State rate plus local rate equals combined rate
The rate a buyer pays is the state base rate plus any local rates layered on top. The state portion is the easy part: a single published number per state, from zero in the no-tax states to 7.25 percent in California. The US sales tax calculator gives you that figure for any state.
The combined rate is what the customer actually pays, and it can run several points higher than the state rate once local taxes are added. In some jurisdictions the combined rate exceeds 10 percent.
Thousands of local jurisdictions
Below the state sit counties, cities, and special districts (transit authorities, stadium districts, and the like), each able to levy its own sales tax. Across the country that adds up to thousands of distinct taxing jurisdictions, and their boundaries do not follow ZIP codes neatly. Two addresses in the same ZIP can fall in different districts and owe different rates.
This is the core reason US sales tax is harder than Canada's. Canada has a clean rate per province; the US has a state rate plus a shifting patchwork of local rates that changes thousands of times a year.
Origin vs destination sourcing
Which local rate applies depends on a state's sourcing rule. Most states are destination-based: you charge the rate at the buyer's location. A few are origin-based for in-state sales, charging the rate at the seller's location. For interstate sales, destination sourcing is the norm. The practical upshot is that you usually need the buyer's full address, not just their state, to get the rate right.
Why a flat or state-only rate under-collects
Charging a single flat rate, or only the state rate, means you under-collect in most jurisdictions and over-collect in a few. Under-collection is the bigger risk: the state still expects the full combined amount on your return, so any shortfall comes out of your margin, plus penalties. This is why our state calculator is explicit that it shows the state portion only, and why accurate checkout needs address-level rates.
Rooftop-accurate combined rates come from a tax engine (Avalara, TaxJar, or similar) that maps a full address to its exact jurisdiction stack in real time. A static rate table cannot keep up with thousands of annual changes.
Getting combined rates right in Odoo
Odoo integrates with rate engines so that each order line is taxed at the exact combined rate for the delivery address, with the result broken out by jurisdiction for filing. That replaces brittle manual rate tables and keeps you current as local rates change. Octura wires up the integration, maps your product tax codes, and validates that collection reconciles to your returns.
Read: Odoo accounting workflows with Avalara →References
- Tax Foundation, State and Local Sales Tax Rates 2026. State rates and population-weighted average local rates by state. taxfoundation.org/data/all/state/sales-tax-rates
- Streamlined Sales Tax Governing Board, rate and boundary databases. The multistate effort to standardize jurisdiction and rate lookups. streamlinedsalestax.org