Sizing an air conditioner is one of the most consequential decisions in an HVAC replacement — and it's one of the most misunderstood. Bigger is not better. A unit that's too large for your home will cost more to run, wear out faster, and leave your house feeling clammy even when the temperature is right. Here's how to think through it correctly.
Why Getting the Size Right Matters in St. Louis
St. Louis summers are hot and humid. On a July afternoon with 95°F heat and 70% humidity, your AC is fighting on two fronts: cooling the air and removing moisture from it. Both jobs matter for comfort — and both depend on your system running correctly sized cycles.
An oversized unit cools the air temperature quickly, then shuts off before it's had time to properly dehumidify. You get a house that hits 72°F but still feels sticky and uncomfortable. An undersized unit runs constantly and can't keep up on the hottest days. Neither is what you want.
The problems caused by wrong-sized equipment
- Short cycles — runs briefly, shuts off, repeats
- High humidity inside even when temperature is met
- Higher energy bills despite fast cooling
- More wear on compressor from frequent starts
- Shorter equipment lifespan
- Runs continuously on hot days
- Can't reach the thermostat setpoint in extreme heat
- Higher energy bills from constant operation
- Overworked compressor — earlier failure
- Uncomfortable upstairs rooms or hot spots
How AC Size Is Measured
Air conditioners are sized in tons — not weight, but cooling capacity. One ton of cooling equals 12,000 BTUs per hour. Residential systems typically run from 1.5 tons to 5 tons, with most homes in the St. Louis area falling somewhere between 2 and 4 tons.
You may also see BTU ratings used directly. A 2-ton unit is 24,000 BTU/hr; a 3-ton is 36,000 BTU/hr; a 4-ton is 48,000 BTU/hr. The terms are interchangeable — contractors typically use tons; equipment specs sometimes list BTUs.
The Rule of Thumb — and Why to Be Careful With It
You'll often see estimates like "1 ton per 500–600 square feet" used as a quick reference. That puts a 1,500 sq ft home around 2.5–3 tons. These numbers are a starting point — not a sizing decision.
The problem with square footage rules is they ignore everything else about your home. Two houses with the same square footage can have drastically different cooling loads depending on:
Ceiling height
9-foot or 10-foot ceilings mean significantly more air volume to cool than 8-foot ceilings at the same square footage.
Insulation quality
Older St. Louis homes from the 1950s–1970s often have little or no attic insulation by modern standards, which dramatically increases cooling load.
Window area and orientation
West-facing windows with direct afternoon sun in July add substantial heat gain. South-facing is similar. North-facing much less so.
Shade and landscaping
A home shaded by mature trees can have a meaningfully lower cooling load than an identical exposed home.
Number of occupants
People generate heat. A household of five produces noticeably more internal heat gain than a household of one.
Ductwork condition and layout
Leaky or undersized ducts reduce delivery efficiency regardless of what the outdoor unit is rated for.
The Right Way to Size: Manual J
The industry standard for HVAC sizing is a calculation called Manual J — a load calculation methodology developed by the Air Conditioning Contractors of America (ACCA). It accounts for all the factors above, plus local climate data (St. Louis design temperatures), construction materials, infiltration rates, and more.
A proper Manual J takes time to do correctly. When a contractor gives you an instant quote based on "what you had before" or square footage alone, they're skipping the math. That's how oversized systems get installed — because contractors default to bigger to avoid the risk of a customer complaining their home doesn't cool.
What to ask any HVAC contractor
Before agreeing to a replacement, ask: "Will you run a Manual J load calculation to confirm the size?"
A contractor who won't do this — or brushes it off — is guessing. That guess might work out, but it also might leave you with a 4-ton unit in a home that actually needs 3 tons, paying 25–30% more in energy costs every summer for the next 15 years.
Rough Sizing Guide for St. Louis Homes
With those caveats in mind, here's a general reference based on typical St. Louis home construction and climate. These are starting points for a conversation — not replacement for a proper calculation.
| Home Size (conditioned sq ft) | Typical Range | Notes |
|---|---|---|
| Under 1,000 sq ft | 1.5 – 2 tons | Small older homes, condos, additions |
| 1,000 – 1,500 sq ft | 2 – 2.5 tons | Ranch homes, bungalows, starter homes |
| 1,500 – 2,000 sq ft | 2.5 – 3 tons | Most common range in St. Louis suburbs |
| 2,000 – 2,500 sq ft | 3 – 3.5 tons | Two-story homes, newer construction |
| 2,500 – 3,500 sq ft | 3.5 – 4 tons | Larger two-story, high ceilings, lots of windows |
| 3,500+ sq ft | 4 – 5 tons (or zoned) | Often better served by a two-zone system |
When the Old Unit Size Isn't the Right Guide
A common assumption is that when it's time to replace, you just match what you have. That's worth questioning. If your old system was oversized from the start, replacing it with the same size locks in the same problems. It's also worth revisiting if your home has changed:
Energy-efficient improvements like new windows and attic insulation can meaningfully reduce your cooling load — sometimes enough to justify going down a half-ton. A contractor who doesn't ask about these changes before recommending a size is leaving money on the table for you.
Efficiency Ratings: SEER2
Once you have the right size, you'll also see efficiency ratings on equipment. The current standard is SEER2 (Seasonal Energy Efficiency Ratio 2). Higher is more efficient. Federal minimums for new equipment in Missouri are 14.3 SEER2 as of 2023.
Most Bryant equipment we install falls in the 16–20+ SEER2 range. The efficiency difference between a 16 and a 20 SEER2 unit is real, but the payback period depends on your usage, local electricity rates, and how long you plan to stay in the home. We'll run the numbers with you — not push the most expensive unit.
A practical take on SEER2
In most St. Louis homes, the jump from minimum efficiency (14.3 SEER2) to mid-range (16–18 SEER2) has a reasonable payback period of 4–7 years in utility savings. Above 20 SEER2, the premium is substantial and payback stretches longer. Unless you have unusual cooling hours or very high electricity costs, we typically recommend mid-range efficiency for most replacements.
What Happens at the In-Home Estimate
When we come out for a replacement estimate, here's what we look at before recommending a size:
Measure and record the home
Square footage, ceiling heights by floor, number and size of windows, orientation of the house.
Assess current insulation
Attic and wall insulation type and approximate R-value — this directly affects heat gain calculations.
Review existing ductwork
Duct sizing, condition, any known leaks, and delivery layout. Undersized or leaky ducts can negate the benefit of correctly sized equipment.
Check the current system's performance history
If you know the old unit's size and whether it was keeping up or short-cycling, that's useful data.
Run the load calculation
Using Manual J methodology with St. Louis design temperatures to arrive at the actual BTU load recommendation.
Ready to figure out the right size for your home?
We've been sizing and installing AC systems in St. Louis homes since 2010. If you're looking at a replacement — or just want a second opinion on what you've been quoted — give us a call. We'll do the load calculation, explain our recommendation, and give you a straight answer on what makes sense for your home and budget.