Duct Size Calculator
Calculate HVAC duct dimensions from airflow (CFM). Supports round and rectangular ducts with velocity validation.
Use Tool →Engineering
Engineering calculators organized into two tracks plus a converter. The HVAC track runs the full residential design pipeline — from IECC 2021 insulation checks and ACCA Manual J loads through equipment sizing, room airflow, and ASHRAE/SMACNA duct sizing. The electrical track covers NEC essentials — conductor ampacity and wire size, voltage drop, conduit and box fill, and dwelling service load. A feet-inches to decimal-feet converter rounds out the set. All calculations run locally in your browser.
This category is organized into two tracks. The HVAC calculators form a complete residential design pipeline: check insulation R-values against IECC 2021, calculate room-by-room heating and cooling loads per ACCA Manual J, size equipment per ACCA Manual S, determine airflow and register sizes, then size ductwork with ASHRAE and SMACNA methods. Used in sequence they take you from the building envelope all the way to a finished duct design, but each tool also works on its own.
The electrical calculators are built on the US National Electrical Code. Size conductors and check ampacity against NEC Table 310.16 while keeping voltage drop within the recommended 3% branch and 5% total limits, verify box fill per NEC 314.16 and conduit fill per NEC Chapter 9, and run a dwelling service-load calculation per NEC Article 220 — the everyday checks from service entrance to device box. A feet-inches to decimal-feet converter supports layout and material takeoffs. Every tool runs locally in your browser, and results are intended for preliminary design, to be verified against your locally adopted code.
HVAC Calculators
HVAC, electrical, and measurement tools: insulation R-value compliance, heating and cooling load calculations, equipment sizing, room airflow, duct sizing, system planning, voltage drop/wire sizing, and feet-inches to decimal feet conversion
Duct System Planner
Free HVAC duct size calculator and system planner. Auto-size ducts by CFM from plenum to registers, calculate friction loss and total pressure drop per ASHRAE and ACCA standards.
Use Tool →Heat Load Calculator
Calculate residential heating and cooling loads room by room. Simplified Manual J methodology with ASHRAE design temperatures.
Use Tool →HVAC Cost Comparison Calculator
Compare annual heating and cooling costs across six HVAC system types. Uses degree-day analysis with real energy prices to find the most affordable and greenest option.
Use Tool →HVAC Repair vs Replace Calculator
Should you repair or replace your HVAC system? Uses the 50% Rule, Rule of 5,000, and ASHRAE lifespan data to give you a clear, data-driven recommendation.
Use Tool →HVAC Sizing Calculator
Calculate the right AC, furnace, or heat pump size for your home. Uses ACCA Manual S sizing limits with standard equipment capacities.
Use Tool →Insulation R-Value Calculator
Calculate insulation R-value for wall assemblies, attics, and floors. Check IECC 2021 code compliance by climate zone.
Use Tool →Mini Split Sizing Calculator
Calculate the right ductless mini split size for each room based on area, insulation, climate zone, and room type.
Use Tool →Psychrometric Calculator
Calculate every moist-air property — dew point, wet bulb, relative humidity, humidity ratio, enthalpy — from any two, with an interactive psychrometric chart.
Use Tool →Refrigerant Charge Calculator
Calculate superheat, subcooling, and look up pressure-temperature values for 10 common refrigerants. Diagnose charge issues for TXV and fixed orifice systems.
Use Tool →Register Size Calculator
Find supply register, return grille, and filter grille sizes for any CFM, with face velocity checked against ACCA Manual D guidance.
Use Tool →Room Airflow Calculator
Calculate CFM airflow requirements and register sizes for each room. Uses ASHRAE 62.2 ventilation rates and load-based airflow calculations.
Use Tool →Electrical Calculators
Five electrical calculators built on the US National Electrical Code (NEC). Size conductors and check ampacity against NEC 310.16 with temperature and conduit-fill derating, keep voltage drop within the recommended 3% on a branch circuit and 5% overall, count box fill per NEC 314.16, check raceway fill per NEC Chapter 9, and run a dwelling service-load calculation per NEC Article 220. Every calculation runs locally in your browser, with nothing uploaded.
