Instructions

1

Know what your local sales tax is. If you live an area that has both state and other local sales tax, add them together to get the appropriate amount.
2

Take the rate and first mentally multiply it times the number of dollars that you have purchased. If the rate is 7% and your purchase is $1 the amount that you will have to pay is 7 cents. For every dollar that you purchase with a sales tax of 7% you need to add this amount. If your purchase is $10 then add 70 cents. If you're living in a state with a 5% sales tax, either multiply by .05 or 5 cents on the dollar or multiply by 10 cents and divide by 2 ($10 X.10=1.00/2=.50).
3

Adjust for the dollars in between. Most taxes will have an equal divisor. If the amount is 5%, then every 20 cents another penny is added. For that 7% rate the amount is 100/7=14 or every 14 cents another penny is added.
4

Add the dollar amount and the cents amount together. If your purchase is $10.42 you have 50 cents for the first $10 in a 5% tax area and since there is 42 cents we would add another 3 cents or 1 cent for the start of every 20 cents, to that for a total of 53 cents.
5

Break down larger amounts to multiply. If a purchase is $2350 and your locality charges a 5% tax, figure $100 for the $2000, $15 for the $300 amount, $2.50 for the $50. Add the $100+15+2.50 for a total of $117.50.