Skip to content
AirPermitHub

Practitioner tool

O2 Correction Calculator (Correct ppm to a Reference Oxygen Level)

Correct a measured concentration to a reference oxygen level such as 3%, 7%, or 15% O2 using the 20.9 correction. Shows the formula and steps.

Emission limits for combustion sources are often written at a reference oxygen level so diluted exhaust can't look cleaner than it is. Enter the measured concentration and O2 (dry basis) and the reference O2 from your permit or rule.

O2 correction calculator

Use the reference O2 your permit or rule specifies.

Corrected to 15% O2
13.532 same unit as measured
Correction factor
0.54128 ×

Steps

  1. Factor = (20.9 − 15) ÷ (20.9 − 10) = 5.9 ÷ 10.9 = 0.54128
  2. Corrected = 25 × 0.54128 = 13.532

Questions engineers ask

Why correct to a reference O2?
Adding excess air dilutes the exhaust and lowers the measured ppm without lowering mass emissions. Correcting every result to the same oxygen level makes results comparable to the limit.
Which reference O2 should I use?
The one written in your permit condition or the rule's emission standard. Different source categories use different reference levels.
Can I correct a wet concentration?
Convert it to a dry basis first (see the wet-to-dry converter) and use O2 on a dry basis.

More converters: ppm to lb/hr, ppm to lb/MMBtu, lb/MMBtu to lb/hr, g/hp-hr to lb/hr, lb/hr to tons/yr, ppm to mg/m³, Wet to dry basis.

How this works

Formula

  • C(ref) = C(measured) × (20.9 − O2(ref)) ÷ (20.9 − O2(measured))
  • Both O2 values in percent, dry basis

Before you rely on the number

  • The 20.9 correction assumes the measured concentration and O2 are both on a dry basis.
  • Check whether your rule specifies a different correction (for example a CO2-based correction) before using this one.
ShareLinkedIn Email

AirPermitHub provides educational, engineering, research, and compliance-support information. Information provided by the platform does not constitute legal advice and should not replace review of applicable regulations, permits, agency guidance, or consultation with qualified professionals. Full disclaimer