Sales Tax Calculator

The underlying calculation

Sales tax is added to the price rather than extracted from it: tax = price × rate, and the total is price × (1 + rate). US sales tax is conventionally quoted and displayed exclusive of tax, which is why the shelf price and the till total differ.

The rate is the sum of every layer that applies at the delivery address: state, county, city, and any special district. Those layers are set independently and there are more than 13,000 combinations in the United States.

An example

A $58 order delivered to an address with a 6.25% state rate, a 1% county rate and a 1.75% city rate, 9% combined. Tax is $5.22 and the total is $63.22.

Move the same order two miles to an address outside the city limits: 7.25% combined, $4.21 of tax, $62.21 total. Identical order, a dollar apart, decided by a boundary the customer cannot see.

That is why rate lookup has to be by full address rather than by ZIP code. A single ZIP frequently spans several taxing jurisdictions, and ZIP-level rates are wrong often enough to matter.

What it does not tell you

Not everything is taxable, and what is exempt varies enormously by state. Groceries, prescription drugs, clothing and digital goods are each treated differently across the fifty states, and applying one rate to a mixed basket overcharges some lines and undercharges others.

The seller is liable for tax that should have been collected and was not. It cannot practically be recovered from customers afterwards, which means an under-collection is paid out of margin that has already been spent.

What this changes

Use address-level rate lookup rather than ZIP tables, and automate it. Maintaining rates by hand is viable in one state and fails everywhere else, because rates change hundreds of times a year across the country.

Then map product taxability per state rather than assuming. A product exempt in one state and taxable in the next is common, and the mapping is a one-time exercise that prevents a recurring error.

Sourcing: where the sale is taxed

Destination sourcing taxes the sale at the buyer's address; origin sourcing taxes it at the seller's. Most states use destination sourcing for remote sellers, and a handful use origin sourcing for sellers located within the state.

That means a business can face two different rules for the same product depending on whether the customer is in its home state or another one. Texas is the clearest example: origin-based for intrastate sales, destination-based for remote ones.

The practical consequence is that a business's home state is often its most complicated one, not its simplest. Remote states apply a single consistent rule; the home state applies whatever local arrangement exists, plus any local registration and filing obligations that come with having a physical presence there.

Several states require a registered seller to file a return even in periods with no sales. Zero returns are compulsory and missing them attracts penalties despite there being no tax due.

That is the hidden ongoing cost of a registration, every state you register in adds a filing calendar that continues whether or not you sell there again.