Engineering Accuracy: The Math and Methodology Behind Our Tax Nexus Calculator

Why We Built This

We didn't build our Nexus calculator because the world needed another tool. We built it because existing tools were failing — specifically, failing to account for the 2026 "Revenue-Only" shift in states like Illinois and Maine, which eliminated transaction count thresholds entirely and moved to pure revenue-based nexus determination.

Most calculators in production in early 2026 still applied the old dual-threshold model (e.g., "$100,000 in revenue OR 200 transactions"). For companies with high average contract values and low transaction counts, this produced false negatives — the tool said "no nexus" when the correct answer was "nexus established."

This document explains how our calculator works, where the data comes from, and the specific methodology decisions we made.

The Data Engine

Our nexus calculator pulls from 153 global jurisdictions and updates on a weekly cadence, with real-time updates pushed for any state that announces a threshold change or reclassification.

Data Sources

We do not rely on a single third-party data vendor. Single-vendor dependency creates a single point of failure — when a vendor's data is wrong, every tool that depends on it is wrong simultaneously. Our architecture cross-references at least two independent sources for every threshold value.

The Weighting Logic

The most technically significant decision in our methodology is the product classification weighting system.

The Problem with Generic Calculators

A generic nexus calculator applies a single threshold to all revenue. This works for physical goods. It fails for digital products because:

1. Some states tax SaaS as a service (lower rate, different threshold triggers)

2. Other states tax SaaS as a tangible digital asset (higher rate, same thresholds as physical goods)

3. A small number of states have bifurcated SaaS rules — cloud-delivered is exempt, locally installed is taxable — requiring product-level classification

A generic calculator that doesn't account for product classification will produce incorrect nexus determinations for any SaaS company with customers in New York, Texas, or Pennsylvania.

Our Classification Model

We utilize a weighting model that differentiates between three product classifications:

Class A: SaaS as a Service

• Fully cloud-delivered, no local install
• Customer accesses software on vendor servers only
• Examples: Pure-play web apps, browser-based tools

Class B: SaaS as a Tangible Digital Asset

• Requires any local installation (agent, desktop app, browser extension with persistent storage)
• Customer has a local software component

Class C: Hybrid/Bundled

• Combination of cloud access and local components
• Or software bundled with professional services on a single invoice

The calculator prompts for classification at the start of the analysis and applies the correct threshold and rate logic for each state based on that classification.

Where $R_s$ is your revenue in state $s$, ${\theta }_s(c)$ is the threshold for your product class $c$ in state $s$, and a score $\ge 1.0$ indicates nexus is established.

Edge Cases We Handle

The Illinois "Lookback" Problem

Illinois uses a rolling 12-month lookback for threshold calculation rather than a calendar year. A company that crosses the threshold in October must register by January — but most calculators apply calendar-year logic and report no nexus until the following January 1.

Our calculator applies state-specific lookback periods for all 14 states that use non-calendar-year measurement windows.

The "First Dollar" States

Four states — Hawaii, South Dakota, Wyoming, and Vermont — have no minimum threshold for certain digital product categories. Any sale creates nexus.

Our calculator flags these states with a "First Dollar Nexus" warning when a user reports any revenue in those states for the applicable product categories.

Marketplace Facilitator Rules

If you sell through Amazon, Shopify, or another marketplace facilitator, the facilitator collects and remits tax on your behalf in most states — which affects whether your sales count toward your own economic nexus thresholds.

Our calculator includes a marketplace facilitator toggle. When enabled, it applies the correct state-by-state rules for which sales count toward your personal threshold and which are covered by the facilitator.

What We Don't Do

Transparency requires stating limitations as clearly as capabilities:

• We do not provide legal advice. Our calculator provides a compliance analysis; final tax determinations require a qualified sales tax attorney or CPA.
• We do not cover Canada GST/HST/PST in the current version. Canadian rules are in the roadmap.
• We do not handle physical goods nexus. Our tool is built specifically for SaaS and digital product companies.
• We do not account for voluntary registration. Some companies register in states below the threshold for business reasons. Our calculator only flags legal obligations.

Accuracy Commitment

We review every state's threshold data weekly. When a state changes its rules — as Illinois, Maine, and New Mexico did in early 2026 — we push an update within 48 hours and notify users who have saved analyses that may be affected.

If you identify a data error in our calculator, contact us directly. We treat data accuracy corrections as P0 issues.