Box Fill Calculator
Calculate electrical box fill in cubic inches and check it against NEC 314.16 — conductors, clamps, devices, and grounds included.
Use Tool →Conduit Fill Calculator
Calculate conduit fill percentage and find the minimum conduit size for your conductors using NEC Chapter 9 tables.
Use Tool →Dwelling Load Calculator
Calculate a single-family dwelling service load and the minimum service size using the NEC Article 220 standard and optional methods.
Use Tool →Voltage Drop Calculator
Calculate voltage drop and select optimal wire sizes for electrical circuits. Supports NEC, IEC, and BS 7671 standards with ampacity derating.
Use Tool →Wire Size & Ampacity Calculator
Find the correct wire size and circuit breaker for any load, or the ampacity of a conductor, using NEC termination and derating rules.
Use Tool →Decimal Feet Converter
Convert between feet-and-inches (with fractions) and decimal feet. Bidirectional with quick reference table.
Use Tool →Frequently Asked Questions
How is the Engineering category organized?
It is grouped into two tracks plus a converter. The HVAC calculators cover the residential design pipeline — insulation, heat load, equipment sizing, airflow, and duct sizing (IECC 2021, ACCA, ASHRAE, SMACNA). The Electrical calculators cover NEC essentials — wire size and ampacity, voltage drop, conduit and box fill, and dwelling load. The feet-inches to decimal-feet converter is a general measurement helper. Each group has its own page so you can go straight to the tools you need.
What code do the electrical calculators use?
They follow the US National Electrical Code (NEC): ampacity from NEC Table 310.16, voltage-drop recommendations of 3% on a branch circuit and 5% overall, box fill per NEC 314.16, conduit fill per NEC Chapter 9, and the standard dwelling load method of NEC Article 220. Always confirm results against the NEC edition adopted by your local jurisdiction, as amendments vary.
What standards are used for duct sizing calculations?
Our duct calculators use the Darcy-Weisbach friction formula and Huebscher equivalent diameter equation as specified in the ASHRAE Handbook of Fundamentals. Recommended duct velocities and standard sizes follow SMACNA (Sheet Metal and Air Conditioning Contractors' National Association) guidelines.
What is the difference between the duct calculator and duct system planner?
The duct calculator sizes individual duct sections based on airflow (CFM), velocity, and friction rate. The duct system planner lets you design a complete duct layout with branches and fittings, automatically sizing all sections and calculating total system pressure drop.
Can I use these tools for professional HVAC design?
These tools use the same formulas and standards (ASHRAE, SMACNA) as professional HVAC software and are suitable for preliminary design calculations. For final engineering submittals, results should be verified by a licensed professional engineer as required by local building codes.
What is the equal friction method?
The equal friction method is the most commonly used duct sizing approach. It selects duct sizes so that the friction rate — the pressure drop per unit length — remains constant throughout the entire duct system. This simplifies design and helps maintain balanced airflow. For residential systems, a typical design friction rate is 0.08 in. w.g. per 100 ft of duct. Commercial systems use varying friction rates depending on available static pressure and system requirements. The method is described in ASHRAE Handbook — Fundamentals, Chapter 21, and is supported by our duct sizing tools.
What duct velocities are recommended?
SMACNA guidelines provide recommended velocity ranges based on application. For residential systems, main ducts should typically run at 600–900 FPM (feet per minute), while branch ducts serving individual rooms should stay between 400–600 FPM to minimize noise. Commercial main ducts generally operate at 1,000–1,800 FPM. Velocities above 2,000 FPM are considered high-velocity and significantly increase both noise levels and pressure drop, requiring more robust duct construction and larger fans. Our duct tools flag velocities outside recommended ranges to help you avoid noise and efficiency problems.
What is the recommended order for using these tools?
Start with the Insulation Calculator to check wall, ceiling, and floor R-values against IECC 2021 code for your climate zone. Then use the Heat Load Calculator for room-by-room or whole-house BTU heating and cooling loads. Feed those loads into the HVAC Sizing Calculator to select the right equipment capacity per ACCA Manual S. Use the Room Airflow Calculator to determine CFM requirements per room and appropriate register sizes. Finally, size your ductwork with the Duct Calculator for individual sections, or plan a complete duct system with branches and fittings using the Duct System Planner.