US Sales Tax Explained

Two formulas, fifty rate tables, one fragmented marketplace

US sales tax is the arithmetic-light side of a system that is anything but light on jurisdictional detail. The sales tax calculator does the math in one line — multiply by the combined rate to add tax, or divide by one-plus-the-rate to extract it — but the answer is only useful if the rate is right and the sale is taxable in the first place. Both of those depend on where the buyer takes delivery, what is being sold, and whether the seller has crossed the destination state's economic-nexus threshold.

This article is the long version of what the calculator handles in two clicks. It walks the two formulas, the structure of the combined state-and-local rate, the NOMAD states, destination vs origin sourcing, the South Dakota v. Wayfair decision and the economic-nexus rules that followed, marketplace-facilitator laws, taxable services, resale certificates, and the working list of edge cases that decide whether sales tax applies at all.

The two formulas

Sales tax is a flat percentage applied to the taxable sale price. The math has exactly two directions:

Add tax: Tax = Subtotal × Rate, Total = Subtotal + Tax
Extract tax: Subtotal = Total ÷ (1 + Rate)

Rate is the combined state-plus-local figure expressed as a decimal — 7% becomes 0.07. A $100 sale at 7% gives $7 of tax and a $107 total. Reversing it, a $107 tax-inclusive total at 7% breaks down as 107 ÷ 1.07 = $100 subtotal and $7 tax. A shortcut for the extract direction is tax = total × rate / (100 + rate); at 7% that ratio is 7/107 ≈ 0.0654, so the tax component is 6.54% of the gross total. The calculator handles both directions; pick “add” when you have a pre-tax subtotal and need the total at the register, and “extract” when you have a tax-inclusive total and need to split it for bookkeeping.

How the combined rate is built

The number you punch into the calculator is rarely the state rate alone. It is the sum of every tax layer that applies at the destination address: state, county, city or municipality, and sometimes a special district — transit authority, stadium district, tourism improvement district, or convention center levy. A few examples of how the layers stack up in real jurisdictions:

• Houston, TX: 6.25% state + 1% city + 1% MTA transit = 8.25% combined
• Seattle (King County), WA: 6.5% state + 3.85% local = 10.35% combined
• Chicago, IL: 6.25% state + 1.75% county + 1.25% city + 1% RTA = 10.25% combined
• New York City, NY: 4% state + 4.5% city + 0.375% MCTD = 8.875% combined
• Phoenix, AZ: 5.6% state + 0.7% county + 2.3% city = 8.6% combined
• Anchorage, AK: 0% state, 0% local = 0% combined (one of the few Alaska cities with no local levy)

The state portion is stable for years at a time. The local portion changes more often — cities raise or lower sales tax through ballot measures, transit authorities sunset and renew their levies, and special districts get created or dissolved. State department of revenue websites are the authoritative source, and most of them publish a downloadable rate table by ZIP code or address; commercial services like Avalara, TaxJar, and the Streamlined Sales Tax Governing Board's rate locator wrap those tables with an API.

Worked example: a Seattle retailer

A Seattle-area retailer rings up a $200 pre-tax sale at the King County combined rate of 10.35%. Plug into the add formula:

Tax = 200 × 0.1035 = $20.70
Total = 200 + 20.70 = $220.70

At the end of the month the retailer remits $20.70 to the Washington Department of Revenue, where the state apportions the local component to King County and the relevant city. Reverse the direction: if the customer had been quoted “$220.70 tax included,” the calculator's extract mode would unwind it the same way:

Subtotal = 220.70 / 1.1035 = $200.00
Tax = 220.70 − 200.00 = $20.70

Same numbers, derived from the gross total instead of the net subtotal. The calculator does both directions; the choice between them depends only on which number you have in hand.

The NOMAD states and other zero-rate territory

Five states have no statewide sales tax. The acronym usually cited is NOMAD: New Hampshire, Oregon, Montana, Delaware, and Alaska. Four of them — New Hampshire, Oregon, Montana, Delaware — have zero statewide rate and no local sales tax, so the combined rate across the entire state is 0%. Alaska is the asymmetric one: the state itself charges nothing, but local boroughs and cities can and do levy their own sales taxes, with effective rates that reach 5% to 7% in parts of the Mat-Su Borough, Wrangell, and Sitka.

