How to Calculate Cubic Feet per Hour
Use this premium calculator to convert airflow and ventilation inputs into cubic feet per hour. Choose a method, enter your values, and instantly calculate CFH from CFM, area and air velocity, or room volume with air changes per hour.
CFH Calculator
CFH means cubic feet per hour. It expresses how much air or gas moves in one hour. Select the method that matches your data.
- From CFM: CFH = CFM × 60
- From area and velocity: CFH = area in ft² × velocity in ft/s × 3600
- From room volume and ACH: CFH = room volume in ft³ × ACH
Expert Guide: How to Calculate Cubic Feet per Hour
Cubic feet per hour, often shortened to CFH, is a volumetric flow rate. It tells you how many cubic feet of air, gas, or another fluid move through a system in one hour. In building ventilation, HVAC design, dust collection, compressed air applications, and industrial process control, CFH is a practical unit because it directly links airflow capacity to time. If you know how much air moves in a minute, per second, or per air change cycle, you can convert it into cubic feet per hour and compare equipment performance on a standardized basis.
The core idea is simple: volume per unit time. A cubic foot is a unit of volume. One hour is the unit of time. So when you calculate cubic feet per hour, you are measuring how much total volume is transferred in a 60-minute period. This matters when evaluating ventilation targets, estimating equipment sizing, determining room flush rates, or comparing fan specifications from different manufacturers.
Why cubic feet per hour matters
CFH is useful because many systems operate continuously, and hourly totals are easier to interpret than minute-by-minute snapshots. For example, if a fan delivers 500 cubic feet per minute, that sounds useful, but if you are evaluating how much air that fan exchanges over an entire work shift, converting to cubic feet per hour gives a more strategic number. The same is true in ventilation planning for offices, classrooms, laboratories, and workshops.
- HVAC: compare supply and exhaust volumes over time.
- Indoor air quality: estimate how much outside air enters a space per hour.
- Industrial systems: size blowers, collectors, and process lines.
- Energy planning: understand how higher flow rates affect heating and cooling loads.
- Room ventilation: convert air changes per hour into actual cubic feet moved.
The basic formula for cubic feet per hour
The most general expression is:
CFH = Volume in cubic feet ÷ Time in hours
If you already know the total volume moved over a measured time period, divide by the number of hours. For example, if a blower moves 9,000 cubic feet in 0.5 hours, then:
CFH = 9,000 ÷ 0.5 = 18,000 CFH
However, in practice, most people do not start with a direct volume-over-hours measurement. Instead, they usually have one of three other common inputs:
- CFM, which means cubic feet per minute
- Area and air velocity
- Room volume and air changes per hour
Method 1: Convert CFM to CFH
This is the easiest conversion because an hour has 60 minutes.
CFH = CFM × 60
Example: A fan is rated at 250 CFM.
CFH = 250 × 60 = 15,000 CFH
This means the fan can theoretically move 15,000 cubic feet of air each hour, assuming actual operating conditions match the rating. Be aware that real-world performance often changes due to static pressure, dirty filters, duct losses, damper positions, and motor speed.
Method 2: Calculate CFH from area and velocity
When you know the cross-sectional area of a duct or opening and the speed of the air moving through it, you can calculate volumetric flow. In imperial units, if area is in square feet and velocity is in feet per second, the formula is:
CFH = area in ft² × velocity in ft/s × 3600
The factor 3600 appears because there are 3600 seconds in an hour.
Example: A vent opening has an area of 2 square feet and average air velocity of 8 feet per second.
CFH = 2 × 8 × 3600 = 57,600 CFH
If your velocity is in feet per minute instead, first calculate cubic feet per minute:
CFM = area in ft² × velocity in ft/min
Then convert to CFH:
CFH = CFM × 60
Method 3: Calculate CFH from room volume and ACH
ACH means air changes per hour. It tells you how many times the total air volume of a room is replaced in one hour. Once you know the room volume in cubic feet, the formula is straightforward:
CFH = Room volume in ft³ × ACH
Example: A room is 20 feet long, 15 feet wide, and 8 feet high.
Room volume = 20 × 15 × 8 = 2,400 ft³
If the target is 6 ACH, then:
CFH = 2,400 × 6 = 14,400 CFH
This means your ventilation system must move 14,400 cubic feet of air per hour to achieve 6 air changes per hour in that room.
How to calculate room volume correctly
Many CFH errors begin with incorrect room volume. For rectangular spaces, use:
Volume = length × width × height
For irregular spaces, break the room into smaller rectangles, calculate the volume of each, then add them together. If your room dimensions are measured in meters, convert to feet before using an imperial airflow formula, or convert the final volume to cubic feet. One cubic meter equals approximately 35.3147 cubic feet.
