Data Accuracy & Methodology
How Sefro sources tax-rate data, how current it is, and what the
accuracy signals in each response mean.
Every response is self-describing
Each rate and calculation response includes a rate_type field:
authoritative — sourced directly from a state
Department of Revenue or an official rate file (state open-data CSV, DOR PDF/XLSX, or
the Streamlined Sales Tax rate files).estimated — an approximation derived from base
geographic data (state base rate or ZIP-level fallback) where an authoritative local
rate is not available.
Responses also carry a disclaimer and, for address lookups, a
confidence (high / medium / low) and
warnings describing how completely the address matched. Use these fields to
decide when to require a full validated address or a human review.
Sourcing tiers
| Tier | Meaning | Examples |
|---|
| A | Official, deterministic, accurate | 23 SST member states (SST rate files), CA (CDTFA open data), NY (Pub 718), CO (CDOR XLSX), TX, FL |
| B | Aggregator, deterministic, accurate for simple states | states with no complex special districts |
| C | Hard — approximate or partial | states whose accurate local rates require GIS/commercial data (e.g. some CO/NM/ID special districts) |
| — | No local sales tax | state base rate is the correct rate (e.g. CT, NJ, DC, and the no-sales-tax states) |
Update cadence
- Authoritative source files are re-fetched weekly by a dedicated
cron service (Sundays, 04:00 UTC) that runs the full fetcher chain and then a
coverage check.
- The data refresh runs only in that cron service, never inside the
API process, so a partial run can never corrupt live rates.
- Per-state import status (last run, last success, row counts, errors) is tracked
internally and surfaced to administrators.
Known limitations
- ZIP codes are not tax jurisdictions. A single ZIP can span
multiple taxing jurisdictions; ZIP-level results are approximate and marked
accordingly. For the most accurate result, provide ZIP + state + county + city or a
full street address.
- Special districts (transit, stadium, tourism) in a few states are
not fully resolved from free official files and may be approximated; these are the
Tier C cases above.
- Product taxability (groceries, clothing thresholds, SaaS,
medicine) is applied from a maintained rules table and reflects general state
treatment; edge cases and local overrides may differ.
Verification
We cross-check our authoritative rates against an independent third-party dataset for
a large sample of ZIP codes and review divergences. This is a verification aid, not a
source.
Rates are provided for informational purposes only and are not tax,
accounting, or legal advice. You are solely responsible for determining, collecting, and
remitting the correct tax for each transaction. See the
Terms.