Outside the NOMAD list, some states have a flat statewide rate with no local add-ons — Connecticut at 6.35%, Michigan at 6%, Maine at 5.5%, Massachusetts at 6.25%, Rhode Island at 7%, Kentucky at 6%, Maryland at 6%, and Indiana at 7%. In those states the calculator's rate field is whatever the state rate is, regardless of where in the state the sale happens. Everywhere else, the combined rate moves with the destination ZIP.

Destination vs origin sourcing

Two sourcing rules decide which jurisdiction's rate applies. Destination sourcing is the more common rule: the rate is set by where the buyer takes delivery, which means an online order shipped to a New Mexico address is taxed at the New Mexico combined rate even if the seller is in California. Origin sourcing is the older minority rule: for in-state sales, the seller charges the rate at the seller's own location, and out-of-state shipments are taxed at the destination.

The states that use origin-based sourcing for in-state sales include Texas, Pennsylvania, Virginia, Arizona, Illinois, Mississippi, Missouri, Ohio, Tennessee, and Utah. California uses a hybrid — origin sourcing for the state-level portion and destination for the local district tax. Everywhere else, for both in-state and out-of-state sales, destination sourcing is the rule. The practical effect is that the calculator needs the buyer's ZIP code, not the seller's, to find the right combined rate for almost every modern e-commerce transaction.

South Dakota v. Wayfair and economic nexus

Before 2018, a state could only require a seller to collect sales tax if the seller had physical presence inside the state — a store, a warehouse, an employee, traveling sales staff, consigned inventory. That rule came from Quill Corp. v. North Dakota, a 1992 Supreme Court decision that pre-dated the e-commerce era and quickly became the most-litigated tax case of the 2000s as states watched a growing share of retail spend migrate online tax-free.

South Dakota v. Wayfair (2018) overturned Quill and validated economic nexus — the rule that a state can require collection based on a seller's economic activity in the state, with no physical presence required. South Dakota's thresholds were $100,000 in sales or 200 transactions per year, and almost every other state adopted similar rules within 18 months. Several have since adjusted: California, New York, Texas, and a handful of others raised the threshold to $500,000, and roughly half of the states have dropped the transaction-count test and kept only the dollar threshold (which avoids capturing small sellers with many low-dollar orders).

The practical effect for an online seller is that the nexus footprint has to be tracked state-by-state. Cross a state's threshold, register with that state's department of revenue, start collecting at the destination rate the calculator works out, and file returns on whatever cadence the state requires — monthly for high-volume sellers, quarterly or annually for smaller ones. Streamlined Sales Tax (SST) membership simplifies the registration and remittance side across the 24 SST member states by giving sellers a single registration that covers all of them.

Marketplace facilitator laws

The companion legislation to economic nexus is the marketplace facilitator law. Every state with a sales tax — and DC, Puerto Rico, and parts of Alaska — now requires the operator of an online marketplace to collect and remit sales tax on the sales made through its platform, instead of the individual sellers doing it themselves. Amazon, eBay, Etsy, Walmart, StubHub, Airbnb, Vrbo, DoorDash, and Uber Eats are the largest examples; every state has its own list of qualifying platforms.

For a seller whose entire business runs through marketplaces, the practical effect is that the marketplace handles the collection and the seller never touches the sales-tax line on a customer-facing invoice. For a hybrid seller with both marketplace and direct-to-consumer sales, the direct sales still require the seller to track nexus, register, collect, and remit — the marketplace coverage applies only to the orders that actually flowed through the marketplace. Most state economic nexus thresholds include marketplace sales in the threshold-counting math, which is why a small DTC seller can find itself over the threshold in a state with no direct sales of its own, simply because Amazon shipped enough of its inventory into the state.

Taxable services, digital goods, and exemptions

The default rule across most states is that tangible personal property is taxable and services are not, but the exceptions have grown wide enough that the default is no longer reliable. Hawaii, New Mexico, South Dakota, and West Virginia tax almost all services on a broad base. California, Illinois, Massachusetts, Nevada, and Virginia tax very few. In between, most states have a list of specifically enumerated taxable services — amusement and recreation, repair and installation, telecommunications, landscaping, dry cleaning, parking, pet grooming — and the list grows whenever a state legislature needs revenue.

