1. Total = Net Price + (Net Price x Tax Rate)
When you know the Net Price and Tax Rate, you can use above formula to calculate the final amount to charge. Your net price should be after deducting any discounts.
2. Tax Rate = (Total ÷ Net Price) -1
Use above formula if you want to know the tax rate. Some times you know the total amount and net price but don't know the tax rate.
3. Net Price = Total ÷ (Tax Rate + 1)
Use above formula when you want to know the Net Price.
4. Tax Amount = Total - (Total ÷ (Tax Rate + 1))
And if you have Total amount and tax rate and want to know about the tax amount then use above formula.