Using online payroll you will have to play around with the numbers to gross it up. If you were using desktop payroll it offers a convenient feature to enter net and it calculates gross. From there you would simply divide by hours.
In a nutshell (Algebra I, 8th grade)
GROSS PAY - FWT - %DEDUCTIONS×GROSS =NET.
As a tentative, look up what FWT should be close to from the IRS tables. Add that to the net. Then divide this by the reduced ,% from the left side. Deductions are FICA plus State plus Local. If 10% you divide the right side by .9
Example you want net of 1000. You estimate FWT at 100, taxes total 10% (FICA alone is 7.65%)
GR - 100 - .1GR = 1000
Combining
.9GR = 1100
GR = 1222.22