---
title: "What margin of error applies to Illinois breath testing?"
description: "Illinois evidentiary breath instruments must meet an accuracy tolerance of plus or minus 10 percent, or 0.01, whichever is greater, during accuracy checks under 20 Ill. Adm. Code 1286. That is the..."
url: https://chicagoduilawyer.net/faq-items/what-margin-of-error-applies-to-illinois-breath-testing/
date: 2026-07-08
modified: 2026-07-08
author: "Chicago DUI Lawyer"
type: faq
lang: en
---

# What margin of error applies to Illinois breath testing?

Illinois evidentiary breath instruments must meet an accuracy tolerance of plus or minus 10 percent, or 0.01, whichever is greater, during accuracy checks under 20 Ill. Adm. Code 1286. That is the machine-level tolerance, not a per-test uncertainty. Actual per-test uncertainty includes breath-to-blood partition variation, temperature effects, and biological variability that can add several hundredths to true uncertainty. Illinois law does not automatically discount a reading by the tolerance, but defense experts routinely testify to reasonable scientific uncertainty around a numerical result. On borderline readings such as 0.08 or 0.09, the margin argument creates reasonable doubt on the per se element. A Chicago DUI lawyer builds this defense with experts and cross-examination.