HVAC Calculators
Free HVAC Calculators
Organized by the job: sizing equipment, ductwork and airflow, charging and diagnostics, the A2L changeover, and decoding a nameplate. Under each job the calculators, charts, and guides are labeled so you grab the right kind fast. Every number cites its source.
Sizing the equipment
Manual J Load Calculator
The ACCA Manual J 8th Edition Form J1 worksheets against the real ASHRAE design conditions for your ZIP's nearest weather station, component by component, with the distance to that station shown. It reports a whole-house HEATING total under four site conditions you declare, each shown beside the number. It reports a whole-house COOLING load as the PAIR Manual J section 5-12 sizes equipment from: the total sensible load (Form J1ae line 20) and the total latent load (line 21). Either is withheld when a printed term of it is unanswered or has no printed route in the abridged edition, and the specific term is named on the result rather than folded in.
Runs in your AIHighest volumeBTU / HVAC Load Calculator
Calculate cooling and heating load by climate zone, insulation, windows, and duct location. Whole-house or room-by-room breakdown with automatic tonnage sizing.
By modelCold-Climate Heat Pump Capacity
What a cold-climate heat pump actually puts out at 5F, per model, from the AHRI-certified NEEP database. MAX capacity at any outdoor temperature, the maintained-% against both denominators, and a balance-point and backup-heat readout.
Runs in your AICharts & tablesBTU per square foot chartHeat pump capacity at 5F chart
Guides & answersWhat size air conditioner do I needHow many BTU to heat a houseWhat size mini splitWhat size garage heater
Ductwork & airflow
The air side of the job: size the duct from CFM, convert a shape, and set the friction rate the duct sizer runs on.
Duct Size Calculator
Round and rectangular duct size from CFM by the equal-friction method (ACCA Manual D / ASHRAE), with the velocity and the rectangular equivalent. Every size computed from the Darcy-Colebrook duct model, not looked up.
NewRound to Rectangular Duct Converter
Convert a duct between round and rectangular, either direction, by equivalent diameter (ASHRAE Fundamentals 21.3.1, Huebscher). Enter a size, get the equal-friction equivalent that carries the same airflow. Computed both ways.
CFM Calculator
Required airflow from nominal tonnage (400 CFM per ton) or from the sensible load (CFM = BTU / (1.08 x deltaT)). The airflow that feeds the duct size calculator.
Static Pressure & Friction Rate
The design friction rate from available static pressure and total effective length (ASP x 100 / TEL), the input the duct sizer needs.
Furnace Temperature Rise
Supply minus return, judged against the rise range on the unit's own rating plate: in range, high or low, with both manufacturer causes for each direction and the airflow that falls out of it.
Charge, test & diagnose
Superheat & Subcooling Calculator
Check charge level with PT interpolation for 10 refrigerants including R-454B and R-32. Four-quadrant diagnostic decision tree with TXV and fixed orifice targets.
Runs in your AISingle intentSubcooling Calculator
Enter your liquid-line pressure and temperature and get subcooling for 10 refrigerants, with target subcooling by refrigerant and a high or low diagnosis. A TXV is charged by subcooling.
NewNitrogen Pressure Test Calculator
Temperature-corrected nitrogen pressure test: separate a real leak from a temperature-driven pressure change with the Gay-Lussac gas law. The pressures it computes come from NIST constants, not a lookup table.
NewVacuum Decay Test & Evacuation Criteria
Your unit's own evacuation acceptance criterion, quoted from its installation instructions, and whether your micron readings met it. Seven criteria covering Amana, Carrier, Daikin, Goodman, Rheem, Ruud, Trane and York, and they disagree: 500 microns held one minute, 1000 after seven, 500 for a full hour. One states no micron pass mark at all and one states no hold time.
Charts: PT tablesR-410A PT chartR-454B PT chartR-32 PT chartR-22 PT chartR-134a PT chartR-404A PT chartR-407C PT chartR-448A PT chartR-449A PT chartR-290 (propane) PT chartSuperheat & subcooling chart
Guides & answersHow to calculate superheatNormal superheat & subcooling valuesSubcooling on a heat pump
The A2L changeover (R-454B & R-32)
The 2025 phase-down replaced R-410A with the A2L refrigerants R-454B and R-32. Reuse a line set, stay under the charge limit, meet the leak rule, and know how the two compare.
Outdoor HVAC GFCI Requirement
Replacing or landing an outdoor condenser? The NEC's GFCI exception for listed HVAC equipment expires 1 September 2026 by its own printed terms, and several states deleted that expiry while others never received it. Answers for the jurisdictions whose amendments we have read from their own primary documents, and refuses with a stated reason for the rest.
