CFM & Static Pressure Converter
Airflow and static-pressure unit reference for duct sizing.
How to use
Two converters in one page: airflow on top, static pressure below. Type in any field, the others update live.
- Enter an airflow value in CFM, L/s, or m³/h.
- Enter a static pressure in inches of water column (in. w.g.) or Pascals.
- Use the reference table to sanity-check sizing.
- Real duct design (Manual D, Manual J) belongs to a qualified HVAC contractor.
Airflow
Static pressure
Reviewed 3 June 2026 · methodology cited
About this converter
HVAC airflow is measured in cubic feet per minute (CFM) in North America and in litres per second (L/s) or cubic metres per hour (m³/h) elsewhere. Static pressure — the resistance the duct system presents to the fan — is measured in inches of water column (in. w.g.) in North America and Pascals (Pa) internationally. This page handles both.
Proper duct design involves much more than unit conversion: it requires a Manual J load calculation, a Manual D duct layout, friction losses by fitting type, balancing dampers, and verification at startup. The reference table below shows typical register airflow per room for sizing reference, but actual design belongs to a qualified contractor.
The conversion factors
Airflow conversions are exact by definition: 1 CFM = 0.471 947 L/s = 1.699 011 m³/h. Going the other way: 1 L/s = 2.118 88 CFM = 3.6 m³/h; 1 m³/h = 0.588 58 CFM = 0.277 78 L/s.
Static pressure: 1 inch of water column at 20 °C and standard gravity = 248.84 Pa. Going the other way: 1 Pa = 0.004 014 6 in. w.g. Residential systems commonly run at 0.5–0.8 in. w.g. external static pressure across the air handler. Higher-efficiency filters and HEPA add 0.2–0.4 in. w.g.; manufacturer fan curves drop off sharply above 1.0 in. w.g.
A worked example: a master-bedroom register sized for 110 CFM converts to 52 L/s or 187 m³/h. If the trunk duct at that point sees a static pressure of 0.4 in. w.g., that is 100 Pa. These are the numbers that flow into a Manual D worksheet.
Common residential register airflow
| Room | CFM | L/s | Typical register |
|---|---|---|---|
| Bedroom (typical) | 100 | 47 | 4×10″ floor / 6″ round |
| Master bedroom | 125 | 59 | 4×12″ floor / 6″ round |
| Living room | 180 | 85 | 6×12″ floor / 8″ round |
| Kitchen | 250 | 118 | 6×14″ floor / 8″ round |
| Bathroom | 75 | 35 | 4×8″ wall / 5″ round |
| Office / den | 110 | 52 | 4×10″ floor / 6″ round |
| Basement (per 500 sq ft) | 200 | 94 | 6×12″ ceiling / 8″ round |
Sizing notes
Rule-of-thumb airflow for residential supply registers: 100 CFM for a typical bedroom, 150–200 CFM for a living room, 75 CFM for a bathroom, 250 CFM or more for a kitchen. Total system airflow is roughly 400 CFM per ton of cooling capacity — a 3-ton system needs around 1,200 CFM through the air handler.
Static pressure adds up: each elbow, transition, and filter contributes to the total external static pressure the fan must overcome. Modern ECM motors maintain CFM as static pressure rises, but they cannot make pressure disappear — at some point the airflow drops anyway, and noise rises. If your installed system runs above the manufacturer's maximum rated static pressure, the duct is undersized, the filter is too restrictive, or both. Real diagnostic work belongs to a qualified HVAC contractor with a manometer.
Frequently asked questions
What is "external static pressure"?
External static pressure (ESP) is the total resistance the duct system, filter, coil, and registers present to the blower, measured outside the air handler. It does not include the internal pressure drop across the blower wheel itself. Manufacturers rate equipment at a maximum ESP — typically 0.5 in. w.g. for older PSC blowers, 0.8 in. w.g. for variable-speed ECM blowers.
Why 400 CFM per ton?
It is the airflow rate at which a standard residential cooling coil removes the rated 12,000 BTU/hr per ton across a typical entering-air temperature difference. Lower flow (350 CFM/ton) increases latent capacity for humid climates; higher flow (450 CFM/ton) increases sensible capacity for dry climates. 400 is the AHRI rating-point baseline.
How accurate is 1 in. w.g. = 248.84 Pa?
It depends on water temperature and local gravity. At 20 °C and standard gravity, 1 in. w.g. = 248.84 Pa exactly. At 60 °F (15.6 °C), 1 in. w.g. = 249.09 Pa — a 0.1 percent difference, negligible for HVAC work. The 248.84 figure is the value used in ASHRAE handbooks.
Is m³/h the same as CMH?
Yes. CMH (Cubic Metres per Hour) and m³/h are the same unit, just written differently. European HVAC catalogues most often use m³/h; the abbreviation CMH appears in some technical literature. 1 m³/h = 0.59 CFM.
Can I size duct with this?
No. Real duct sizing requires Manual D — the equal-friction or static-regain method applied to the specific layout, with fitting losses, branch take-offs, and balancing. This is a unit-conversion utility, not a Manual D calculator.
How much CFM should each register deliver?
It depends on the room's heat gain or loss — that is the output of a Manual J load calculation. The reference table on this page lists typical values, but actual values vary by climate, insulation, window area, and use. Always start from a Manual J, not a rule of thumb.