Digital goods are the fastest-moving category. Roughly half of states tax downloaded software, streaming subscriptions, and e-books as if they were the physical equivalent. The other half either exempt them entirely or treat them under a different category with a different rate. SaaS — software accessed over the internet rather than downloaded — is taxed in about 20 states and exempt in the rest, with several states still litigating whether SaaS is a service (exempt by default) or a digital good (taxable).

Common exemptions across most states: grocery food (often taxed at a reduced rate rather than fully exempt), prescription drugs, medical devices, residential utilities, agricultural inputs, and manufacturing inputs. Most states also run sales-tax holidays — usually a weekend in August for back-to-school clothing and supplies, sometimes another for Energy Star appliances or hurricane-preparedness goods — during which specified categories are tax-free up to a per-item cap. The calculator handles only the rate arithmetic; whether a particular sale is taxable at that rate is a question for the state taxability matrix.

Resale certificates and the single-stage design

Sales tax is single-stage by design — charged only once, at the final sale to an end consumer. A retailer buying inventory from a wholesaler does not pay sales tax on the wholesale invoice; the wholesaler does not collect sales tax from the retailer. The mechanism is the resale certificate: the retailer gives the wholesaler a state-issued certificate (or a Streamlined Sales Tax exemption certificate that covers multiple states), and the wholesale invoice goes out tax-free. The retailer collects sales tax from the end customer at the retail sale instead, and remits it to the state.

This is the practical difference between US sales tax and the VAT systems used almost everywhere else in the world. VAT is multi-stage and invoice-credit: every business in the supply chain charges output tax on its sales and reclaims input tax on its purchases, with the net difference paid to the tax authority. The end customer ends up paying the same effective tax in both systems on a like-for-like sale, but the compliance machinery is completely different. The arithmetic the calculator uses — add or extract a flat percentage from a price — is identical in both systems, which is why the same calculator works for VAT or GST when you enter the relevant rate.

Common mistakes

Using the seller's rate on a destination-sourced sale

The most common e-commerce error. The rate the calculator needs is the combined rate at the buyer's shipping address, not the seller's location. Charging the wrong rate under-collects in high-rate destinations and over-collects in low-rate ones, and the state expects the right amount remitted regardless of what was actually charged on the invoice.

Forgetting the local component

Punching the state rate into the calculator and skipping the county, city, and special-district add-ons typically under-collects by 1% to 4%. The combined rate in Chicago is 10.25%; the Illinois state rate alone is 6.25%. The difference is real money on volume, and the state can come back for the shortfall during a sales-tax audit.

Treating economic nexus as a one-time setup

Thresholds reset every calendar year in most states, and a seller's nexus footprint moves with its sales mix. The once-a-year audit of where you crossed thresholds is part of the recurring compliance cycle, not a one-time onboarding task.

Charging sales tax on a resale

A retailer-to-retailer sale with a valid resale certificate is tax-free at the wholesale stage. Charging it anyway means the downstream retailer either eats the cost or has to chase a refund. The certificate has to be on file before the sale, and most states require the seller to verify the certificate number with the state's online lookup tool.

When to bring in a CPA or sales-tax software

The calculator handles the arithmetic for a single sale once you know the right rate. The cases where the arithmetic is the easy part and a tool or professional earns its fee:

• Multi-state e-commerce above the lowest economic-nexus threshold ($100,000 in any single state) — registration, monthly filings, and rate-table maintenance across dozens of states gets unwieldy fast.
• Sales of mixed taxable and exempt items, where the line items on a single invoice need different treatment — invoicing systems with a built-in taxability matrix are worth the subscription.
• Sales of services in states that tax some services and exempt others — the taxability question often outranks the rate question in dollar impact.
• Marketplace plus direct-to-consumer hybrid models where the nexus math includes marketplace sales but the collection duty does not.

Related calculators

• Sales Tax Calculator — add or extract US sales tax at any combined rate
• VAT Calculator — add or remove UK and EU value-added tax
• Tip Calculator — split a bill and calculate gratuity
• Markup Calculator — set a selling price from cost and markup percentage
• Commission Calculator — sales commission and total earnings
• Discount Calculator — sale price and savings from a list price