Duct Friction Loss Chart — Per 100 Ft by CFM & Duct Size
What a straight run of round metal duct costs in static pressure: friction loss in inches of water column per 100 feet, for every practical pairing of airflow and duct diameter. Computed from the Darcy-Weisbach equation at ASHRAE standard air in galvanized duct — the inverse read of the duct sizing chart. Cells past 2,500 FPM are dashed rather than endorsed.
Friction loss — in. w.g. per 100 ft of straight duct
| CFM | 4" | 5" | 6" | 7" | 8" | 9" | 10" | 12" | 14" | 16" | 18" | 20" | 24" |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 50 | 0.171 | 0.058 | 0.024 | 0.012 | 0.006 | — | — | — | — | — | — | — | — |
| 75 | 0.357 | 0.12 | 0.05 | 0.024 | 0.012 | 0.007 | — | — | — | — | — | — | — |
| 100 | 0.605 | 0.202 | 0.083 | 0.039 | 0.021 | 0.012 | 0.007 | — | — | — | — | — | — |
| 150 | 1.284 | 0.425 | 0.174 | 0.082 | 0.043 | 0.024 | 0.015 | 0.006 | — | — | — | — | — |
| 200 | 2.202 | 0.724 | 0.294 | 0.138 | 0.072 | 0.041 | 0.024 | 0.01 | — | — | — | — | — |
| 250 | — | 1.097 | 0.444 | 0.208 | 0.108 | 0.061 | 0.036 | 0.015 | 0.007 | — | — | — | — |
| 300 | — | 1.546 | 0.623 | 0.29 | 0.151 | 0.085 | 0.051 | 0.021 | 0.01 | 0.005 | — | — | — |
| 400 | — | — | 1.067 | 0.495 | 0.256 | 0.143 | 0.086 | 0.035 | 0.017 | 0.009 | — | — | — |
| 500 | — | — | — | 0.752 | 0.387 | 0.216 | 0.129 | 0.053 | 0.025 | 0.013 | 0.007 | — | — |
| 600 | — | — | — | 1.06 | 0.544 | 0.303 | 0.18 | 0.074 | 0.035 | 0.018 | 0.01 | 0.006 | — |
| 800 | — | — | — | — | 0.935 | 0.52 | 0.308 | 0.125 | 0.059 | 0.031 | 0.017 | 0.01 | — |
| 1,000 | — | — | — | — | — | 0.791 | 0.467 | 0.189 | 0.089 | 0.046 | 0.026 | 0.016 | 0.006 |
| 1,250 | — | — | — | — | — | — | 0.712 | 0.287 | 0.134 | 0.069 | 0.039 | 0.023 | 0.01 |
| 1,500 | — | — | — | — | — | — | — | 0.404 | 0.188 | 0.097 | 0.055 | 0.033 | 0.013 |
| 2,000 | — | — | — | — | — | — | — | — | 0.322 | 0.166 | 0.093 | 0.055 | 0.023 |
| 2,500 | — | — | — | — | — | — | — | — | 0.491 | 0.252 | 0.141 | 0.084 | 0.034 |
| 3,000 | — | — | — | — | — | — | — | — | — | 0.356 | 0.198 | 0.117 | 0.048 |
| 4,000 | — | — | — | — | — | — | — | — | — | — | 0.341 | 0.201 | 0.082 |
| 5,000 | — | — | — | — | — | — | — | — | — | — | — | 0.307 | 0.124 |
From chart to static budget
The chart prices only the straight duct. A real external-static budget stacks this on top of every component the air crosses — coil, filter, dampers, registers — and adds fittings as equivalent lengths of straight duct. Work the whole budget with the static pressure calculator, size new runs from the duct sizing chart, and convert rectangular duct through the equivalents chart before reading this one. Verify against the equipment's blower data — friction you don't budget comes out of the airflow.
Common questions
How do I read a friction loss chart?
Find your airflow row and duct-size column; the cell is the static pressure the duct itself eats per 100 feet of straight run. Multiply by your actual effective length over 100 — a 60-foot effective run of 10" duct at 500 CFM loses about 0.6 × 0.129 ≈ 0.08 in. w.g. Fittings are added on top as equivalent lengths.
What is a good friction loss for ductwork?
Most designs land between 0.05 and 0.10 in. w.g. per 100 ft for supply mains — low enough to stay inside a typical fan's external static after coil, filter, and register losses. There is no virtue in ultra-low friction (it just buys bigger duct); the budget method — available static divided by effective length — gives the right number for your system.
Why are some cells blank?
Cells above 2,500 FPM are dashed because that pairing runs faster than recommended for ducted mains — the loss is calculable but the duct would be loud and wasteful; step up a size. Cells below 0.005 in/100 ft are dashed because the duct is far oversized for the flow and the reading carries no design signal.
Does this chart work for flex duct or rectangular duct?
No — it is computed for rigid galvanized round duct. Flex duct runs materially higher friction even when fully stretched, and rectangular duct is handled by converting to its equivalent round diameter first (the duct equivalent chart), then reading this chart at that diameter.
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