Unit conversions you should know
- 1 hour = 60 minutes = 3600 seconds
- 1 cubic meter = 35.3147 cubic feet
- 1 square meter = 10.7639 square feet
- 1 square foot = 144 square inches
- 1 mile per hour = 1.46667 feet per second
- 1 meter per second = 3.28084 feet per second
Comparison table: common cubic feet per hour formulas
| Scenario | Known inputs | Formula | Example result |
|---|---|---|---|
| Convert CFM to CFH | 250 CFM | CFH = CFM × 60 | 15,000 CFH |
| Area and velocity | 2.0 ft², 8 ft/s | CFH = area × velocity × 3600 | 57,600 CFH |
| Room volume and ACH | 2,400 ft³, 6 ACH | CFH = volume × ACH | 14,400 CFH |
| Measured total volume | 9,000 ft³ in 0.5 hr | CFH = volume ÷ time | 18,000 CFH |
Real ventilation statistics and benchmarks
To make CFH meaningful, you need context. The U.S. Environmental Protection Agency states that indoor air can be more polluted than outdoor air in many settings, which is one reason ventilation rates matter. The U.S. Department of Energy also notes that heating and cooling account for a major share of home energy use, so moving more air is not just a comfort issue but also an efficiency issue. In ventilation design, balancing adequate fresh air with energy performance is critical.
| Reference statistic | Value | Why it matters for CFH |
|---|---|---|
| Minutes in one hour | 60 | This is why CFM is multiplied by 60 to convert to CFH. |
| Seconds in one hour | 3,600 | This is why ft² × ft/s is multiplied by 3,600 to get CFH. |
| 1 cubic meter in cubic feet | 35.3147 ft³ | Essential for converting metric room volume or airflow data into CFH. |
| 1 square meter in square feet | 10.7639 ft² | Needed when velocity-area calculations start with metric dimensions. |
| Approximate share of home energy used for heating and cooling according to DOE Energy Saver | About 43% | Ventilation and airflow decisions can directly affect energy consumption. |
Statistics and guidance should always be interpreted in the context of the specific building type, occupancy level, contaminant source, code requirements, and equipment performance data.
Step-by-step process to calculate cubic feet per hour
- Identify your starting data. Do you have CFM, room dimensions and ACH, or area and velocity?
- Check the units. Make sure dimensions are in feet if you want a direct cubic feet per hour answer.
- Use the correct formula. Avoid mixing formulas from different measurement methods.
- Complete any unit conversions first. Convert meters to feet, square inches to square feet, or m/s to ft/s as needed.
- Perform the CFH calculation. Keep enough decimal places during the math, then round at the end.
- Interpret the answer. Compare the result with room volume, target ACH, fan rating, or process requirement.
Common mistakes when calculating CFH
- Forgetting the time conversion. Many people leave a CFM value unchanged and mistakenly label it CFH.
- Using the wrong area unit. Square inches must be converted to square feet before applying ft/s velocity formulas.
- Ignoring actual operating conditions. A fan’s catalog rating may not match installed performance.
- Confusing ACH with CFM or CFH. ACH is a room replacement rate, not a direct flow rate until volume is included.
- Failing to use average velocity. Air speed often varies across the face of a duct or grille.
When CFH is more useful than CFM
CFM is common in fan specifications because it shows minute-by-minute flow. CFH becomes more useful when discussing total hourly air movement, ventilation targets over time, gas consumption patterns, or room air replacement goals. If a compliance document, process log, or ventilation analysis is written on an hourly basis, CFH aligns better with the reporting period.
Practical example: converting a classroom ventilation target
Suppose a classroom measures 30 feet by 25 feet by 10 feet. The room volume is:
30 × 25 × 10 = 7,500 ft³
If the desired ventilation target is 4 ACH, then the required flow is:
7,500 × 4 = 30,000 CFH
To express this in CFM, divide by 60:
30,000 ÷ 60 = 500 CFM
This illustrates how CFH, ACH, and CFM are closely linked. Once you understand one, you can move easily among all three.
Practical example: duct opening and measured velocity
Assume a rectangular opening is 24 inches by 18 inches. The area is 432 square inches. Convert to square feet:
432 ÷ 144 = 3.0 ft²
If average measured air speed is 900 feet per minute, then:
CFM = 3.0 × 900 = 2,700 CFM
CFH = 2,700 × 60 = 162,000 CFH
This approach is common in balancing and troubleshooting airflow systems.
Authoritative sources for ventilation and airflow context
- U.S. EPA: Introduction to Indoor Air Quality
- U.S. Department of Energy: Heating and Cooling
- Cornell University: Indoor Air Quality Guidance
Final takeaway
If you want to know how to calculate cubic feet per hour, the answer depends on what data you already have. If you know CFM, multiply by 60. If you know area and velocity, convert the dimensions and speed into compatible units, then multiply by the number of seconds in an hour. If you know room volume and ACH, multiply them directly. The key is to keep the units consistent and choose the formula that matches the real measurement method. Once you do that, CFH becomes a powerful way to compare airflow, judge ventilation performance, and make informed equipment decisions.