Cooling Load Calculator
Screen the load — envelope, people and the half-ton round-up, then the 400-CFM-per-ton airflow that follows it.
Cooling Load Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Screening rule — no glazing, orientation or envelope detail
- Fixed climate constant; hot-humid and mild run either side
In short: 2,000 ft² at the 20 BTU/h per square foot screening rule carries 40,000 BTU/h of envelope, and three occupants add 1,800 more: 41,800 BTU/h, or 3.483333 tons. Equipment rounds UP to the nearest half-ton — 3.5 tons, 42,000 BTU/h — and the airflow rule hangs 1,400 CFM off it. This is the screening arithmetic; the Manual J it previews is the authority.
Formula
load = 20 × ft² + 600 × people; tons = load ÷ 12,000, rounded up to the ½ ton; CFM = tons × 400
The screening rule prices a house's cooling in two lines: about 20 BTU/h per square foot of envelope (climate-adjusted — hot-humid runs higher, mild coastal lower) and about 600 BTU/h per occupant of internal gain. Twelve thousand BTU/h make a ton; equipment is chosen at the nearest half-ton AT OR ABOVE the load, never below. The airflow rule then asks 400 CFM per ton (350 for variable-speed) — the number the duct page sizes the trunks for. A real Manual J replaces every constant here with your walls, windows and orientation.
Worked Example
- Enter the conditioned area and the design occupancy.
- Read the load, the tonnage round-up and the airflow it implies.
- Treat the answer as the screening number a Manual J must confirm.
Defaults: 3.5 tons at 1,400 CFM. Drive to 1,000 ft² and one occupant: 20,600 BTU/h = 1.716667 tons, rounds to 2.0 tons, 800 CFM — the smallest residential system still clears it.
Strengths & Limits Of This Model
Where this engine is strong
- Load → tons → CFM in one chain — the sizing sequence honest
- The Manual J caveat is doctrine, not fine print
Where it stops
- A real load calc can move the answer a half-ton either way
Practical Use Cases
System shopping
the tonnage before the quotes
Duct planning
the CFM the trunks must carry
Quote checks
is the proposed tonnage honest
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Cooling Load Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many tons do I need per square foot?
The screening rule lands near one ton per 600 ft² (20 BTU/h·ft² × 12,000 ≈ 600 ft²/ton) before occupants. It is a screen: a sunny two-story with old windows can run double the envelope line of a shaded one of the same size.
Why round UP to the half-ton?
Equipment ships in half-ton steps, and an undersized system loses the setpoint on design days. Oversizing has its own tax — short cycling and dead humidity control — which is why the round is to the nearest half-ton above, not two sizes above.
Where does 600 BTU/h per person come from?
An adult at light activity sheds roughly 230 BTU/h sensible plus a few hundred latent, depending on activity. Six hundred is the round screening number; a dining room of dancers rewrites it.
What is Manual J and why does it outrank this page?
It is the ACCA room-by-room load method: U-values, R-values, glazing, orientation, infiltration, internal gains — your actual house instead of a per-square-foot constant. This page is the screening arithmetic Manual J confirms or corrects.
Why does airflow follow the tonnage?
Because coils are rated at a design airflow — about 400 CFM per ton for standard equipment, 350 for high-efficiency variable-speed. Three and a half tons is 1,400 CFM, and the duct page turns that into trunk sizes.
Do humidity and latent loads live here?
Only implicitly — the 20 BTU/h·ft² screen blends sensible and latent the way field rules do. A Manual J splits them, because latent loads decide runtime and dehumidification, not just temperature.
What about heat pumps?
The cooling side reads the same; the heating side needs the balance-point analysis — the load line against the capacity line at your design temperature. Size cooling per this page, then check heating separately.
Is 3.5 tons right for my 2,000 ft² house?
It is right for the screening inputs you entered. Two identical footprints can differ 50% in real load — shade, glazing, air sealing. Let this number bracket the quotes, and let the Manual J pick inside it.