Creosote gets graded by how thick it is: a dusting, a crust, or a hardened glaze. Put the measurement in a cell and let a nested IF write the level on the inspection report the same way every time.
Under an eighth of an inch is Level 1 sooty buildup; a quarter inch or more of hardened glaze is Level 3 and needs more than a brush.
The example
Three flues measured on the same route, thickness recorded in inches.
| A | B | C | |
|---|---|---|---|
| 1 | Flue | Thickness (in) | Rating |
| 2 | 101 Oak St | 0.06 | Level 1 |
| 3 | 44 Elm Ct | 0.19 | Level 2 |
| 4 | 7 Ridge Rd | 0.31 | Level 3 |
The formula
The outer IF catches the thinnest deposits; anything it does not catch falls through to the inner IF:
How it works
Nested IFs read like a staircase — test the lowest band first:
IF(B2<0.125,...)checks the eighth-inch line. Anything under it is Level 1, sooty and brushable.- If that test fails, Excel evaluates the second IF, which checks the quarter-inch line for Level 2.
- If both tests fail the value must be a quarter inch or more, so the final argument returns Level 3 without needing a third test.
- Order matters: test from the smallest threshold upward or the bands overlap and everything lands in the first one.
On Excel 365 the same logic reads more cleanly as =IFS(B2<0.125,"Level 1",B2<0.25,"Level 2",TRUE,"Level 3").
Try it: interactive demo
Enter a measured deposit thickness in inches.
Variations
Add a recommended action
Return the next step rather than the label so the report writes itself.
Colour the rating cell
Conditional formatting on the thickness column makes a route sheet scannable.
Pitfalls & errors
These thickness bands are a common field convention, not a substitute for the CSIA/NFPA inspection levels, which describe how far you inspect rather than what you find. Keep both on the report.
Record the thickest point you measured, not an average. One glazed section is what starts the fire.
Practice workbook
Frequently asked questions
Why not use VLOOKUP with a band table?
What if the cell is blank?
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