Forty-five out of the 50 states in the United States charge sales tax on goods. If you are either a consumer or business owner in any of these states, it's helpful to know not only how to calculate the sales tax in dollar form, but also to know how to round the figure up the correct way. As a consumer, you'll know off the bat whether you're being overcharged. If you're a small merchant who collects sales without the use of a cash register, which automatically rounds figures, this is also helpful information.

Determine the sales tax rate in your state and the total amount of the sale. In this example, use a 6.875 percent sales tax rate and a total sale amount of $100.

Convert the sales tax rate to a decimal figure (0.06875) and then multiply it by the total sales. In this example the result is $6.875. There's no such thing as half a penny, so you have to round up the figure to the next penny.

Evaluate whether the third digit after the decimal point is greater than, less than or equal to five. If five or greater, round the sales tax dollar amount up to the next penny and remove the extra digits. In this case, the total sales tax is $6.88. If the figure is less than five, just remove any extra numbers following the second digit after the decimal point to get your sales tax amount. For instance, if the figure were $6.871, the final sales tax figure is $6.87.

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- 3D dollar sign with multiple smaller dollar signs. image by Steve Johnson from Fotolia.com