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
HVAC calculators and tools
Every calculator runs in your browser, prints cleanly and asks for nothing. Grouped by the part of the system they measure.
Which tool for which job
The tools follow the order a job runs in. A survey sets the load; the load picks the equipment; the equipment sets the airflow the ducts have to carry; commissioning checks that what went in performs the way the numbers said it would. Each calculator page explains its method and cites the sources its figures come from, so a number can be checked rather than trusted.
| Stage | The question | Tool |
|---|---|---|
| Survey | How big is the load, room by room? | Manual J load calculator |
| Survey | Is the attic insulation pulling the load up? | Attic insulation R-value calculator |
| Survey | How leaky is the house, and how much fresh air does it need? | Air changes per hour calculator |
| Equipment | How many tons of cooling? | AC unit size calculator |
| Equipment | What furnace input, at this altitude? | Furnace size calculator |
| Equipment | Will a heat pump carry the design day? | Heat pump sizing calculator |
| Equipment | One room, one window unit or dehumidifier? | Window AC size calculator |
| Ducts | How much airflow for these tons? | CFM per ton calculator |
| Ducts | What size trunk and branches? | Duct size calculator |
| Commissioning | Is the blower fighting the ducts? | Static pressure calculator |
| Commissioning | Is the refrigerant charge right? | Superheat and subcooling calculator |
| Commissioning | What saturation temperature is this gauge reading? | R-410A pressure chart |
| Selling | What will the efficiency upgrade save? | SEER2 savings calculator |
| Selling | What should the replacement cost? | HVAC replacement cost calculator |
| Paperwork | Invoice or estimate for the visit | HVAC invoice template |
Sizing & Load
8 tools, 4 guides
Manual J load, BTU/h, tonnage, duct sizing and static pressure — the numbers that decide what equipment goes in.
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
Heating
3 tools, 5 guides
Furnaces, heat pumps and the efficiency ratings that price them — AFUE, HSPF2 and the cold-climate question.
Furnace size calculator
Input BTU/h and output capacity from load, climate zone and AFUE.
BTU/h · AFUE
Heat pump sizing calculator
Capacity at design temperature, balance point and the supplemental-heat gap.
BTU/h · balance point
AFUE fuel cost calculator
Annual heating cost from AFUE, fuel price and load — gas against electric.
$/yr · AFUE
Cooling
6 tools, 5 guides
Air conditioners, mini splits, refrigerant and SEER2 — the cool side of the manifold.
SEER2 savings calculator
Annual cooling cost and payback when you move from one SEER2 rating to another.
SEER2 · $/yr
Mini split sizing calculator
BTU/h per zone and how many heads a house actually needs.
BTU/h · zones
Superheat and subcooling calculator
Target superheat and subcooling from pressures and temperatures, with the charge verdict.
°F · psig
R-410A pressure chart
Saturation pressure by temperature for R-410A, with R-454B bubble and dew columns.
psig · °F
Window AC size calculator
Room air conditioner BTU/h from the ENERGY STAR chart, with sun, shade and kitchen adjustments.
BTU/h · room
Dehumidifier size calculator
Pints per day by room size and dampness, and old ratings converted to the 2019 test.
pints/day
Service & Costs
6 tools, 6 guides
Replacement cost, maintenance, filters and the credits that move the price — what the job costs and what keeps it running.
HVAC replacement cost calculator
Installed cost band by system type, tonnage and region, with the line items.
$ · installed
Air filter size chart
Nominal against actual filter dimensions, searchable, with the common sizes.
in. nominal
HVAC maintenance checklist
A printable spring and fall checklist with the readings a tech should record.
spring · fall
MERV rating chart
MERV 1–16 against particle size, pressure drop and what each rating catches.
MERV 1–16
Air changes per hour calculator
CFM to ACH and back, blower-door ACH50 against the 2021 IECC, and the ventilation rate.
ACH · ACH50
HVAC invoice template
A fill-in service invoice or estimate with equipment, refrigerant pounds, labor and tax.
invoice · estimate