Duct Slide Rule Planning Guide
A duct slide rule helps designers compare airflow, duct size, velocity, and friction. This calculator follows that same field idea. It gives fast values without turning a physical wheel. It is useful during early HVAC planning, duct checks, and site discussions.
Why Duct Sizing Matters
Airflow must move through a duct with reasonable speed. If the duct is too small, velocity rises. Noise can increase. Friction loss also rises. Fans may need more pressure. If the duct is too large, material cost increases. Space can also become difficult. A balanced size supports comfort, cost control, and service access.
Round and Rectangular Duct Checks
Round ducts usually have lower friction for the same airflow. They are simple to seal and size. Rectangular ducts fit tight ceiling spaces better. They also need aspect ratio review. Very flat ducts can create higher losses. This tool estimates an equivalent round size. That makes mixed layouts easier to compare.
Using Results in the Field
Enter airflow in CFM first. Then choose whether to evaluate a known duct or size from a target. Use target velocity for quick sizing. Use target friction when equal friction design is preferred. Add run length and fitting loss. The result shows velocity, friction, pressure loss, and velocity pressure. These values support quick decisions before final drawings.
Practical Limits
This calculator is for planning and checking. Final ductwork should follow project standards. Local codes, fan data, acoustic limits, leakage class, insulation, and balancing needs still matter. Treat the output as a guide. Confirm critical systems with approved design methods and measured data.
Better Inputs Give Better Choices
Use realistic airflow from room loads or an equipment schedule. Do not guess from duct size alone. Enter actual duct length, not only straight distance. Elbows, transitions, dampers, boots, and grilles add resistance. Put known extra loss in the fitting field. Keep units consistent. Review every branch separately. A main trunk and a branch can need different limits. For quiet rooms, lower velocity is often better. For short utility runs, higher velocity may be acceptable. Always leave room for installation, hangers, insulation, and future service. Good notes also make review easier during approval and balancing. Record assumptions. Update revisions.