Sales Tax Explained: What Is Taxed, Nexus, and How to Calculate It

Sales tax is a consumption tax charged on the sale of goods and some services, collected by the seller and remitted to the state and local governments. In the United States there is no national sales tax: each state sets its own rate and its own base — what is taxed and what is exempt — and local jurisdictions add their own rates on top. The arithmetic is one multiplication. The complexity is entirely in the rate and the base: which rate applies, to which sale, in which place, and what is exempt. This guide covers the arithmetic, the rate structure, nexus and the marketplace rules that decide who collects, and the exemptions — method only, with no state-rate table, because rates change and a table in an article goes stale.

The arithmetic: one multiplication, two directions

Adding tax: tax = price × rate; total = price × (1 + rate). At 8.25% on $100: tax $8.25, total $108.25. Removing tax from a tax-inclusive price: price = total ÷ (1 + rate); tax = total − price. A $108.25 total at 8.25% gives a pre-tax price of $100.00 and tax of $8.25. The two directions are inverses, and the common error is to remove tax by subtracting the rate — $108.25 minus 8.25% is $99.32, not $100.00 — because the rate applies to the pre-tax price, not to the total. The Sales Tax Calculator does both directions and shows the split, so the error is visible before it is made.

Rounding follows the jurisdiction’s rules, and the rules vary: round per item, per transaction, or per line, and round half up or to the nearest cent by the statutory method. The differences are cents per transaction and dollars per audit, and a seller’s system must follow the jurisdiction’s method, not a generic one. Discounts complicate the base: a manufacturer’s coupon usually does not reduce the taxable price, while a store coupon usually does, and the distinction varies by state. The taxable price is the price after the discounts the state allows, not the price the customer perceives.

Multiple rates in one transaction are handled line by line. A basket with taxable goods, exempt groceries, and a taxable service is not taxed at one rate on the total; each line is taxed at its own rate, and the exempt lines are not taxed at all. A single combined rate applied to the whole basket overstates the tax whenever any line is exempt or differently rated. The calculator’s per-line treatment is the correct model, and a receipt that shows one tax line for a mixed basket should be checked line by line.

Rates, base, and the local layer

The combined rate is the state rate plus every local rate that applies at the place of sale: county, city, transit district, and special districts. State rates run from 0% in the five states with no statewide sales tax — Oregon, Montana, New Hampshire, Delaware, and Alaska, though Alaska allows local sales taxes — to over 7% at the state level, and combined rates exceed 10% in some localities. The same goods, bought a mile apart across a city boundary, can carry different combined rates, and the boundary is the taxing jurisdiction’s, not the shopper’s intuition of the neighbourhood.

The place of sale is the delivery address for shipped goods in destination-based states, which are the majority, and the seller’s location in origin-based states, which are the minority. A business shipping to customers in many states must therefore know the destination rate for each shipment, not one rate for its own location. The distinction is the reason sales-tax software exists: the rate table is thousands of jurisdictions deep, it changes monthly, and a business that guesses it will guess wrong. This guide quotes no rate table, because a table printed in an article is stale the month a locality changes its rate, and a stale rate is worse than no rate — it is a wrong rate believed.

The base — what is taxed — varies more than the rate. Most states tax tangible personal property; most exempt groceries and prescription drugs, in whole or in part; services are taxed selectively, with digital goods, software, and streaming taxed in some states and not in others; clothing is exempt in a few states and taxed in most. The base is where the money is: a state with a 6% rate on a broad base collects more than a state with a 7% rate on a narrow one, and a buyer who assumes the base of one state applies in another will miscalculate. Check the state’s base for the category before calculating, not after.

Nexus, marketplaces, and who collects

Nexus is the connection that obliges a seller to collect a state’s sales tax. Before 2018, nexus generally required a physical presence. Since South Dakota v. Wayfair (2018), economic nexus — sales above a threshold, commonly $100,000 or 200 transactions in the state — obliges remote sellers to collect, even with no physical presence in the state. A seller with customers in 30 states may have nexus in all 30, and must register, collect, and remit in each, at the destination rate for each sale. The thresholds and the definitions vary by state, and the compliance burden is the reason marketplace and software solutions exist.

