contractor · 2026-05-01

HVAC tonnage sizing calculator

Manual J shortcut: estimate AC tonnage and BTU requirement for a residential space based on square footage, climate, insulation, and orientation.

AC tonnage
5.8
Round up to nearest 0.5 ton

Inputs

Conditioned sqft2,200
Climate zoneHot-humid (TX, FL, deep South)
Insulation + windowsAverage (1990s-2000s build)
Sun exposureAverage
Occupants4
Kitchens (heat-generating)1

Supporting metrics

Total BTU/hr cooling load69,600
BTU/hr per sqft32
Rounded ton size to install6.0

About this calculator

HVAC sizing — the rule of thumb that's wrong, and the one that's right

The old rule "1 ton per 500-600 sqft" oversizes most homes by 30-50%. A right-sized AC runs longer cycles at lower output, dehumidifies properly, and lasts 5-10 years longer than an oversized unit short-cycling itself to death.

What actually matters

  1. Climate zone — the design temperature delta drives base BTU/sqft. Hot-humid Florida needs 28-32 BTU/sqft; mixed Ohio is 23-26.
  2. Envelope — insulation level and window quality. Post-2015 tight homes lose 30-40% less heat than 1980s builds.
  3. Solar gain — west-facing glass at 5pm in July is brutal. Shaded north-facing rooms are easy.
  4. Internal loads — bodies (~600 BTU/person), cooking (~1,200 BTU per active kitchen), unvented gas appliances.

The sizing rule

This calc is a Manual J shortcut. For homes in tight bands of envelope/climate the answer is within 10-15% of a full Manual J — close enough for replacement-in-kind decisions. For new construction or a major renovation, always do a full Manual J because the cost of getting it wrong is a 20-year headache.

Why oversized AC fails

An oversized unit hits the thermostat setpoint fast on temperature, but never runs long enough to pull humidity out of the air. You get cold + clammy houses, mold growth, and short-cycling that wears out compressors. Right-sized = longer cycles = drier air = comfort.

FAQ

Why is my installed unit always rounded up?

AC compressors come in half-ton increments (1.5, 2, 2.5, 3, 3.5, 4, 5). You round to the nearest half-ton at or above the calculated load. Don't round to 'just one more ton to be safe' — that's where short-cycling and humidity problems live.

Should I replace like-for-like or downsize?

If your home has been weatherized since the original AC was installed (new windows, attic insulation, air sealing), downsize. A house that needed 4 tons in 1995 often needs 2.5-3 tons after a deep retrofit. Run this calc with the new envelope numbers, then trust the answer.

Is a heat pump sized the same way?

Cooling capacity is sized the same. Heating capacity needs separate analysis — modern variable-speed heat pumps can be sized to the heating load (which is often higher in cold climates) without oversizing for cooling, because they modulate down. Have your installer run both calcs.

What about really humid days?

If you're in a hot-humid zone, prioritize equipment with a high SHR (sensible heat ratio) closer to 0.7-0.75 instead of 0.8+. Variable-speed inverter units handle dehumidification far better than single-stage. Sizing alone won't fix humidity — you also need the right equipment.