Blower Airflow from Fan Table Calculator
Type in your furnace or air handler’s blower table, enter the measured total external static pressure, and read the airflow for each speed tap — with CFM per ton.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Speed 1
- 1,398CFM
- Speed 2
- 1,248CFM
- Speed 3
- 1,098CFM
At 0.25 in. w.c.: speed 1 1,398 CFM, speed 2 1,248 CFM, speed 3 1,098 CFM. Speeds 2, 3 fall in the typical 350–450 CFM per ton range.
- A blower table is only as good as the static pressure reading and the filter and coil conditions it assumes (wet or dry coil, filter in or out). Read the table’s footnotes.
Airflow at the measured static pressure
- Speed 1
- 1,398CFM
- Speed 2
- 1,248CFM
- Speed 3
- 1,098CFM
CFM per ton
- Speed 1
- 466CFM/ton
- Speed 2
- 416CFM/ton
- Speed 3
- 366CFM/ton
Show the working
- Speed 1: interpolate between 0.1 and 0.3 in. w.c.1,450 + (0.25 − 0.1) ÷ (0.3 − 0.1) × (1,380 − 1,450) = 1,398 CFM
- Speed 2: interpolate between 0.1 and 0.3 in. w.c.1,300 + (0.25 − 0.1) ÷ (0.3 − 0.1) × (1,230 − 1,300) = 1,248 CFM
- Speed 3: interpolate between 0.1 and 0.3 in. w.c.1,150 + (0.25 − 0.1) ÷ (0.3 − 0.1) × (1,080 − 1,150) = 1,098 CFM
How the method works
Every furnace and air handler comes with a blower table: airflow at a series of external static pressures for each speed tap or setting. Measure total external static pressure, find it in the table and read the airflow. When the reading falls between two columns, the calculator interpolates in a straight line between them.
It is one of the standard field ways to estimate airflow, alongside a duct traverse or a flow hood.
Comparing speed taps
Enter up to four rows to compare taps at the same static pressure. With a tonnage, each tap’s CFM per ton is shown; the typical comfort-cooling range is 350–450 CFM per ton, but the equipment’s own airflow requirement comes first.
Getting a good answer
- Measure static pressure where the manufacturer says, and check the table’s footnotes: some assume a dry coil or no filter
- Do not extrapolate beyond the table; the calculator refuses readings outside it
- The numbers in the form are an example table, not data for any product
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Measured total external static pressure
- 0.25 in. w.c.
- Speed taps in your table
- 3 (compare taps)
- Static pressure columns
- 5
- Column 1: external static pressure
- 0.1 in. w.c.
- Column 2: external static pressure
- 0.3 in. w.c.
- Column 3: external static pressure
- 0.5 in. w.c.
- Column 4: external static pressure
- 0.7 in. w.c.
- Column 5: external static pressure
- 0.9 in. w.c.
- Speed 1: airflow at column 1
- 1,450 CFM
- Speed 1: airflow at column 2
- 1,380 CFM
- Speed 1: airflow at column 3
- 1,300 CFM
- Speed 1: airflow at column 4
- 1,210 CFM
- Speed 1: airflow at column 5
- 1,110 CFM
- Speed 2: airflow at column 1
- 1,300 CFM
- Speed 2: airflow at column 2
- 1,230 CFM
- Speed 2: airflow at column 3
- 1,150 CFM
- Speed 2: airflow at column 4
- 1,060 CFM
- Speed 2: airflow at column 5
- 960 CFM
- Speed 3: airflow at column 1
- 1,150 CFM
- Speed 3: airflow at column 2
- 1,080 CFM
- Speed 3: airflow at column 3
- 1,000 CFM
- Speed 3: airflow at column 4
- 910 CFM
- Speed 3: airflow at column 5
- 810 CFM
- Nominal cooling capacity (optional)
- 3 tons
Result
At 0.25 in. w.c.: speed 1 1,398 CFM, speed 2 1,248 CFM, speed 3 1,098 CFM. Speeds 2, 3 fall in the typical 350–450 CFM per ton range.
Assumptions
- Airflow varies linearly between neighbouring static-pressure columns of the table you enter.
- The 350–450 CFM per ton band is the typical comfort-cooling range (400 nominal); the equipment manufacturer’s requirement takes precedence.
Limitations
- Uses only the table you type in; the site publishes no manufacturer data.
- Outside the table’s pressure range no result is given.
Frequently asked questions
How do I find blower CFM from static pressure?
Measure total external static pressure, then read the airflow for your speed tap at that pressure in the blower table in the installation manual. This calculator does the interpolation.
Which speed tap should I use?
The one that gives the airflow the equipment requires for cooling or heating at your measured static pressure — often around 350–450 CFM per ton for cooling, but follow the manual.
What if my static pressure is off the table?
Then the table cannot tell you the airflow, and the static pressure is probably too high. Find out why (filter, coil, ducts) before choosing a higher speed.
Sources
- 400 CFM per Ton, or Is It? — ACCA HVAC Blog (M. Akins). 400 CFM/ton is a rule of thumb; about 350 suits humid climates and about 450 drier climates; follow OEM airflow tables.
- Measure and Interpret Static Pressures (National Comfort Institute, hosted by ENERGY STAR) — National Comfort Institute / ENERGY STAR. Total external static pressure is found by adding the supply and return static pressure readings at the equipment (shown for upflow, downflow, horizontal and package units).
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