Manual J Calculator (Simplified)
Room-by-room cooling and heating load calculator with sensible, latent, and total cooling, heating BTU, and per-room CFM. Add as many rooms as your project needs.
Project
Climate zone applies to all rooms.
Whole-house totals — Zone 4 — Mixed (DC, Nashville, Seattle)
| Room | Sensible | Latent | Total cool | Heat | CFM |
|---|---|---|---|---|---|
| Living room | 13,596 | 3,319 | 16,915 | 14,884 | 629 |
| Kitchen | 6,338 | 1,668 | 8,006 | 7,875 | 293 |
| Master bedroom | 8,346 | 2,069 | 10,415 | 10,631 | 386 |
Simplified Manual J — applies climate-zone rates with envelope (insulation, exterior walls, windows), sun, ceiling, and occupant adjustments. For permitting or rebate documentation, run a full ACCA-approved Manual J via certified software.
Visualization
Block load vs room-by-room
The diagram below shows the two Manual J methods side by side. Block load gives one whole-house number. Room-by-room gives every room its own load and airflow — required for zoned systems, ductless, and any project where Manual D ductwork follows.
Block load — fast and approximate
- One whole-house BTU number
- Best for single-zone systems < 2,000 sq ft
- Cannot identify under-served rooms
- Cannot drive Manual D duct sizing
Formula
What Manual J calculates per room
Latent ratio: 30% in humid zones (1–3), 20% otherwise.
20°F is the typical supply air ΔT in residential cooling.
Reference
The 8 building factors Manual J analyzes
| Factor | Why it matters | Typical input source |
|---|---|---|
| Envelope assemblies | Conduction path for heat | Construction docs or visual inspection |
| Insulation R-value | Resists heat flow | Spec sheet or audit |
| Windows (U + SHGC) | Largest single variable | NFRC label or product cut sheet |
| Infiltration | Air leakage drives heating + latent cooling | Blower door test or estimated ACH |
| Internal gains | People, lights, appliances | Occupancy schedule + appliance list |
| Duct losses | Unconditioned-space ducts add 15–25% to load | Duct location + R-value |
| Outdoor design temp | Sets ΔT magnitude | ASHRAE handbook by ZIP |
| Indoor design temp | Cooling: 75°F, heating: 70°F (typical) | Project specification |
Workflow
From Manual J to a finished install
- Manual J — calculate room-by-room cooling and heating loads (this tool)
- Manual S — select equipment that matches the load curve at your design conditions
- Manual D — design supply and return ducts using per-room CFM
- Manual T — select supply air registers and grilles by velocity and throw
- Manual RS — verify design comfort criteria (humidity, draft, stratification)
ACCA design sequence. Each step depends on the prior; skipping any one introduces sizing or comfort errors that show up after install.
Manual J FAQ
Quick answers to common HVAC sizing questions.