The formula
Sales tax is added at checkout: total = shelf price × quantity × (1 + rate). The rate here is the combined rate for the place of purchase or delivery, which can include state, county and city parts. In Canada, GST, HST or provincial tax depend on the province.
Comparing with tax-inclusive prices
US and Canadian list prices leave this tax out, while prices in VAT countries normally include it. Add the tax before comparing, or the pre-tax market looks cheaper than it is. The price tables on this site mark US and Canadian observations as incomplete for this reason. See why US and Canadian prices exclude tax and sales tax vs VAT on price tags.