Pick the right calculator.

Use this if you're:

• Replacing or upgrading a central AC, furnace, or central heat pump
• Wondering whether your existing 3-ton or 4-ton system was sized correctly
• Planning a whole-house electrification (gas furnace → heat pump conversion)
• Comparing competing install bids and wondering if anyone did the load math

What it computes: total cooling tonnage and heating BTU/h for the whole house, accounting for climate zone (CZ3 coast, CZ4 South Bay, CZ12 inland), square footage, insulation vintage, window area, infiltration, and duct condition. Calls out short-cycling risk if you're considering an oversized system.

What it tells you: a single recommended tonnage (e.g., "3-ton heat pump, 36,000 BTU/h cooling, 38,000 BTU/h heating") for the home as a whole.

Use this if you're:

• Tracing weak airflow at one register in a central HVAC system
• Checking a long run or an upper-level room before changing branch size
• Working with measured CFM, static-pressure allowance, and effective length
• Comparing a supply-duct concern with return-air or equipment restrictions

What it computes: a conditional one-room branch screen, plus whole-inch round and rectangular metal sizes based on velocity and friction limits. The measured-input view also reports equivalent diameter and straight-section pressure loss.

What it tells you: whether a reference 6-inch branch deserves attention, what to measure next, and which duct dimensions satisfy the entered pressure and air-speed limits. Final sizing still belongs in an ACCA Manual D design.

Use this if you're:

• Adding heating/cooling to a room that doesn't have ducts (ADU, garage conversion, sunroom, addition)
• Considering a ductless system for the whole home because existing ducts are poor or absent
• Planning zone-by-zone temperature control (main floor cool, bedrooms warm)
• Building or buying an ADU and need to spec the right outdoor + indoor combination

What it computes: BTU/h load for each room individually, the right indoor head size per room (9k, 12k, 18k, 24k), and a recommended outdoor unit that handles the combined connected load with proper diversity factors.

What it tells you: a per-zone equipment list (e.g., "kitchen: 12k wall-mount, master bedroom: 9k wall-mount, ADU: 18k ceiling cassette, outdoor: 36k 3-zone") plus 2026 Bay Area rebate context and a brief explainer on BAAQMD Rule 9-4 gas-ban deadlines.