Central AC Tonnage Calculator Guide

What is a "Ton" of Air Conditioning?

A ton of AC capacity is 12,000 BTU/hour—the amount of cooling needed to melt one ton (2,000 pounds) of ice in 24 hours. It's a historical unit from when ice blocks cooled buildings, but the term stuck in HVAC sizing.

Common Tonnage Sizes:

• 1 ton: 12,000 BTU/hr → ~400-600 sq ft
• 1.5 tons: 18,000 BTU/hr → ~600-900 sq ft
• 2 tons: 24,000 BTU/hr → ~900-1,200 sq ft
• 2.5 tons: 30,000 BTU/hr → ~1,200-1,500 sq ft
• 3 tons: 36,000 BTU/hr → ~1,500-1,800 sq ft
• 3.5 tons: 42,000 BTU/hr → ~1,800-2,100 sq ft
• 4 tons: 48,000 BTU/hr → ~2,100-2,400 sq ft
• 5 tons: 60,000 BTU/hr → ~2,400-3,000 sq ft

Important: Square footage is a rough guide only. Actual capacity depends on insulation, windows, ceiling height, orientation, and climate. Always calculate BTUs using our BTU calculator before selecting tonnage.

BTU to Tonnage Conversion Formula

Tonnage = BTU ÷ 12,000

Examples:

• 22,000 BTU ÷ 12,000 = 1.83 tons → Round UP to 2 tons
• 28,000 BTU ÷ 12,000 = 2.33 tons → Round UP to 2.5 tons
• 34,500 BTU ÷ 12,000 = 2.88 tons → Stay at 2.5 tons or round to 3 tons
• 38,000 BTU ÷ 12,000 = 3.17 tons → Do NOT round to 3.5 tons—too much oversizing

Use our BTU-Tonnage converter tool for quick calculations with rounding recommendations.

Rounding Rules: When to Go Up or Down

General Rule: Round to Nearest Half-Ton

Residential central AC systems come in half-ton increments (1.5, 2, 2.5, 3, 3.5, 4, 5 tons). You can't buy a 2.3-ton unit—you choose 2 or 2.5 tons.

Rounding Decision Matrix:

If calculated tonnage is within 10% of next size DOWN → Go smaller

Example: 2.42 tons (29,000 BTU) calculated

• 2.5 tons = 30,000 BTU (3.4% over calculated)
• 2 tons = 24,000 BTU (17% under calculated)
• Choose: 2.5 tons (slight oversizing acceptable)

If calculated tonnage is within 5% of next size UP → Go smaller unless extreme climate

Example: 2.88 tons (34,500 BTU) calculated

• 3 tons = 36,000 BTU (4.3% over calculated)
• 2.5 tons = 30,000 BTU (13% under calculated)
• Choose: 2.5 tons in moderate climates (Zones 3-5), 3 tons in extreme heat (Zones 1-2)

If calculated tonnage exceeds next size by 15%+ → Round up

Example: 2.08 tons (25,000 BTU) calculated

• 2.5 tons = 30,000 BTU (20% over calculated)
• 2 tons = 24,000 BTU (4% under calculated)
• Choose: 2 tons (under-sizing by 4% is fine; home might reach 74°F on hottest days instead of 72°F)

Why Tonnage Matters

1. Cost Scales with Tonnage

Equipment and installation costs increase ~$800-1,200 per half-ton:

• 2 tons: $4,000-5,500 installed
• 2.5 tons: $4,800-6,500 installed
• 3 tons: $5,500-7,500 installed
• 3.5 tons: $6,300-8,500 installed

Buying a 3-ton system when you need 2.5 tons wastes $700-1,000 upfront.

2. Operating Costs Increase with Oversizing

Larger systems consume more electricity per cycle. An oversized 3-ton system costs $150-300/year more to operate than a properly sized 2.5-ton system, even though both cool the same space.

Over 15 years: $2,250-4,500 in wasted energy costs.

