HVAC Calculators
Design HVAC heating and air-distribution checks from airflow: furnace BTU from area, load factor, AFUE and temperature rise; duct size by velocity or equal friction; room ACH and required ventilation flow; sensible BTU/h from CFM and ΔT; total external static pressure; and grille/register free-area sizing.
Furnace BTU
Screen furnace BTU/h from floor area, heating load factor, design temperature difference, AFUE and supply airflow.
Duct Size
Size round and rectangular HVAC ducts from airflow by target velocity or friction rate, with pressure drop and velocity checks.
Air Changes per Hour
Required airflow from room volume and target ACH, plus actual air changes per hour from design or measured airflow.
HVAC Airflow & BTU Load
Sensible BTU/h or kW from airflow and temperature difference, with required CFM, cooling tons and CFM per ton.
HVAC Static Pressure
Total external static pressure from duct, filter, coil, terminal and accessory drops, with margin against blower available static.
Grille Size
Size HVAC grilles, registers and diffusers from airflow, target face velocity and free-area ratio.
Refrigerant Line Charge
Estimate added refrigerant charge from line-set length, factory-included length and manufacturer charge rate or liquid-line volume.
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This HVAC cluster covers the early heating and air-distribution checks that usually happen before a final duct layout or equipment submittal. Start with the furnace BTU calculator when the question is equipment size: it screens heat loss from floor area, BTU/h per square foot, design temperature difference, AFUE and sizing margin, then estimates supply airflow from temperature rise. For coil or airflow checks, the HVAC airflow and BTU load calculator uses q = ρ·cₚ·Q·ΔT — the same relation behind BTU/h ≈ 1.08 × CFM × Δ°F — and reports required CFM, required ΔT, cooling tons and CFM per ton.
Once airflow is known, the duct size calculator turns CFM into round and rectangular duct dimensions by either target velocity or equal-friction sizing. It also calculates straight-duct velocity, pressure drop and friction rate, so a branch or trunk can be screened before fittings, transitions and terminals are added. The grille size calculator then sizes a register or diffuser from airflow, target face velocity and free-area ratio, and checks actual face velocity for an installed grille size.
For the pressure budget, the HVAC static pressure calculator sums return duct, filter, coil, supply duct, terminal and accessory drops into total external static pressure and compares it with available blower static. The air changes per hour calculator converts room volume and airflow into ventilation rate. For installation arithmetic, the refrigerant line charge calculator estimates added charge from line-set length beyond the factory-included length, using either the manufacturer charge rate or a line-volume/density fallback.
These are screening tools for HVAC designers, contractors and building owners; final designs still need ASHRAE/ACCA/SMACNA methods, manufacturer fan and terminal data, refrigerant line-size tables, fittings, leakage, balancing, acoustics and local code checks.