AC Sizing Guide Tonnage vs BTU: Pick the Right System Every Time

A hot living room and the “tonnage vs BTU” mystery

You’re not imagining it some rooms roast while others feel fine. When a system is too small, it runs nonstop and never catches up. When it’s too big, it cools fast but doesn’t dehumidify, so you feel clammy. The cure starts with understanding tonnage vs BTU. In HVAC, BTUs measure heat energy, and tons describe cooling capacity. Your goal is simple: match your home’s heat load to a system that can remove that heat smoothly and efficiently. In this guide we’ll translate the jargon, share on-the-job shortcuts, and help you pick real equipment not guess. If you like, keep The Furnace Outlet’s handy Sizing Guide open as we go. Pro tip: Jot down your square footage, insulation quality, window count, and sun exposure. We’ll use those to fine-tune your pick at the end.

BTUs explained like we’re standing in your hallway

A BTU is the amount of heat needed to raise (or remove from) one pound of water by 1°F. Your AC’s job is simply to remove BTUs from your house each hour. That’s why units are rated in BTU/hr. If your house gains 24,000 BTUs of heat each hour on a hot day, you need a system that can remove about that much—otherwise indoor temperatures creep up. Think of BTUs as the “speed” at which we scoop heat out. More BTUs per hour, more scooping. Pro tip: When comparing models, check both the BTU/hr and the efficiency rating (SEER2 or EER). Two systems with the same BTU/hr can have very different energy bills. When you’re ready to browse, peek at our efficient R32 condensers.

So what’s a “ton” in HVAC and how is it 12,000 BTU?

In HVAC talk, one ton = 12,000 BTU per hour of cooling. No, it’s not the unit’s weight. A 2-ton system removes 24,000 BTU/hr; a 3-ton removes 36,000 BTU/hr; and so on. We still use “tons” because it’s a quick, uniform yardstick for larger systems. Many pros think in tons first, then convert to BTUs when checking specs. Helpful modern conversions:

• 1 ton = 12,000 BTU/hr

• 1 ton ≈ 3.517 kW

• 1 ton ≈ 4.71 HP

If you’re mixing and matching components, ensure the air handler/coil and condenser are capacity-matched. That pairing protects performance and warranties. You can explore compatible air handlers.

The ice-trade backstory: why “12,000” stuck

The 12,000 number isn’t random—it’s history plus physics. Before modern AC, buildings were cooled with blocks of harvested ice. Melting ice absorbs heat: about 144 BTUs per pound (latent heat of fusion). One ton of ice is 2,000 pounds. So melting a ton absorbs 144 × 2,000 = 288,000 BTUs. Spread over 24 hours, that’s 12,000 BTU/hr the origin of one ton of cooling. In the late 1800s, the ice trade was huge, moving millions of tons worldwide. The term “ton” stuck as engineers moved from ice to compressors because it made capacity easy to picture. If you like numbers, keep 12,000 BTU/hr per ton in your head. It’s the fastest way to sanity-check spec sheets and sales quotes.

From ice blocks to standards: Shipley, ASRE, and today

As refrigeration matured, the industry needed consistent ratings. In 1903, Thomas Shipley (York Manufacturing) led testing to define common conditions for capacity. That work helped form the American Society of Refrigerating Engineers (ASRE) a predecessor to ASHRAE and cemented the ton as a standard. Fast-forward: we still use tons and BTU/hr because they let homeowners and pros compare apples to apples across brands and system types. You’ll see both measurements on spec sheets and product pages. Pro tip: When browsing, look for both capacity and efficiency on the same page. We keep that front-and-center across our product lines and check our ductless mini-splits for clear capacity listings.

Quick-and-honest rough sizing (then we refine it)

Rule-of-thumb math helps you get in the ballpark fast, then we tighten it with your home’s details. A common starting point is 500–600 sq ft per ton in average U.S. conditions. Examples (very rough):

• ~1,000 sq ft → ~2 tons (24k BTU/hr)

• ~1,500 sq ft → ~3 tons (36k BTU/hr)

• ~2,000 sq ft → ~3.5–4 tons (42–48k BTU/hr)

• ~2,500 sq ft → ~4–5 tons (48–60k BTU/hr)

But climate, insulation, windows, and ducts can swing sizing by 20–40%. Pro tip: Use the rough tonnage as a starting bracket, then adjust using the checklist in Section 8. Want a second pair of eyes? Send photos and basics with our Quote by Photo.

Why oversizing and undersizing both feel bad

Undersized systems run constantly, struggle in heat waves, and wear out early. Oversized systems short-cycle: they blast cold air, shut off, and never pull moisture out. The result is that sticky, “cold but damp” feeling. Side effects:

• Higher bills (inefficient run patterns)

• Uneven rooms (temperature swings)

• More maintenance (hard starts, stress)

In humid regions, a slightly smaller correctly matched system with good airflow often feels more comfortable than a larger short-cycling unit. If you’re battling humidity or room-to-room differences, consider zoning or ductless heads; our wall-mounted mini-splits let you target problem spaces without redoing ducts.

The real-world checklist that changes your tonnage

Before you lock in size, walk your home with this list:

1. Climate & sun: South/west exposure, big glass, and top-floor rooms gain more heat.

2. Insulation & air sealing: Attic levels matter. Poor sealing increases required BTU/hr.

3. Windows & shading: Single-pane vs low-E, blinds, and trees alter loads fast.

4. People & appliances: Kitchens and busy rooms run hotter.

5. Ducts & returns: Kinks, leaks, or missing returns starve airflow.

6. Ceiling height: Large volumes need more capacity.

If ducts are tired, fix them before you upsize the AC. Often a simple duct repair saves a half-ton of capacity. For duct projects, see accessories you may need during install.

Matching components: coils, air handlers, and efficiency

Your outdoor unit (condenser/heat pump) needs the right indoor coil or air handler to deliver its rated tons/BTUs and efficiency. Mismatching can drop performance, increase noise, and void warranties. Steps we use on installs:

• Pick the capacity first (tons/BTU/hr).

• Match the AHRI-rated coil/air handler.

• Verify airflow (CFM): roughly 350–450 CFM per ton.

• Confirm refrigerant type (e.g., R-32) and line-set sizing.

If you’re pairing a new outdoor with an older indoor coil, double-check compatibility. When in doubt, choose a matched set like our R32 AC + Air Handler Systems to keep it simple and efficient.

Ducted vs ductless: which sizing path fits your house?

Ducted systems serve the whole home from a central unit—great if your ducts are solid and you want one thermostat. Consider packaged options if space is tight (see package units). Ductless shines for additions, bonus rooms, and homes without good ducts; you size each head to the room.

A simple, step-by-step mini load estimate (at your kitchen table)

Here’s a quick DIY approach we use for first-pass estimates:

1. Square footage: Total conditioned area ÷ 550 ≈ base tons.

2. Adjust for insulation:

• Poor attic/walls: +10–20%

• Great sealing/low-E windows: −10–15%

3. Sun & glass: Big west/south glass: +5–10%

4. Ceiling height over 8 ft: Add 5% per extra foot average.

5. High-use kitchens/people: +3–7%

6. Convert tons × 12,000 = BTU/hr target.

Curious about payments? See HVAC financing. Keep your notes square footage, sun, insulation, duct status—and compare two or three matched systems. When specs are clear, the choice is easy. And if you hit a snag, we’re right here at The Furnace Outlet.