Sizing & Load

8 tools, 4 guides, 1 comparison

Manual J load, BTU/h, tonnage, duct sizing and static pressure — the numbers that decide what equipment goes in.

Every equipment decision downstream starts here. Work in this order: the load first — the Manual J worksheet room by room, or the BTU calculator for a quick check — then the envelope inputs that move it, such as the attic R-value. Once the load is known, the CFM per ton calculator turns tons into airflow, and the duct calculator turns airflow into sizes.

The last step is the one most jobs skip: measuring what was installed. Static pressure and temperature rise tell you whether the duct system delivers the airflow the load called for. The guides explain why square-foot rules of thumb oversize, and the flex vs rigid comparison covers the installation detail that most often wrecks a good duct design.

Thermal-toned room showing supply and return airflow — Supply into the room, return out of it: every sizing question lives between those two grilles.

Tools

BTU calculator for heating and cooling

Room-by-room heating and cooling load in BTU/h from area, climate zone and envelope.

BTU/h · Manual J

AC unit size calculator

Tonnage for a whole house, with the oversizing check most rules of thumb skip.

tons · sq ft

Duct size calculator

Round and rectangular duct sizes from CFM and friction rate, with equivalent diameter.

CFM · in. w.c./100 ft

Manual J load calculator

A room-by-room worksheet that follows the ACCA Manual J method, printable.

BTU/h · room by room

Static pressure calculator

Total external static pressure from supply and return readings, against the rated maximum.

in. w.c. · TESP

CFM per ton calculator

Target airflow per ton, or measured CFM from a furnace's temperature rise.

CFM/ton · °F rise

Wet bulb temperature calculator

Wet bulb and dew point from dry bulb and RH, in °F or °C.

°F WB · dew point

Attic insulation R-value calculator

Your zone's ENERGY STAR attic R-value, what is up there now, and the inches to add.

R-value · inches

Sizing reference

The screening bands the tools in this section start from, before vintage, shading and storey-count corrections.

Load intensity and area per ton by IECC climate zone
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 air conditioner

Why square-foot rules of thumb oversize, and what to measure instead.

9 min read

HVAC tonnage per square foot

The per-zone square-feet-per-ton bands, and where they break down.

8 min read

How to size ducts

CFM per ton, friction rate, and the duct calculator method step by step.

10 min read

Static pressure explained

What total external static pressure is, how to read it, and what a high number costs.

9 min read

Comparisons

Flex duct vs rigid duct

Friction, bends, support spacing, sag and leakage, and where each belongs.

in sag · ft span

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.

6 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.

6 tools, 6 guides