Calculate the right AC capacity in BTU and tonnage for any room or whole-house cooling. Returns the recommended size and equipment class (window, portable, mini split, or central).
Reviewed May 22, 2026
Enter the space characteristics, then click Calculate to see the recommended AC size, the Manual S tolerance band, sensible vs latent cooling for your climate, equipment recommendations, operating cost by efficiency tier, and the math step-by-step.
+600 BTU per person above 2
Enter your inputs above, then click Calculate
Result will appear here with the recommended AC tonnage, Manual S tolerance band, sensible vs latent split, equipment options, and operating cost estimate.
Climate zone is the single most important input in any HVAC sizing decision — it drives both heating and cooling design temperatures and the equipment-class recommendation. The reference card below covers all eight US climate zones with sample cities and design temperatures.
The calculator output tells you how many BTU/hr of cooling capacity you need. The other half of the decision is the efficiency tier — what SEER2 rating to buy. For the same recommended tonnage, the cheapest federal-minimum equipment and the top-tier inverter equipment can differ by $250-$540 per year in operating cost in long-runtime climates, but only $50-$100 per year in short-runtime climates. The chart below shows the spread.
The decision rule that comes out of this: match efficiency tier to runtime, not to the salesperson's margin. In Phoenix or Houston, the 22 SEER2 inverter pays back a $1,500 efficiency premium in 7-8 years; in Seattle or Boston, the same upgrade may never pay back over the equipment's 15-year useful life. See the AC reference hub for the full SEER2 / EER2 / HSPF2 explanation.
The default state shows the calculator's answer for a typical 2,000 square foot home in IECC climate zone 4 (mid-Atlantic, Ohio Valley), with 8-foot ceilings, average insulation, mixed sun, four occupants, treated as a living-room equivalent (the open whole-house treatment).
The math:
Air conditioning equipment is sold in standard BTU sizes; the calculator rounds the raw result to the nearest one. Use the table to map a target BTU to the equipment class typically available at that size.
| BTU | Tons | Typical equipment class |
|---|---|---|
| 5,000 | 0.42 | Window AC |
| 6,000 | 0.5 | Window AC |
| 8,000 | 0.67 | Window or portable |
| 10,000 | 0.83 | Window or portable |
| 12,000 | 1 | Window, portable, or mini split |
| 14,000 | 1.17 | Window or mini split |
| 18,000 | 1.5 | Mini split or large window |
| 24,000 | 2 | Central or mini split |
| 36,000 | 3 | Central AC |
| 48,000 | 4 | Central AC |
| 60,000 | 5 | Central AC |
Portable AC nameplate BTU ratings often overstate real-world cooling output. Single-hose portable units exhaust hot air using conditioned indoor air, which reduces effective cooling by 20-30%. Dual-hose portable units avoid this and deliver closer to nameplate.
The practical rule: if you're buying a portable AC, size up one tier from the calculator's recommendation. If the calculator says 10,000 BTU, buy a 12,000 BTU portable. If it says 12,000 BTU, buy a 14,000 BTU portable.
AC oversizing produces short cycling, poor humidity control, and accelerated equipment wear. AC undersizing means the unit can't keep up on hot summer days. The Goldilocks zone is the calculator's recommendation; a 10-20% margin is fine, but 30%+ oversizing starts to hurt.
For the full discussion of oversize vs undersize penalties, see the AC short cycling article. For variable-speed (inverter) AC equipment, the modulation range tolerates moderate oversizing better than single-stage units.
The calculator output is the total cooling capacity needed. That total splits into sensible cooling (dropping temperature) and latent cooling (removing water vapor), and the split depends on your climate. In dry climates (zones 2B, 3B — Phoenix, Las Vegas, Albuquerque) almost all of the cooling work is sensible; latent capacity requirements are minimal. In humid climates (zones 1A, 2A, 3A — Miami, Houston, Atlanta) about 25-35% of the cooling work is latent.
The Sensible Heat Ratio (SHR) measures this split: SHR = sensible cooling ÷ total cooling. A humid-climate cooling load might be SHR 0.70 (30% latent); a dry-climate load is closer to SHR 0.90 (10% latent). An oversized AC in a humid climate cools to setpoint quickly, shuts off before condensing enough moisture, and leaves the house at 65%+ relative humidity — comfort complaints follow even though the thermostat says "satisfied". Variable-speed inverter equipment is the right answer in humid climates because it runs long cycles at lower capacity, removing more moisture per BTU of cooling delivered.
