900, the tax rate is 25%.

Married/Joint earning in this tax bracket $18,651 to $75,900, the tax rate is 15%.

2018 RETURNS FILED IN 2018:

Single earning in this tax bracket $38,701 to $82,500, the tax rate is 22%.

Married/Joint earning in this tax bracket $19,051 to $77,400, the tax rate is 12%.

Hope this helps! :)