About BTU Size

How we build, check, and update these HVAC tools.

BTU Size is a working set of HVAC calculators for homeowners, technicians, and DIYers. The math comes from the codes contractors actually pull on permitted jobs: ACCA Manual J for sizing, NEC Article 440 for branch circuits, NFPA 54 for gas piping, IMC 307 for condensate. Cost numbers come from current contractor quote aggregators. Every page lists who reviewed it and what their credential is.

What you can do here

The tools cover the work that comes up on a typical residential or light commercial HVAC job. Sizing a new AC or heat pump. Checking what a replacement should cost in your region. Reading SEER2 and HSPF2 nameplates. Running Manual D on a duct system. Charging an A2L system to the right superheat and subcooling on a humid day. Sizing wire, breakers, and gas pipe before pulling a permit.

The calculators show their work. You get the number, the formula behind it, and a plain-English read of what the number means for the job in front of you. When the answer depends on something only you can see, like your utility rate, the nameplate on your specific unit, or the inspector's interpretation of a borderline code call, the result panel says so instead of making something up.

The reviewer roster

Every calculator on the site has a named technician credited at the top of the page. You can see who reviewed the math, what their credential is, and which other tools they signed off on. The people below have between 9 and 18 years of field experience each, and they review tools in the area they actually work in every day.

Marcus Reilly

EPA 608 Universal, NATE-certified, 14 years HVAC

Service tech running residential and light commercial calls out of the Midwest. Marcus reviews the sizing tools and refrigerant work on this site, which is the math he uses on every charge and recommissioning job.

Sam Ortiz

HVAC installer, ACCA Manual J trained, 9 years field work

Installer who has put in roughly 800 residential systems across the southwest. Sam handles the tools that touch new equipment selection and gas-fired install work, including the Manual J and AC tonnage calculators.

Dana Okafor

HVAC contractor & estimator, ACCA member, 11 years

Owns a small residential HVAC contracting business and writes hundreds of replacement estimates a year. Dana reviews the cost calculators, the heat pump rebate finder, and the repair-versus-replace decision tools.

Priya Natarajan

P.E. Mechanical, LEED AP, energy modeling consultant

Licensed mechanical engineer who builds calibrated energy models for residential retrofits and small commercial projects. Priya checks the efficiency-rating math: SEER2 and HSPF2 conversions, COP curves, and annual kWh estimates.

Tom Hendricks

Sheet metal journeyman, SMACNA, 18 years ductwork

Sheet metal journeyman who fabricates and installs residential and light commercial duct systems. Tom reviews everything in the ductwork category and the airflow-side diagnostic tools like delta-T and condensate drain sizing.

Luis Arroyo

EPA 608 Universal, RSES CMS, refrigeration specialist

Refrigeration specialist working commercial and supermarket systems on R-454B, R-32, and CO2 transcritical equipment. Luis reviews the subcooling and refrigerant charge calculators because the underlying thermodynamics are what he troubleshoots every day.

Jen Whitaker

Master electrician, NATE-certified, HVAC electrical

Master electrician focused on HVAC branch circuits, compressor starting, and control wiring on light commercial jobs. Jen reviews the wire size, breaker, and capacitor tools against NEC Article 440.

Methodology and sources we cite

Every calculator on this site cites the standard, code section, or data source it implements. Generic rules of thumb are flagged as such. The references below cover every category of tool on the site.

System sizing: ACCA Manual J Eighth Edition residential load procedure. DOE 2023 climate zone map. ASHRAE Fundamentals Handbook design temperature data. Manufacturer extended-performance tables for heat pump sizing at design temperature (Carrier, Trane, Mitsubishi, Daikin).
Cost data: Aggregated contractor quote data verified quarterly. EIA Annual Energy Outlook for electricity and natural gas residential rate projections. DOE energy-rebate program guidance and state utility portals. DSIRE database for ZIP-level incentive lookups.
Efficiency ratings: DOE 2023 M1 test procedure for SEER2, EER2, and HSPF2. AHRI 210/240 reference conditions. EIA Residential Energy Consumption Survey for the 3.84 kWh/sqft national HVAC intensity benchmark. ENERGY STAR certified-product specifications.
Ductwork and airflow: ACCA Manual D simplified residential procedure. ASHRAE Darcy-Weisbach friction equation for round and rectangular duct. SMACNA HVAC Duct Construction Standards Metal and Flexible.
Refrigerant work: ASHRAE Standard 34 refrigerant designation and safety classification. Chemours Opteon, Honeywell Solstice, and Daikin technical bulletins for R-454B and R-32 saturation properties. NFPA 54 / IFGC for combustion appliance interaction.
Pro tools (codes): NEC 2023 (NFPA 70) Article 440 for HVAC branch circuits, Table 310.16 for ampacity, 240.6 for standard breaker sizes, 110.14(C) for terminal temperature limitation. IMC 307 / IRC M1411 for condensate disposal. NFPA 54 / IFGC Appendix A longest-length method for gas piping.

What these calculators cannot do

These tools help you check a contractor's quote, plan a project, or troubleshoot a running system. They do not replace a technician walking through your home with a manometer and a clamp meter. A real Manual J load calculation looks at every window orientation, attic insulation, duct condition, and infiltration rate specific to your house. This site uses good defaults; an ACCA-certified contractor uses your house.

The same goes for electrical and gas work. Sizing wire to NEC Table 310.16 is one thing; pulling a permit, passing inspection, and bonding a CSST gas line correctly is another. If a calculator on this site disagrees with what your local code inspector says, the inspector wins. Always.

Cost numbers age fast. The figures on this site come from current contractor quote aggregators and energy price data, but pricing swings 20 to 40 percent across regions, seasons, and brands. Use our cost calculators to know what the ballpark looks like before you sit down with a contractor, not to argue an exact dollar figure on the kitchen table. And get three quotes on any install over $5,000, every time.