For the full SHR discussion and how Manual S equipment selection accounts for climate-specific latent load, see the Manual S reference hub.
Existing AC equipment installed before about 2010 was typically sized using rules of thumb that produced 20-40% oversizing relative to current Manual J methodology. When the original system needs replacement, simply "matching the old unit" perpetuates the oversize. Modern Manual J accounts for tighter modern construction (post-2009 IECC code), high-performance windows, and improved infiltration measurement — all of which reduce cooling loads versus 1990s assumptions.
Manual S — the ACCA equipment selection standard — allows installed equipment to exceed the Manual J cooling load by 15% for single-stage units and 25% for variable-speed inverter equipment. Equipment within that tolerance is "Manual S compliant"; equipment beyond it is technically out of compliance and produces the short-cycling, humidity-control, and durability problems documented in the AC short-cycling article. The calculator result includes both the recommended Manual J number and the Manual S tolerance band, so you can immediately see whether a proposed unit falls inside or outside it. See the Manual S reference hub for the full tolerance-by-equipment-type breakdown.
For permit-grade central AC sizing, a Manual J load calculation is the right tool. Manual J accounts for orientation, room-by-room loads, ductwork, infiltration measured with a blower door, and internal gains from specific equipment, none of which this calculator captures. For an expensive central AC install (15-20 year equipment lifespan, $5,000-15,000 cost), the difference between a chart-grade estimate and a Manual J calculation is worth the small extra effort. See the Manual J methodology article for the underlying calculation.
Permit applications, HEEHRA and state rebate documentation, manufacturer warranty claims, and post-retrofit equipment selection (after envelope upgrades change the load) all require a full Manual J performed by a credentialed contractor using ACCA-approved software (Wrightsoft Right-J, Cool Calc Manual J, Elite RHVAC, EnergyGauge USA). Output from any free planning-grade tool — including this one — is not eligible for those uses.
Most central-AC sizing errors fall into one of five categories. Avoiding them puts the proposed tonnage within Manual S tolerance of the actual load.
A 4-ton AC installed in 1998 may have been correctly sized for a 1998 envelope. If you have since added attic insulation, replaced windows, or air-sealed, the current cooling load may be 2.5-3 tons. Replacing the old 4-ton with another 4-ton perpetuates the oversize. Run the calculator with current envelope inputs before signing a contractor proposal.
A 2,000 sq ft home in Phoenix needs about 4 tons of cooling; the same home in Minneapolis needs about 2.5 tons. The "600 sq ft per ton" rule misses climate, envelope, and occupancy variation by 50%+ in either direction.
AC oversizing beyond 15% (single-stage) or 25% (variable-speed) hurts humidity control, increases wear, and reduces effective efficiency. Manual S caps oversizing at the published tolerance band for these reasons.
A 3-ton AC connected to undersized ducts delivers about 2-2.4 tons of actual cooling capacity because the air handler cannot move design CFM at the duct system's pressure drop. Manual D duct design must accompany Manual S equipment selection; neither alone suffices.
In zones 1A-3A, latent (moisture removal) load matters as much as sensible load. Two equally-sized units (same nominal tons) can deliver very different humidity control because their AHRI-rated latent capacity differs. In humid climates, check the latent capacity number on the AHRI Certificate, not just the nominal tonnage.
Pre-computed AC sizing for typical rooms, apartments, and houses. Each example shows the recommended tonnage, equipment class, and full BTU calculation.
0.5 tons (5,000 BTU)
0.5 tons (5,000 BTU)
0.5 tons (7,000 BTU)
0.75 tons (10,000 BTU)
1 ton (12,000 BTU)
1.75 tons (21,000 BTU)
2 tons (24,000 BTU)
2.5 tons (30,000 BTU)
3 tons (36,000 BTU)
3.5 tons (42,000 BTU)
4 tons (48,000 BTU)
5 tons (60,000 BTU)
5 tons (60,000 BTU)
1.75 tons (21,000 BTU)
0.75 tons (10,000 BTU)
The full reference chart and adjustment-factor methodology this calculator implements.
For garage-specific sizing, the load is 2-3× higher per sq ft than interior spaces.
Why oversizing matters: an oversized AC short cycles, hurting humidity control.
Reviewed May 22, 2026