You can calculate the difference or you can have Turbo Tax do the calculation for you.
To calculate the difference:
The standard deduction for a Head of Household status is $9,300. You can find the total itemized deductions that you took on line 40 of your form 1040. Subtract $9,300 from the total itemized deductions. This your increase in taxable income. You then need to multiply the difference by your tax rate. See screenshot below to get your tax rate (just use the percentage). The result of multiplying the difference by your tax rate is the amount your refund will decrease by switching to a standard deduction.
Use Turbo Tax to do the calculation for you:
You can create a new sign on and enter all your tax return information except that you will select that you want to use the standard deduction instead. You will not file this tax return, so you will not be charged a fee.