Calculators
Duct and airflow calculators
CFM per room and per ton, duct sizes, velocity, return grilles, registers, equivalent lengths, air changes and exhaust fans.
Airflow is the half of an HVAC system that nobody sizes. The equipment arrives with a rating, but the ducts that carry its air are often whatever fitted the joists, and a 3-ton system pushing air through ducts sized for 2 tons loses capacity, makes noise and freezes its coil. The calculators here cover the whole air path: how much air each room needs, how big the duct must be to carry it at a sensible friction rate, how fast the air moves, how large the return grilles and registers have to be, and how much resistance the fittings add.
The method behind them is the equal-friction approach used in residential design: pick a friction rate, typically 0.08 to 0.10 inches of water per 100 feet of duct, and size every run so it loses pressure at that rate. ACCA Manual D is the full procedure and includes the equipment's available static pressure, which these calculators ask you to check rather than assume.
9 calculators
CFM per Room Calculator
Splits the system airflow between up to eight rooms by area and exposure, with registers per room.
Duct Size Calculator
Round and rectangular duct size for an airflow at a friction rate, checked against the velocity limit.
Duct Velocity Calculator
Air speed in a round or rectangular duct, its friction rate, and how it compares with residential limits.
Return Air Grille Size Calculator
Free area and nominal size of the return grilles for a system airflow at a quiet face velocity.
Supply Register Size Calculator
Floor, ceiling or sidewall register size and count for a room's airflow at a target face velocity.
Duct Equivalent Length Calculator
Total equivalent length of a duct run with its fittings, the available static pressure and the design friction rate.
Air Changes per Hour Calculator
ACH from a room's airflow, or the CFM for a target ACH, against common ventilation and exhaust targets.
Bathroom Exhaust Fan Calculator
Fan CFM from the code minimum, the HVI area or fixture rule and 8 air changes, with the duct run checked.
Range Hood CFM Calculator
Hood airflow from the burner BTU and hood width rules, the makeup-air threshold and the duct run.
Guides for this topic
Whole-house ventilation: the ASHRAE 62.2 formula
Tight houses save energy and trap whatever is in the air. ASHRAE 62.2 says how much outdoor air a home needs, and the arithmetic fits on one line.
Bathroom and kitchen exhaust: the code minimums and the better numbers
The code sets a floor for exhaust fans. The HVI rules set a better target, and the duct decides whether the fan ever gets there.
Duct leakage: the 20% you pay for and never feel
Ducts in an attic or crawl space typically leak a fifth or more of the air the system makes. Sealing them is one of the best paybacks in a house.
Duct sizing basics: friction rate, velocity and equivalent length
A duct is sized from three numbers, the airflow it carries, the pressure the blower can spare, and the speed the air can travel quietly. Here is how they fit together.
MERV filters: filtration vs airflow, and the replacement schedule
A better filter catches more and restricts more. The answer is rarely the cheapest or the highest MERV; it is the right rating in a filter deep enough for the system.
Return air: why undersized returns wreck a good system
Every cubic foot of air pushed into the rooms has to come back to the blower. When the return path is too small, the whole system suffers, and it rarely gets blamed.
Questions people ask
- How many CFM per ton does a system need?
- 400 CFM per ton is the usual design figure, giving 800 CFM for a 2-ton system and 1,200 CFM for 3 tons. Humid climates run 350 CFM per ton for better dehumidification; hot dry climates can run 450. The number matters because every duct, grille and register downstream is sized from it, and a system starved of air loses capacity and efficiency.
- What size duct do I need for 400 CFM?
- At a friction rate of 0.1 inches of water per 100 ft, 400 CFM calls for a 10-inch round duct, with air moving at about 730 feet per minute. The rectangular equivalents are about 8 × 12 or 6 × 16 inches. A 9-inch duct would carry the same air at 900 FPM and a higher friction rate, which is noisier and costs fan energy; this is why the duct size calculator shows velocity next to the diameter.
- How big should a return air grille be?
- Size returns for a face velocity of 300 to 500 feet per minute so they stay quiet; many designers use 400. A 3-ton system moving 1,200 CFM needs about 3 sq ft of free area at 400 FPM, and because a stamped grille is only about 70% open, that means roughly 4.3 sq ft of grille, for example two 20 × 20 inch returns or one 24 × 30. Undersized returns are the most common duct fault in houses.
- What is the maximum velocity for residential ducts?
- Common residential limits are 700 to 900 feet per minute in supply trunks, 600 to 700 in branch runs, and 500 to 700 in returns; registers and grilles are kept lower still so they do not whistle. Commercial systems run faster because noise is tolerated and ducts are stiffer. The duct velocity calculator compares your duct to these limits.
- How many air changes per hour should a bedroom have?
- Comfort and code targets differ. For whole-house ventilation, ASHRAE 62.2 works out to roughly 0.35 air changes per hour of outdoor air in a typical house. Bathrooms are often sized for 8 air changes per hour of exhaust. A conditioned-air supply of 4 to 6 air changes per hour is typical for a bedroom served by a central system. The air changes calculator converts between CFM and ACH for any room.
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