3. Comfort Degrades with Wrong Tonnage

• Oversized: Short-cycles, poor humidity removal, temperature swings, noisy on/off cycling
• Undersized: Can't reach target temp on hottest days (minor issue), runs continuously (good for dehumidification)

Slight undersizing (5-10%) is preferable to slight oversizing in humid climates. See our humidity control guide for details.

Common Tonnage Sizing Mistakes

Mistake #1: "1 Ton Per 600 Square Feet" Rule

This crude formula ignores insulation, windows, orientation, ceiling height, and climate. A 1,800 sq ft home doesn't automatically need 3 tons.

Reality check: Well-insulated 1,800 sq ft home in Zone 5 (cool climate) may need only 2 tons. Poorly insulated 1,800 sq ft home in Zone 1 (hot-humid) may need 3.5 tons.

Always start with BTU calculator, then convert to tonnage.

Mistake #2: Replacing "Same Size as Old Unit"

Your old 3-ton AC might have been oversized from day one. Or you've since upgraded insulation, replaced windows, or added shade trees—all reduce load.

Best practice: Recalculate load from scratch using current home conditions. Don't assume old size was correct.

Mistake #3: "Biggest Unit That Fits Budget"

Bigger is NOT better. A 4-ton system in a 2-ton load home will short-cycle constantly, fail to dehumidify, and wear out faster (more on/off cycles stress compressor).

Mistake #4: Ignoring Climate Zone

A 1,500 sq ft home in Phoenix (Zone 2, 110°F summers) needs 2.5-3 tons. Same home in Seattle (Zone 4, 85°F summers) needs 1.5-2 tons. Square footage alone doesn't determine tonnage.

Check your zone with our climate zone guide and use zone-specific design temperatures in calculations.

How Contractors Should Calculate Tonnage

Step 1: Perform Manual J Load Calculation

Professional load calc accounts for:

• Room-by-room dimensions
• Window sizes, types (single/double pane), and orientations
• Insulation R-values (attic, walls, floors)
• Number of occupants
• Internal heat sources (appliances, lighting)
• Infiltration (air leakage)
• Design temperatures (99% summer temp for your zip code)

Learn more in our Manual J simplified guide.

Step 2: Convert BTU to Tonnage

Manual J result: 26,800 BTU → 26,800 ÷ 12,000 = 2.23 tons

Step 3: Round Appropriately

2.23 tons rounds to 2.5 tons (next available size). Oversizing by 12% is acceptable.

Step 4: Verify with Homeowner

Quality contractors share load calc and explain reasoning. Red flag if contractor can't explain how they determined size.

Use our contractor bid comparison guide to evaluate proposals.

Adjusting Tonnage for Special Situations

Two-Story Homes

Upper floors get hotter. Two options:

1. Single system: Size for total load, use zoning dampers to balance airflow
2. Two systems: Smaller unit for 1st floor, larger for 2nd floor (typically 60% of load upstairs)

Calculate floor-by-floor loads with multi-room BTU planner.

Additions or Renovations

If adding 400 sq ft to your home, don't just tack on 0.5-1 ton. Recalculate entire home load—existing system may already be oversized, or addition has better insulation/windows than original home.

Future-Proofing for Solar Panels

Installing solar later? Don't oversize AC now to "use the extra solar power." Right-size for current load. Oversized systems waste electricity even with free solar (short-cycling reduces panel ROI by running at peak demand rates before panels fully offset load).

High Ceilings (10+ feet)

Add 10-20% to load for ceiling heights above 10 feet. Air stratifies (hot air rises), increasing cooling demand. Use ceiling fans to mitigate.

Tonnage by Home Size (Realistic Ranges)

Next Steps: From Tonnage to Equipment Selection

1. Calculate BTUs: Use BTU calculator with your home specs
2. Convert to tonnage: Divide BTU by 12,000, round to nearest half-ton
3. Verify climate factors: Check climate zone adjustments
4. Get multiple bids: Share BTU requirements with contractors, see bid comparison guide
5. Choose equipment type: Single-stage vs variable-speed (affects dehumidification, see humidity guide)
6. Consider efficiency: SEER2 ratings balance upfront cost with operating savings