Practitioner tool
NSR Baseline & Project Emissions Calculator
“Is this project a major modification? Which 24-month baseline period helps us most?”
1. Project and lookback
Enter the project date to limit windows to the lookback. Until then every window in your data is treated as eligible.
2. Monthly emissions by unit (tons per month)
Enter values already adjusted as the rule requires (for example, excluding non-compliant emissions and emissions above limits that now apply). Leave a month blank if it lacks adequate data; windows that include it are skipped.
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Rows shaded green are the highest eligible window.
One row per month, one column per unit (up to 8). A month label column and a header row are ignored. Set the first month of data first.
3. Baseline actual emissions
No eligible window yet. You need at least 24 consecutive months with data for every unit, inside the lookback.
4. Project emissions increase (Step 1)
For each unit in the project, enter projected actual emissions (or its potential to emit). New units have a baseline of zero, so the sum below is the hybrid test when the project has both.
| Unit | Type | Baseline (tpy) | Projected actual or PTE (tpy) | Uses PTE | Could have accommodated (tpy) | Increase | Remove |
|---|
Sum of unit increases
0 tpy
Enter the significance value from the rule that applies to compare.
How this works
Baseline actual emissions
- Every consecutive window of the length you set is checked. A window counts only if it sits inside the lookback before the project date and every unit has data for every month in it.
- Each unit's baseline is the window's total tons × 12 ÷ window length, the average annual rate.
- All units in the project share one window, and the window with the highest combined rate is shown first. You may pick a different eligible window; the top five are listed.
- Values must already be adjusted the way the rule requires. The tool does not remove non-compliant emissions for you.
Step 1: the project emissions increase
- For each existing unit: projected actual emissions minus baseline, minus emissions the unit could have accommodated during the baseline period that are unrelated to the project.
- Electing potential to emit in place of projected actual emissions removes that exclusion.
- New units have a baseline of zero, so a project with new and existing units is summed the same way (the hybrid test).
- The comparison uses the significance value you enter. A result at or above it is only Step 1; netting (Step 2) decides whether the project is a major modification.
Rule text this tool follows
“Baseline actual emissions means the rate of emissions, in tons per year, of a regulated NSR pollutant”
40 CFR 52.21(b)(48)
“during any consecutive 24-month period selected by the owner or operator within the 5-year period”
40 CFR 52.21(b)(48)(i)
“within the 10-year period immediately preceding either the date the owner or operator begins actual construction”
40 CFR 52.21(b)(48)(ii)
“only one consecutive 24-month period must be used to determine the baseline actual emissions for”
40 CFR 52.21(b)(48)(ii)(d)
“A different consecutive 24-month period can be used”
40 CFR 52.21(b)(48)(ii)(d)
“The average rate shall not be based on any consecutive 24-month period for which there is inadequate information”
40 CFR 52.21(b)(48)(ii)(e)
“the unit's emissions following the project that an existing unit could have accommodated”
40 CFR 52.21(b)(41)(ii)(c)
“A significant emissions increase of a regulated NSR pollutant is projected to occur if the sum of the difference”
40 CFR 52.21(a)(2)(iv)(c)
Excerpts quoted verbatim from 40 CFR 52.21 (2020 edition, govinfo.gov). Your state's SIP-approved rule may differ; confirm against the current text.
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