AC Size Calculator
How many BTUs and tons does a space need to stay cool? This estimator turns square footage, climate, ceiling height and sun into a cooling load and a system size, for HVAC contractors and homeowners comparing replacement bids.
Free, no sign-up. Runs in your browser; nothing you type is stored unless you choose to email yourself the results.
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Email me these results. We'll send your numbers, plus how to turn this calculator into a lead magnet on your own website.
Three steps, about a minute
Step 1
Type the Area to cool, counting only the conditioned rooms this system will serve.
Step 2
Pick a Climate and a Sun exposure, and change Ceiling height if the rooms aren't 8 ft.
Step 3
Read the Estimated cooling load in BTU/h and the Nearest system size in half-ton steps.
How the ac size calculator works
The base is a rule of thumb: 20 BTU per hour for every square foot. Three adjustments follow. Climate multiplies the load by 0.9 for cool northern areas, 1.0 for moderate and 1.15 for hot southern areas. Ceiling height ÷ 8 scales it for taller rooms holding more air. Sun exposure applies 0.9 for mostly shaded, 1.0 for average and 1.1 for very sunny. Dividing the load by 12,000, the BTU/h in one ton of cooling, and rounding to the nearest half gives the system size. The default 1,800 sq ft in a moderate climate, 8 ft ceilings and average sun comes to 36,000 BTU/h, which is 3 tons. Insulation, window area, duct leakage and the number of occupants are not in this math. Before you size equipment you'll install, run a Manual J load calculation.
| Area to cool | 1,800 sq ft |
|---|---|
| Climate | Moderate |
| Ceiling height | 8 ft |
| Sun exposure | Average |
| Estimated cooling load | 36,000 BTU/h |
|---|---|
| Nearest system size | 3 tons |
| Per square foot | 20 BTU |
Rule of thumb of about 20 BTU per square foot, adjusted for climate, ceiling height and sun. A Manual J load calculation is the professional standard.
Using the numbers like a pro: tips for HVAC contractors
Use this number to start the conversation, not to pick the unit. Offering a Manual J with every replacement quote separates you from bids that simply match the old unit's tonnage.
When a customer asks to go bigger just to be safe, explain short cycling. An oversized unit shuts off before it pulls moisture out, leaving rooms cool but clammy.
Quote good, better and best options at the same tonnage, changing efficiency and features rather than size. Customers compare like for like, and you avoid a race to the cheapest box.
Put a calculator like this on your own website
Someone searching for AC sizes is often replacing a system or doubting a quote they already have. A sizing tool on your cooling page answers the question on the spot, then offers a free in-home load calculation for a phone number and address. That converts a curious visitor into a booked estimate. If you want one on your site, we build them to match your brand and service area.
Get a free audit of your marketing
Website, Google Business Profile and a Google Maps rank scan across your area. Plain English, no obligation.
AC Size Calculator questions
What size AC do I need for 1,800 square feet?
Using the common rule of thumb of 20 BTU per square foot, 1,800 sq ft needs about 36,000 BTU/h, which is a 3-ton system. Hot climates, very sunny rooms or tall ceilings push that up, and a tight, well-insulated home may need less. A Manual J load calculation gives the real answer for a specific house.
How many BTU is a ton of air conditioning?
One ton of cooling equals 12,000 BTU per hour, a unit that traces back to the heat needed to melt a ton of ice in 24 hours. A 2-ton system is 24,000 BTU/h, a 3-ton is 36,000 BTU/h and a 5-ton is 60,000 BTU/h. Home systems are typically sold in half-ton steps.
What is a Manual J load calculation?
Manual J is the ACCA procedure for calculating a home's heating and cooling loads room by room. It accounts for insulation, window size and orientation, air leakage, duct location, local design temperatures and occupants. Many jurisdictions reference it in code, and it is the standard way to avoid oversizing or undersizing equipment.
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