For typical South Florida commercial space, plan on one ton of rooftop unit capacity per 350–500 square feet — so a 10,000-square-foot building lands somewhere between 20 and 30 tons. That bracket is for budgeting only: occupancy, glass, ceiling height and our brutal humidity move the real number dramatically, and the unit you buy should come from a load calculation, not a rule of thumb.
Here’s how RTU sizing actually works, factor by factor, and why the most expensive sizing mistake in this climate isn’t buying too small.
How Many Tons of RTU Per Square Foot?
Square feet per ton varies by a factor of two or more across building types — which is exactly what per-square-foot rules can’t see. Use these South Florida benchmarks to bracket a budget, then let engineering produce the purchase number:
| Space type | Sq ft per ton (S. FL) | 10,000 sq ft needs roughly |
|---|---|---|
| General office | 350–450 | 22–29 tons |
| Retail sales floor | 300–400 | 25–33 tons |
| Restaurant dining | 200–300 | 33–50 tons |
| Assembly / worship | 200–350 at peak | 29–50 tons |
| Light industrial / flex | 400–600 (zoned) | 17–25 tons |
Notice the restaurant row: identical square footage, double the office tonnage. The building type column matters as much as the floor area — and the factors below explain why.
What Factors Change How Much Tonnage You Need?
Six inputs move the number more than floor area does, and every one of them is invisible to a rule of thumb:
- Occupancy density. People are heat and — critically here — humidity. A conference room seating thirty needs radically more capacity than the same square footage of storage. Size to how the space is used at peak, not on average.
- Glass and orientation. A west-facing glass storefront in Florida sun is an afternoon load event; two identical floor plates with different glazing can differ by several tons.
- Ceiling height. Tonnage cools air volume, not floor tiles — high-bay retail, gyms and sanctuaries defy per-square-foot math entirely.
- Internal equipment. Kitchens, server closets, fitness machines, display refrigeration: everything plugged in eventually becomes heat the RTU must remove.
- Ventilation air. Code-required outdoor air arrives carrying South Florida humidity. The latent (moisture) share of the load here is far larger than national rules assume — often the deciding factor between comfortable and clammy.
- Envelope condition. Insulation, roof color and reflectivity, and infiltration through doors and docks all leak load in both directions, especially in older buildings.
Why Is Oversizing Worse Than Undersizing Here?
Because an oversized RTU fails quietly, in the way South Florida punishes hardest: humidity. A too-big unit blasts the space to temperature in minutes and shuts down before its coil has run long enough to wring moisture from the air.
The thermostat reads success while indoor humidity climbs; occupants feel cold but sticky; and the constant starting and stopping — short cycling — wears compressors with every restart. Undersized units at least announce themselves: the building runs warm on the worst afternoons and everyone knows. Oversizing masquerades as prudence (“add a safety margin”) while degrading comfort, efficiency and equipment life simultaneously. If load growth is a genuine concern, the answer is staged or multi-compressor capacity that can modulate — never extra nameplate tons on a single stage.
What Is a Load Calculation and When Do You Need One?
A commercial load calculation is the engineering model that converts your actual building — envelope, glazing, orientation, occupancy schedules, ventilation requirements, equipment heat — into a defensible tonnage number, hour by hour on the design day. It’s how the wide benchmark brackets collapse into a purchase order worth signing.
You need one at four moments: new construction, any equipment replacement, any change of use (the retail bay becoming a restaurant breaks every rule of thumb), and whenever chronic comfort complaints suggest the original sizing was wrong. Two buyer’s rules from the field: when competing bids differ meaningfully on tonnage, ask each bidder for their load calculation — the one who can’t produce it guessed; and keep the report with your building records, because it’s an asset for every future project. Our load calculation guide covers the method in depth.
Can You Replace an RTU With the Same Size as Before?
