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This tool applies a standard formula from the relevant domain. The computation is deterministic — given the same inputs, the result is always identical. The formula accounts for the primary variables that determine the output.
Total load = Conduction + Solar + Internal + Infiltration + Ventilation (Manual J simplified)The calculation requires: Floor Area, Wall R-Value, Window Area, Design ΔT. Each parameter contributes to the final result according to the formula above. Accuracy depends on the precision of your input measurements.
Updated: July 2026
A typical use case demonstrating the calculator with representative values.
→ Calculated result based on the formula and provided inputs
Enter your own values to get a personalized result for your specific situation.
→ Result calculated from your specific input values
Check the expected unit for each input field (shown as suffix). Converting between imperial and metric units before entering values prevents calculation errors.
When a formula combines multiple measurements, all inputs must use compatible units. Convert everything to the same system (metric or imperial) before calculating.
Simplified Manual J heat gain/loss calculation for HVAC sizing. Free online HVAC calculator with instant, accurate results. It applies the calculation formula (Total load = Conduction + Solar + Internal + Infiltration + Ventilation (Manual J simplified)). For example: 2000 sq ft, r-13 walls, 200 sq ft windows, 30°f δt — A typical use case demonstrating the calculator with representative values.