Calculator 09 / HVAC
HVAC Calculator
Cooling load scales with floor area first, then adjusts for ceiling height, insulation, how many people use the room, and sun-exposed windows. Get a quick BTU load estimate for a room, or a recommended round duct diameter for a target airflow.
How this calculator works
BTU load uses a common quick-estimate formula (25 BTU per sq ft baseline, adjusted for ceiling height, insulation, occupants, and sun exposure) — useful for a ballpark before calling in a contractor, not a substitute for a full Manual J calculation that accounts for windows, orientation, climate zone, and construction details room by room. Duct size uses a target air velocity to keep noise and static pressure reasonable, then rounds up to a size you can actually buy.
What this estimate cannot tell you
<strong>This is a square-footage rule of thumb, and it returns a single number for what are really two different calculations.</strong> A cooling load and a heating load are not the same quantity: cooling is driven largely by solar gain, humidity and internal heat, while heating is driven by the temperature difference to your design outdoor temperature. A house can easily need very different figures for each. Treat the result as a sanity check on somebody else's number, not as a basis for buying equipment.
A proper load calculation — ACCA Manual J, with duct design to Manual D and equipment selection to Manual S — accounts for a long list of things no area-based formula can see:
| Factor | Why it moves the number |
|---|---|
| Climate zone and design temperatures | The single largest driver, and entirely absent here |
| Orientation and shading | A west-facing glass wall behaves nothing like a north one |
| Window U-value and SHGC | This tool counts windows; it cannot know their performance |
| Air infiltration and tightness | Often a third of the heating load in older housing |
| Duct location and leakage | Ducts in an unconditioned attic can lose a great deal |
| Latent load (humidity) | Separate from sensible load and ignored entirely here |
Two worked examples
Load: a 14 × 12 ft room, 8 ft ceiling, average insulation, two occupants, two sun-exposed windows.
- Floor area = 14 × 12 = 168 sq ft
- Base = 168 × 25 = 4,200 BTU
- Height and insulation adjustments = ×1.0 (no change)
- Occupants over two = +0 BTU
- Windows = 2 × 1,000 = +2,000 BTU
- Total = 6,200 BTU, about 0.52 tons
Duct: 120 CFM at a 700 FPM design velocity.
- Duct area = 120 ÷ 700 = 0.171 sq ft = 24.7 sq in
- Diameter = √(4 × 24.7 ÷ π) = 5.6 inches
- Rounded up to a size you can buy = 6 inch round
Always round duct up, never down. An undersized duct raises velocity, and velocity is what you hear — the whistle from a register is almost always a duct that is one size too small.
Duct velocity and what each size carries
Velocity is the trade-off between duct size and noise. Sizing by a target velocity, as this tool does, is the quick method; a full design instead works to a friction rate across the whole system, which is what Manual D sets out. For a single branch run the two land in much the same place.
| Run | Typical velocity | Notes |
|---|---|---|
| Supply trunk | 700–900 FPM | Higher is tolerable away from living space |
| Supply branch | 600–700 FPM | The default here |
| Return trunk | 600–700 FPM | Returns are usually sized generously |
| Return branch | 400–600 FPM | Keep low; returns telegraph noise |
Above roughly 900 FPM in occupied rooms, air noise becomes noticeable, and it gets worse at every fitting and register. The figures below assume the 700 FPM branch default.
| Round duct | Approx. CFM at 700 FPM |
|---|---|
| 4" | 61 |
| 5" | 95 |
| 6" | 137 |
| 7" | 187 |
| 8" | 244 |
| 10" | 382 |
| 12" | 550 |
BTU, tons, and why bigger is not better
A "ton" of cooling is 12,000 BTU per hour, a unit inherited from how much ice it once took to do the same job. Equipment is sold in half-ton steps, so a load lands between two sizes more often than not.
| Tons | BTU/h |
|---|---|
| 1.0 | 12,000 |
| 1.5 | 18,000 |
| 2.0 | 24,000 |
| 2.5 | 30,000 |
| 3.0 | 36,000 |
| 4.0 | 48,000 |
| 5.0 | 60,000 |
When the load falls between sizes, the instinct is to go up. For cooling that instinct is usually wrong. An oversized unit satisfies the thermostat quickly and shuts off — short cycling — which wears the compressor, and worse, it never runs long enough to pull moisture out of the air. The result is a house that is cold and clammy rather than comfortable, and it is one of the most common complaints about residential air conditioning. Heating is more forgiving of oversizing, which is another reason the two loads deserve separate calculations.
Where HVAC estimates go wrong
- Sizing equipment from a rule of thumb. Including this one. It is a sanity check, not a specification — get a Manual J done before anything is bought.
- Sizing the box and ignoring the ducts. A correctly sized unit on undersized ductwork underperforms and fails early. The duct system is half the design.
- Rounding duct down to fit. If the size does not fit the cavity, change the route or use a rectangular equivalent — do not squeeze the air.
- Forgetting the return path. Supply air has to get back. A closed door with no return path or undercut can undo the whole design.
- Treating one number as both loads. Cooling and heating are separate calculations, and in most climates one dominates.
Where these numbers come from
Rules of thumb size equipment badly, and oversized equipment short-cycles and dehumidifies poorly. These are the methods that replace the rule of thumb.
- ACCA standards — Manual J (room-by-room load), Manual S (equipment selection) and Manual D (duct design) — the recognized sizing procedures.
- ASHRAE — Outdoor design conditions, ventilation rates and the standards most mechanical codes reference.
- ENERGY STAR — Equipment efficiency ratings, and what a given SEER or HSPF figure actually certifies.
Why does duct velocity change by duct type?
Supply trunk lines can run faster since noise is less noticeable further from the room; supply branches and returns run slower to keep register and grille noise down.
Is a bigger BTU number always better?
No — an oversized AC unit cools too fast without properly removing humidity, leaving a room cold and clammy. Right-sizing (not over-sizing) is the goal, which is exactly why a real Manual J calculation matters for final equipment selection.
Can I save, share, or print a calculation?
Yes — use the Save, Copy link, and Print buttons under your results. Saved calculations can be grouped into a named project on the My Projects page for a combined job estimate with cost totals. Everything is stored only in your browser, never uploaded — see the Privacy Policy for details.
Estimates only. Confirm final equipment and duct sizing with a licensed HVAC contractor.