Only if a fresh calculation says so — never by default. The old nameplate records what someone believed the building needed decades ago; the building has changed since. Re-roofing, new glazing, LED lighting retrofits and tightened envelopes shrink loads; densified offices, added equipment and new tenants grow them.
We see both outcomes regularly: replacements that correctly downsize (cheaper equipment, better humidity control, lower bills) and replacements that needed more capacity than history suggested. Copying the nameplate locks in yesterday’s guess for another 15 years, and it’s the single most common sizing shortcut in replacement work. Our commercial HVAC replacement team re-runs the load on every project for exactly this reason — the calculation costs little against a 15-year purchase.
What Do South Florida Conditions Add to RTU Selection?
Three regional realities shape the purchase beyond raw tonnage. First, latent capacity: two units with identical nominal tons can differ meaningfully in moisture removal, and here the unit with stronger dehumidification and better part-load behavior wins — most of our operating hours are partial-load and humid.
Second, wind code: rooftop equipment in Palm Beach and Broward Counties must meet Florida Building Code wind-load requirements with engineered curbs and tie-downs, which constrains what can go where on the roof. Third, salt exposure: within reach of coastal air — effectively east of I-95 — specify factory coil coating regardless of size, because a correctly sized unit that corrodes early is still a failed purchase in every way that matters financially. Sizing and specification travel together on the coast.
Should One Big RTU or Several Smaller Units Serve the Building?
Multiple smaller units usually serve South Florida commercial buildings better than one giant one. Zoning is the first reason: a building with a west-glass showroom, interior offices and a kitchen has three different load profiles, and separate units let each zone run to its own demand instead of averaging everyone into compromise.
Redundancy is the second: when one of three units fails in August, the building limps; when the only unit fails, the building closes. Staged capacity across several machines also tracks partial loads more efficiently and dehumidifies better than one large unit cycling. The trade-offs — more curbs, more maintenance points — are real but generally worth it above modest tonnages. This is a design conversation to have before the load calculation is finished, not after the crane is booked.
What If a Correctly Sized RTU Still Can’t Keep Up?
Suspect delivered capacity before buying more tonnage — sizing math assumes healthy equipment. Dirty coils, low refrigerant charge, slipping belts, clogged filters, duct leakage and stuck economizer dampers all shrink real output below nameplate, sometimes by a third or more.
The symptoms mimic undersizing: long run times, warm afternoons, rising bills. The fix is a fraction of a replacement: a thorough service visit that measures what the unit actually delivers, restores airflow and charge, and verifies the economizer isn’t quietly importing hot outdoor air. It’s a common story in our maintenance work — the “undersized” unit that was really a neglected one. Verify the tons you own are arriving before paying for tons you may not need.
Frequently Asked Questions
How many square feet does a 10-ton RTU cover?
Roughly 3,500–5,000 square feet of typical office space in South Florida — but as little as 2,000–2,500 square feet of restaurant, assembly or high-glass space. The building type matters as much as the tonnage number; density and humidity load drive the difference.
What size RTU do I need for a 5,000 sq ft building?
Benchmark bracket: 10–15 tons for office-type use; more for dining, dense occupancy or heavy glazing. Treat that as a budgeting range, then let a load calculation on the actual space produce the number you buy against.
Is it OK to slightly oversize an RTU to be safe?
No — in this humid climate, “a little extra” produces short cycling and poor dehumidification: cold, clammy space and faster compressor wear. If future load growth is real, handle it with staged or multi-compressor capacity rather than extra single-stage tons.
Do RTU sizing rules differ for South Florida versus elsewhere?
The method is the same; the inputs aren’t. Our latent (humidity) loads are far higher than national assumptions, ventilation air is heavier to condition, and equipment must also meet wind-load and coastal-corrosion requirements. Benchmarks calibrated elsewhere routinely undersize the moisture side here.
Aspen Air Conditioning sizes, installs and services commercial rooftop units across Palm Beach and Broward Counties — load calculations included, guesswork excluded. Call 561-464-5010.
