HVAC & Comfort Calculators
20 free hvac & comfort calculators — standards-based, with formulas, worked examples, and no sign-up.
Every HVAC decision is a sizing decision, and oversizing fails as surely as undersizing — an oversized air conditioner short-cycles and never dehumidifies. So the tools here follow the professional sequence: load first, equipment second, distribution third. The load end is ASHRAE Manual J-style heating and cooling analysis with sensible/latent split and equipment tonnage, ventilation rates per ASHRAE 62.1 and EN 16798 with demand-controlled options, and solar gain through 20 glazing types for the envelope contribution.
Distribution and plant have their own bench of tools. Pipe and duct sizing runs Darcy-Weisbach with fitting losses; chilled and hot water flow converts Q = m·cp·ΔT between kW, GPM, and pipe velocity; air mixing does the mass-weighted outdoor/return blend; cooling coils split sensible from latent with SHR and condensate rate; heat exchangers get both LMTD and effectiveness-NTU methods; cooling towers report range, approach, and makeup water; and the fan laws tool answers the perennial retrofit question — what happens to flow, pressure, and power when you change speed (power goes with the cube). Heat pump COP, geothermal borehole lengths, expansion tank acceptance volume, and NFPA 54 gas pipe sizing cover the plant room.
The household cluster deliberately uses the published consumer tables rather than reinventing them: window AC capacity from the ENERGY STAR room-size table with its sun/occupancy adjustments, water heaters from the DOE first-hour-rating worksheet, ceiling fans from the ENERGY STAR span-by-room-area guidance, attic ventilation from the IRC 1/150 and 1/300 rules, whole-house fans by CFM per square foot with an attic-NFA check, and firewood by the NIST-defined cord with BTU per species.
All HVAC & Comfort Tools
Daylight / Solar Gain
Solar heat gain, visible light transmittance, daylight factor, and shading analysis for glazing systems.
Pipe & Duct Sizing
Pipe and duct sizing calculator using Darcy-Weisbach equation with pressure drop, fitting losses, and material databases.
HVAC Load Calculator
Heating and cooling load sizing with sensible/latent analysis, psychrometrics, and equipment tonnage.
Ventilation Rate Calculator
Required airflow calculation per ASHRAE 62.1, EN 16798, and CIBSE Guide B with demand-controlled ventilation.
Heat Exchanger LMTD Calculator
LMTD and effectiveness-NTU sizing/rating for counter-flow and parallel-flow heat exchangers.
Fan & Pump Affinity Laws Calculator
Affinity laws for new flow, pressure, and power from speed or impeller diameter changes.
Cooling Tower Calculator
Range, approach, heat rejection, evaporation, blowdown, and makeup water with thermal efficiency.
Heat Pump COP Calculator
Carnot and actual COP, EER, running cost, and savings versus electric resistance heating.
Expansion Tank Sizing Calculator
Hydronic expansion tank sizing by the ASHRAE closed-loop method: water expansion volume, acceptance factor, and required tank volume.
Chilled & Hot Water Flow Calculator
Hydronic chilled-water and hot-water flow rate from a cooling or heating load via Q = m·cp·ΔT: mass flow, L/s, m³/h and US GPM, plus pipe velocity and a suggested pipe size for a target velocity.
Air Mixing Calculator
Adiabatic mixing of two moist-air streams such as outdoor and return air: mass-weighted mixed dry-bulb, humidity ratio, enthalpy, relative humidity and dew point on a dry-air basis.
Cooling Coil Load Calculator
Cooling coil total load split into sensible and latent components from the entering and leaving air conditions using ASHRAE psychrometrics, with dry-air mass flow, cooling in kW and refrigeration tons, the sensible heat ratio (SHR), and the condensate removal rate.
Geothermal Borehole (GSHP) Sizing Calculator
Ground-source heat pump vertical borehole sizing: ground load from the building heating or cooling load and COP, total bore length from the W/m heat-transfer rate, and the number and depth of boreholes per ASHRAE / IGSHPA rule-of-thumb design.
Gas Pipe Sizing Calculator (NFPA 54)
Low-pressure natural gas pipe sizing by the NFPA 54 / IFGC longest-length method: BTU/hr load (or a per-appliance list) to CFH demand, Schedule 40 capacities from Table 402.4(2) at the rounded-up run length, the smallest adequate size with utilization and next size up, and a Spitzglass formula cross-check.
Ceiling Fan Size Calculator (Blade Span & Downrod Length)
Ceiling fan sizing from the room dimensions and ceiling height per the ENERGY STAR / American Lighting Association buying guidance: blade span band from the floor area (29–36 in up to 75 ft², 36–42 in to 144 ft², 44 in to 225 ft², 50–54 in to 400 ft², and 56 in or larger — or two fans — beyond the published table), the band decided on the displayed whole-square-foot area so the badge never contradicts the printed number, downrod length from the verified ceiling-height ladder (flush under 8 ft, 3 in at 8 ft, 6 in at 9 ft through 120 in at 20 ft, non-integer heights rounding down to the step), the clearance checklist of blades at least 7 ft above the floor with 8–9 ft optimal and 18 in from walls, and a qualitative CFM shopping note — all guidance, since no building code governs fan sizing.