Runs in your AIA2L transitionA2L Line Set Calculator
Size the liquid and suction line set by tonnage and length, and get a REUSE, FLUSH, or REPLACE verdict for an existing R-410A set with R-454B or R-32. Charge, drier, and evacuation specs cited.
Runs in your AIA2L complianceA2L Charge Limit Calculator
Maximum R-454B and R-32 charge for a room, or minimum room area for a charge, per UL 60335-2-40 and ASHRAE 15.2. Verified against ASHRAE 15.2's published Table 9-3 and Table 9-4.
Runs in your AINew rule 2026EPA Refrigerant Leak Rate Calculator
The AIM Act leak repair rule took effect January 1, 2026. Annualizing and rolling average methods, 10/20/30 percent thresholds, repair deadlines, and ALD checks per 40 CFR 84.106.
Charts & tablesA2L properties (GWP & class)
Guides & answersR-410A phase-out & timelineR-454B vs R-410AR-32 vs R-410AR-32 vs R-454B
Converting to a heat pump? Switchover and the gas left behind
The gas-furnace-to-heat-pump changeout spans the whole job: what the heat pump can deliver in the cold, when to hand a cold hour back to the furnace, and the combustion air the water heater still left in the space now needs on its own.
Dual-Fuel Switchover Calculator
The outdoor temperature where the furnace becomes cheaper than the heat pump, by your actual rates, with the thermal balance point beside it so a lockout never strands the load. The switchover moves with your rates: there is no universal number.
NewGas Pipe Sizing Calculator
The gas side of the same job: every section of the piping system sized from the code's own equation rather than one pipe off a chart. Natural gas or undiluted propane, longest length and branch length beside each other, and the code address for whichever book your jurisdiction adopted.
NewCombustion Air Calculator
When the furnace leaves, the atmospheric water heater it shared the space with may now be short on combustion air. Confined-vs-unconfined verdict and the required opening size under IFGC 304, for every gas appliance left in the room.
Sizing the panel for the new heat pump and its backup?Can My Panel Handle Itchecks the electrical side of the conversion.
More calculatorsWhat am I looking at? Decode the nameplate
Age from the serial and tonnage from the model, before you quote the job or pull a part.
Serial Number Lookup
How old is that unit? Decode the manufacture date from the serial for 50+ brands, with the refrigerant era and warranty context for the decoded year.
4 brandsWater Heater Age Decoder
How old is that tank? Decode the manufacture date from a water heater serial, read from the manufacturers' own date-code documents, with an honest refusal when the code is genuinely ambiguous.
34 OEM documentsTonnage From Model Number
Paste a residential AC or heat pump model number and get nominal tonnage, BTU/h, and SEER and refrigerant hints. Brand schemes for 40+ brands, read at each maker's documented capacity position where one exists, and a pattern-based estimate labelled as such where it does not. Where we hold the manufacturer's own nomenclature document we name it so you can check it, and where we do not we say what the decode rests on instead.
The circuit your equipment needs
Sized the equipment? Cross over to the electrical side for the breaker, wire, and panel capacity it needs.
Frequently Asked Questions
Are these HVAC calculators free?
Yes. Every calculator is completely free: no login, no email gate, no ads. We build these tools because they help trade contractors do better work. If a number is wrong we want to hear about it. Accuracy is the whole point.
How accurate is the BTU calculator?
The BTU calculator uses a simplified square-foot method on IECC 2021 climate zones. The base BTU-per-square-foot values are calibrated planning constants, not figures any standard publishes. It is an estimate for planning purposes, not a substitute for a full Manual J calculation required for permits and equipment selection. We do not publish an accuracy percentage against a detailed Manual J. What we have measured so far is one-sided: against a partial Manual J reconstruction a small share of cases already come out low, worst in the hottest zones, and no case has yet been shown accurate. We will print a figure when we can show the working behind it.
Which refrigerants does the superheat calculator support?
The calculator includes R-410A, R-407C (with zeotropic glide handling), R-134a, R-404A, R-22 (legacy), R-454B (Puron Advance), R-32, R-448A, R-449A, and R-290 (propane). PT tables are sourced from published manufacturer charts (Chemours, Honeywell, Hudson Technologies, National Refrigerants, Carrier) and every table is cross-checked against a second published source.
More math off your plate.
Size the system with the BTU calculator, verify the charge with superheat and subcooling, then make sure the invoice actually covers your overhead.