How do I calculate the amount of sales tax that is included in a sales amount?

To calculate the sales tax that is included in the total amount of a sale, you can divide the total amount by 1 + the sales tax rate.

To illustrate, let’s assume that the sales tax rate is 7%. An item that is subject to the sales tax was sold for a total amount of $481.50 including sales tax. We can express this total amount as the [selling price before sales tax] + [the sales tax on the amount before sales tax] = the total amount. Algebraically it would be: Let S = the selling price before sales tax.  S+0.07S=$481.50;  1.07S=$481.50; S=$481.50 divided by 1.07; S=$450.00. The proof is selling price of $450 + the sales tax of $31.50 ($450 X 0.07) = $481.50. 

Let’s try another example. If the total amount including a 7% sales tax is $32,100 what is the actual sales and what is the sales tax? $32,100 divided by 1.07 = $30,000 the true sales amount. The sales tax payable will be 0.07 X $30,000 = $2,100. In general journal form it will be: debit Cash $32,100; credit Sales $30,000; credit Sales Tax Payable $2,100.