HVAC Load Calculator
The whole-house screen: zone factor, ceiling and sun adjustments, occupant gains — then tons, the next standard unit, and the honesty card about rules of thumb.
HVAC Load Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Screening only — not a room-by-room Manual J.
- Cooling load modeled; heating needs the design ΔT.
- No orientation, glazing U-value or infiltration inputs.
In short: A 1,800-ft² home, moderate zone, 8-ft ceilings, normal sun, four occupants: 36,000 base + 1,200 occupant = 37,200 BTU/h = 3.10 tons → the 3.5-ton standard unit, inside Manual S’s 115% at 42,000 ≤ 42,780. The hot-zone version of the same house: 46,200 BTU/h → 4.0 tons.
Formula
base = ft² × zone factor × (H ÷ 8) × sun
load = base + 600 × max(0, occupants − 2)
tons = BTU/h ÷ 12,000
Manual S: equipment ≤ 115% of load
[('20', 'BTU/h per ft², the moderate baseline'), ('600', 'BTU/h per occupant past two'), ('12,000', 'BTU/h per ton, exact'), ('115%', 'Manual S single-stage ceiling')]
Worked Example
- Measure conditioned area; pick the climate factor.
- Scale for ceiling height and sun exposure.
- Add 600 BTU/h per regular occupant past two.
- Divide by 12,000; step to the next half-ton unit.
- Demand a real Manual J before signing.
1,800 ft², factor 20, 8-ft, normal, 4 occupants: 37,200 BTU/h = 3.10 tons → 3.5-ton unit (42,000 ≤ 42,780 at 115%). Hot zone (25): 46,200 → 3.85 → 4.0 tons. Legacy rule: 3.6; Manual-J average: 1.26.
Strengths & Limits Of This Model
Where this engine is strong
- Screen and honesty cards in one answer
- Manual S 115% checked on the proposed unit
- Legacy rule shown next to the load, never instead of it
Where it stops
- No envelope component detail
- Cooling only
Practical Use Cases
Equipment quotes
A number to hold the bid against.
Replacements
Is the old tonnage sane?
Additions
Screen the new load before the duct run.
New builds
Pre-Manual-J ballpark for budgeting.
Efficiency audits
Spot the oversized short-cycler.
Methodology & Editorial Standards
The screen multiplies conditioned area by a free-entry zone factor (15-30 band enforced), ceiling scaling H/8, and the ENERGY STAR sun factor (0.7-1.3 band); occupants past two add 600 BTU/h each. Tons = BTU/h ÷ 12,000; the next half-ton unit is checked against Manual S’s 115% ceiling. The legacy 500 ft²/ton figure and the real Manual-J average (~1,431 ft²/ton) are printed as context cards — the page claims to screen, not to replace the load calculation permits require. Areas under 100 ft² (room territory) and over 50,000 refused.
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.
HVAC Load Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many BTU per square foot for cooling?
About 20 for a moderate climate with 8-ft ceilings; hot-humid regions run 20-25% more. The screen multiplies the factor by ceiling height and sun, then adds people.
How do I convert BTU to tons?
Divide by 12,000 — one ton of cooling is 12,000 BTU/h, exact. 37,200 BTU/h is 3.10 tons, and real equipment steps in half tons: 3.0, 3.5, 4.0.
Why do contractors say 500 square feet per ton?
Legacy habit from leakier houses. Modern load calcs average about 1,431 ft² per ton — the old rule oversizes 2-3×, and oversized cooling short-cycles, leaving air cold but clammy.
What is Manual S?
The ACCA equipment-selection standard: single-stage cooling capacity may exceed the calculated load by no more than 115%. The screen checks the next standard unit against that ceiling.
How much heat does a person add?
About 600 BTU/h for room-screening purposes; a full Manual J carries 230 sensible + 200 latent per occupant and defaults occupancy to bedrooms + 1.
Do ducts in the attic change this?
Yes — they typically add 15-25% to the system requirement. Sealed-and-conditioned ducts are why two identical floor plans can need different equipment.
HVAC Load or BTU page — which one?
Whole house, tons, climate factors: this page. One room, a window or mini-split unit, the ENERGY STAR chart: the BTU page. They share the 20 BTU/ft² grammar and link each other.
What is a Manual J load calculation?
The ACCA standard — a room-by-room accounting of envelope, glazing, infiltration and internal gains under design temperatures. Most jurisdictions require it for permits; this screen is its five-minute preview.