HVAC Heat Load BTU Calculator

Manual J heat load calculator. BTU sizing, ASHRAE climate zones.

HVAC Heat Load BTU Calculator helps HVAC technicians, engineers, and contractors calculate accurate cooling and heating loads. Calculate accurate cooling and heating loads using a simplified ACCA Manual J methodology based on room dimensions, insulation levels, window types, and ASHRAE climate zones — the industry standard for professional air conditioning system sizing.

LOAD CALCULATION MODES:
Cooling Mode — Calculate BTU/h cooling requirement for air conditioning
Heating Mode — Calculate BTU/h heating requirement for furnace and heat pump sizing
Automatic Tonnage Conversion — Get AC tonnage (0.5 to 5+ tons) from load results
CFM Requirement — Airflow from the supply-air temperature difference, around 400 CFM per ton

ROOM PARAMETERS:
Room Dimensions — Enter length, width, and ceiling height
Exterior Walls — Count how many of the room's walls face outdoors
Insulation Levels — Poor, Average, Good, or Excellent, mapped to assembly U-factors
Window Types — Single Pane, Double Pane, Triple Pane, Low-E, with U-factor and SHGC
Window Orientation — North, East, South, West or mixed exposure for solar gain
Floor Type — Over conditioned space, over crawlspace/garage, or slab on grade
Climate Zones — 5 zones with ASHRAE design temperatures, overridable by hand
Duct Location — Add duct gain/loss when the ducts run through unconditioned space
Occupant Load — Sensible and latent heat from building occupants

COMPONENT BREAKDOWN:
Wall conduction load
Window conduction and SHGC-based solar gain, shown separately
Ceiling and roof heat gain
Floor and slab-perimeter load
Infiltration load from envelope tightness
Occupant sensible and latent heat
Duct gain/loss in unconditioned space
Sensible and latent totals, tonnage, airflow and BTU/h per sq ft

OPTIONAL PRO UPGRADE — $2.99 ONE-TIME IN-APP PURCHASE
The following features require a separate one-time in-app purchase. They are not required to use the calculator:
• Unlimited saved calculation history
• PDF export of results
• Dark theme
• Ad-free experience

WHY HVAC PROS USE THIS CALCULATOR:
Faster Manual J estimates than spreadsheets or online calculators
Accurate BTU sizing prevents oversized/undersized systems
Reduces callbacks from incorrect system selection
Quick cooling load checks on service calls
ASHRAE-compliant Manual J methodology for commercial work
Improves customer confidence with detailed technical reports

PERFECT FOR:
HVAC design engineers sizing air conditioning systems
HVAC field technicians quick-checking load requirements
AC contractors preparing accurate equipment bids
HVAC apprentices learning Manual J methodology
Building maintenance engineers planning replacements
Residential and commercial HVAC system sizing
Energy auditors estimating cooling needs

Based on ACCA Manual J design methodology: assembly U-factors, SHGC-based solar gain, ASHRAE design conditions, and no arbitrary safety padding.

Part of HVACTools Suite — the professional HVAC technician app for field calculations, refrigeration diagnostics, and air conditioning system design.