What is the difference in taxes between single and head of household in 2018?

no one can say for sure without knowing your tax income and its component, dependents, credits,  and additional taxes

If you want to know the standard deduction amounts for 2018:

 

2018 Standard Deductions:

Single   $12,000  (+ $1600 65 or older)

Married Filing Separately    $12,000  (+ $1300 65 or older)

Married Filing Jointly  $24,000  (+ $1300 each spouse 65 or older)

Head of Household  $18,000  (+ $1600 65 or older)

Look at line 8 of your Form 1040 to see your standard or itemized deductions

 

 

But that will not tell you the difference in the refund or tax due you would have for 2018-----there are many variables as already mentioned---the number of dependents, tax withheld, ages of the children, etc. etc. etc,