BTU Calculator
Sizing & LoadRoom-by-room heating and cooling load in BTU/h from area, climate zone and envelope.
BTU/h · Manual J
Sizing & Load
5 tools, 4 guides
Manual J load, BTU/h, tonnage, duct sizing and static pressure — the numbers that decide what equipment goes in.
Tools
AC Unit Size Calculator
Sizing & LoadTonnage for a whole house, with the oversizing check most rules of thumb skip.
tons · sq ft
Duct Size Calculator
Sizing & LoadRound and rectangular duct sizes from CFM and friction rate, with equivalent diameter.
CFM · in. w.c./100 ft
Manual J Load Calculator
Sizing & LoadA room-by-room worksheet that follows the ACCA Manual J method, printable.
BTU/h · room by room
Static Pressure Calculator
Sizing & LoadTotal external static pressure from supply and return readings, against the rated maximum.
in. w.c. · TESP
Sizing reference
The screening bands the tools in this section start from, before vintage, shading and storey-count corrections.
| Zone | Cooling BTU/h·ft² | Heating BTU/h·ft² | ft²/ton |
|---|---|---|---|
| 1 | 32 | 22 | 375 |
| 2 | 30 | 26 | 400 |
| 3 | 27 | 32 | 445 |
| 4 | 24 | 38 | 500 |
| 5 | 22 | 45 | 545 |
| 6 | 20 | 52 | 600 |
| 7 | 18 | 60 | 665 |
Guides
How to Size an AC Unit
Sizing & LoadWhy square-foot rules of thumb oversize, and what to measure instead.
9 min read
HVAC Tonnage per Square Foot
Sizing & LoadThe per-zone square-feet-per-ton bands, and where they break down.
8 min read
How to Size Ducts
Sizing & LoadCFM per ton, friction rate, and the duct calculator method step by step.
10 min read
Static Pressure Explained
Sizing & LoadWhat total external static pressure is, how to read it, and what a high number costs.
9 min read
Other sections
Heating
Furnaces, heat pumps and the efficiency ratings that price them — AFUE, HSPF2 and the cold-climate question.
3 tools · 5 guides
Cooling
Air conditioners, mini splits, refrigerant and SEER2 — the cool side of the manifold.
3 tools · 5 guides
Service & Costs
Replacement cost, maintenance, filters and the credits that move the price — what the job costs and what keeps it running.
4 tools · 5 guides