What Size AC Unit Do I Need for a Shop? Sizing Guide

A shop’s AC size comes from a cooling-load calculation that counts ceiling height, insulation, sun, people, and heat from tools — not square footage alone.

A 1,200-square-foot shop with a 12-foot ceiling, a welder, and west-facing windows can need nearly twice the cooling of a same-size insulated office. Square-footage charts assume typical room conditions, so on a shop floor they routinely undersize equipment. Getting the number right takes about ten minutes of measuring and one honest equipment selection.

Why Square Footage Alone Undersizes A Shop AC

Floor-area charts work for ordinary rooms and fail on shops because they assume ordinary rooms. ASHRAE, which writes U.S. cooling-load standards, identifies separate nonresidential procedures because commercial spaces carry loads a room chart never sees. Four factors blow past the chart estimate:

• Ceiling height.
• Internal heat. Compressors, welders, ovens, motors, and lighting dump heat into the space; commercial methods count those gains directly.
• Envelope and sun. Poor insulation, metal walls, and large west or south glass add load charts ignore.
• Ventilation and infiltration. Open bay doors, exhaust fans, and outdoor-air requirements pull in humid outside air that must be cooled and dried.

One planning rule cites roughly 300–400 square feet per ton of cooling, with a wider spread of 230–520 square feet per ton. In heat output terms that lands near 30–40 Btuh per square foot, with a broader range of 23–52 Btuh per square foot. That spread is the point: the same 1,000-square-foot shop can land at 1.5 tons or 3 tons depending on what is inside it.

How To Calculate Your Shop’s Cooling Load

A proper load calculation adds the envelope, internal, and outdoor-air loads, then picks equipment to match. ASHRAE’s Handbook — Fundamentals chapter on nonresidential cooling and heating load calculation procedures — sets out the heat balance (HB) and radiant time series (RTS) methods, and the Load Calculation Applications Manual supplies spreadsheets for RTS-based work.

You do not need engineering software to get close. Work in this order:

1. Measure length and width for total floor area. If two open bays share one space, measure them as one.
2. Note ceiling height and volume. Tall spaces break square-foot charts.
3. List every internal heat source — people, lights, compressors, welders, ovens — and add their gains.
4. Add envelope load: window area, sun exposure, and roof and wall conduction.
5. Add ventilation and infiltration load, including open doors and exhaust fans.
6. Total the zone load, then select equipment, applying diversity when different tools rarely run together.

ASHRAE publishes the residential counterpart as the Air Conditioning Contractors of America’s Manual J — the room-level method most homeowner charts loosely derive from. It is the right tool for a house, not a shop floor.

A room-style chart commonly maps 350–550 square feet to about 9,800–12,500 BTU, and can serve as a sanity check. Treat it as a floor, not a verdict — it understates tall, solar-loaded, or equipment-heavy shops every time. Consumer Reports publishes a window-unit sizing guide explaining the basic BTU-to-area relationship.

Once the load number is firm, matching it to the right equipment model matters as much as tonnage. Our breakdown of the best AC unit for a shop covers ductless splits, mini-split heads, and portable options by capacity band.

Common Sizing Mistakes That Cost You Comfort

Oversizing is the most common and most expensive error, because it cools the air fast and leaves humidity behind. An oversized unit short-cycles, never runs long enough to pull moisture out, and leaves a shop clammy at a temperature that reads fine. In humid climates the latent load is the whole ballgame — a shop with open bay doors in summer carries a heavier latent load than a sealed one, and undersizing fails both temperature and humidity control. Other regular misses:

• Sizing from square footage while ignoring ceiling height, insulation, and equipment heat.
• Treating a consumer room chart as the final answer for a shop.
• Skipping ventilation and latent load entirely.
• Forgetting that two zones open to each other act as one larger cooled area.

Commercial and workshop cooling often needs equipment beyond a basic room AC. When your calculated load exceeds what a portable or window unit can deliver, a ductless split or mini-split is the realistic next step — it holds temperature and humidity far better than stacking two smaller units.