Window AC BTU Calculator (ENERGY STAR Room Sizing)
Window air conditioner sizing from the published ENERGY STAR room-AC table: 14 verified rows mapping room area to cooling capacity (100–150 ft² → 5,000 BTU/h through 2,000–2,500 ft² → 34,000 BTU/h, each boundary starting the higher row and the row decided on the displayed whole-square-foot area), the published adjustments in their published order — heavily shaded −10% of the base, very sunny +10%, +600 BTU/h per person beyond two occupants, +4,000 BTU/h for a kitchen — with a step-by-step breakdown from base to adjusted capacity, the closest representative retail unit sizes on both sides of the target, a smallest-unit note with an oversizing caveat for rooms under 100 ft², and an honest warning that oversized units short-cycle and dehumidify poorly — whole-house loads pointed at a full load calculation instead.
Attic Ventilation Calculator (IRC 1/150 & 1/300 Rule)
Required attic net free ventilating area per IRC 2021 R806.2: 1/150 of the attic floor area, or 1/300 when both exception conditions are met (Climate Zone 6-8 ceiling vapor retarder, 40-50% of the NFA in upper-portion ventilators), split 50/50 into upper exhaust and eave intake targets. Converts each half into ridge vent lineal feet, 8x16 soffit vents, continuous soffit strip, box vents, gable louvers, or turbines from representative NFA ratings, and a check mode grades an existing vent mix with pass/fail, deficit, and an intake/exhaust balance warning.
Water Heater Sizing Calculator (FHR & Tankless GPM)
Water heater sizing both ways the DOE Energy Saver method recommends: tank mode fills in the DOE Worksheet for Estimating Peak Hour Demand/First Hour Rating with the published averages — shower 20 gal per use, shaving 2, hand dishwashing or food prep 3, automatic dishwasher 7, clothes washer 25 (top-loader) or 15 (H-axis) — and matches the total to a storage or heat-pump model with a first hour rating at or just above the demand (size by the EnergyGuide FHR, never by tank volume); tankless mode sums representative simultaneous fixture flows (shower 2.0 gpm, kitchen sink 1.5, lavatory 1.0, clothes washer 2.0, dishwasher 1.5) and computes the required temperature rise from groundwater (regional presets 40-70 °F) to the fixed 120 °F delivery, compared against a typical gas unit of about 5 gpm — and an electric unit of about 2 gpm — at a 70 °F rise.
Whole House Fan Sizing Calculator (CFM & Attic NFA)
Whole house fan sizing from published DOE and manufacturer practice: target airflow at 2, 2.5, or 3 CFM per square foot of living area (the QuietCool good/better/best sizing ladder, inside the PNNL Building America 1.5–3 CFM/ft² band), an air-changes-per-hour cross-check from house volume against the 12–23 ACH effective range with a tall-ceiling size-up flag, the attic exhaust net free area the fan needs at 1 ft² per 750 CFM in ft² and in² (doubled to gross area for screened or louvered vents, with a link to attic-ventilation for vent selection), the open-window guideline of 4 ft² per 1,000 CFM, and a good/better/best CFM comparison chart — windows-open night-flush ventilation cooling, not a substitute for load-based HVAC design.
Firewood Cord Calculator (Cords, Face Cords & BTU)
How much wood a firewood stack really is: full cords from the statutory 128 ft³ (NIST Handbook 130 §2.4, wood ranked and well stowed) via stack length × height × piece length ÷ 12 ÷ 128, face-cord equivalents at the stack's own piece length with the fraction-of-a-cord explainer (a 16″ face cord is exactly 1/3 of a full cord, 24″ = 1/2, 12″ = 1/4 — the classic buyer-beware unit), total heat in million BTU from a 10-species table verified against USU Forestry Extension (white oak 29.1 down to cottonwood 15.8 MBTU per dry cord, seasoned-wood basis), and an optional $/cord price converted to total cost and dollars per million BTU for fair fuel comparison.
Frequently Asked Questions
Do I really need a full load calculation for one room?
For a single room served by a window unit, no — the ENERGY STAR table in the window AC tool, with its shade/sun and occupancy adjustments, is the appropriate fidelity. For whole-house equipment (furnace, heat pump, central AC), yes: rule-of-thumb tonnage per square foot is how houses end up with short-cycling, humid, unevenly conditioned interiors. That is the HVAC load calculator's job.
My duct system was quiet at low speed but roars after a fan upgrade. What happened?
The fan affinity laws: flow scales with speed, pressure with speed squared, and power with speed cubed. A 20% speed increase costs about 73% more power and raises duct velocities enough to cross into audible range. Run the fan laws calculator before specifying a speed change, and the pipe & duct tool to check whether the ductwork itself is the real bottleneck.
Whole-house fan or window AC for summer cooling?
Different climates, different physics. A whole-house fan exchanges indoor air for outdoor air — brilliant where evenings cool off, useless in humid heat, and it needs the attic net-free vent area the calculator checks. A window AC actually removes heat and moisture regardless of outdoor conditions. Dry-summer climates often want the fan (pennies per hour to run); humid ones need refrigeration.