Marketplace facilitator laws shift the collection duty for sales through a marketplace to the marketplace itself: a sale through a large online marketplace is collected by the marketplace, not by the individual seller, in the states with such laws — which is now nearly all of them. The seller’s own website sales remain the seller’s duty. The distinction matters for the small seller: the marketplace sales are handled, the direct sales are not, and the direct sales’ nexus must still be tracked. A seller who assumes the marketplace covers everything will miss the direct channel.

Use tax is the companion: where sales tax was not collected — an out-of-state purchase, a business purchase for use — the buyer owes use tax to their own state at the same rate. Consumers rarely pay it and rarely are pursued for it; businesses are audited for it, and the audit looks at purchases, not sales. The use tax is the reason “no sales tax charged” is not “no tax owed,” and the reason a business’s purchasing records matter as much as its sales records.

Exemptions, holidays, and the checks

Exemptions fall into three kinds: exempt goods (groceries and prescriptions in most states, clothing in a few), exempt buyers (nonprofits, government, resellers buying for resale with a certificate), and exempt uses (manufacturing inputs, agricultural inputs, goods shipped out of state). Each requires documentation: a resale certificate for a reseller, an exemption certificate for a nonprofit, and the seller who accepts a certificate in good faith is generally protected. An exemption claimed without the certificate is a liability, and the liability lands on the seller if the certificate is missing and on the buyer if it is false.

Sales tax holidays — days or weekends when specified goods, usually clothing, school supplies, and computers below a price cap, are exempt — are a timing opportunity, not a saving strategy. The caps and categories are specific, the dates are set annually, and the saving is the tax on the qualifying goods only. Buying early or late to catch a holiday saves the tax; buying more to catch a holiday spends more than the tax saved. The holiday’s terms are published by each state’s revenue department, and they change yearly.

Checks before calculating. The rate is for the delivery jurisdiction, at the current date, from an authoritative source — the state’s rate lookup, not an article’s table. The base is checked for the category: taxable, exempt, or reduced-rate. The seller’s nexus and marketplace status decide who collects. And the rounding follows the jurisdiction’s method. A calculation that passes all four is right; one that fails any of them is a guess with arithmetic.

Worked basket: taxable, exempt, and reduced-rate lines together

A basket shows why the per-line treatment matters. Groceries of $80.00, exempt in the state. Clothing of $50.00, taxable at 8.25%. A repair service of $30.00, exempt, because the state does not tax that service. The tax is charged only on the clothing: $50.00 × 8.25% = $4.125, rounded to $4.13 by the jurisdiction’s method. The total is $80.00 + $50.00 + $30.00 + $4.13 = $164.13. A single combined rate applied to the whole $160.00 of goods and services would give $13.20 of tax — more than three times the correct figure — because it taxes the exempt lines.

The rounding follows the jurisdiction’s method, and the method should be visible on the receipt. Per-line rounding rounds each line’s tax to the cent and adds the rounded lines; per-transaction rounding adds the unrounded taxes and rounds the total. The two differ by cents, and the difference is the jurisdiction’s choice, not the seller’s: a system that rounds per line in a per-transaction state overcharges by the drift, and a system that rounds per transaction in a per-line state undercharges by it. The receipt that shows the tax per line, with the rate stated, lets the buyer check the method; the receipt that shows one tax line for a mixed basket does not.

Discounts and coupons change the base before the rate applies, and the change depends on the coupon’s kind. A store coupon — the seller’s own discount — usually reduces the taxable price: a $50.00 item with a $10.00 store coupon is taxed on $40.00. A manufacturer’s coupon — a third party’s payment — usually does not: the same item with a $10.00 manufacturer’s coupon is taxed on $50.00, because the seller still receives $50.00, $40.00 from the buyer and $10.00 from the manufacturer. The distinction varies by state, and the basket’s taxable lines must be priced after the discounts the state allows, not after the discounts the buyer perceives. Check the state’s coupon rules before assuming the receipt is right.

Total with tax: $164.13. A combined rate on the whole basket would wrongly give $13.20.

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