Sizing guide
Estimate the BTU size each room needs, see where it lands on the climate-zone chart, and shop the systems that fit. Confirm the final size with a Manual J load calculation before you buy.
BTU calculator
Answer from the room itself. Every input starts at a typical value, the estimate updates as you go, and each adjustment is shown so you can see where the number comes from.
Sizing chart
Find your zone's colour, follow its column down to your floor area, and read the size on the left. Your estimate is marked here once you enter a floor area.
| CapacityFloor area, sq ft | Zone 1Hot | Zone 2Warm | Zone 3Your zone | Zone 4Cool | Zone 5Cold |
|---|---|---|---|---|---|
| 9,000 BTU0.75 ton | Up to 340 sq ft | Up to 350 sq ft | Up to 370 sq ft | Up to 390 sq ft | Up to 410 sq ft |
| 12,000 BTU1 ton | 341–450 sq ft | 351–470 sq ft | 371–500 sq ft | 391–520 sq ft | 411–550 sq ft |
| 18,000 BTU1.5 tons | 451–680 sq ft | 471–710 sq ft | 501–750 sq ft | 521–780 sq ft | 551–830 sq ft |
| 24,000 BTU2 tons | 681–900 sq ft | 711–950 sq ft | 751–1,000 sq ft | 781–1,050 sq ft | 831–1,110 sq ft |
| 36,000 BTU3 tons | 901–1,360 sq ft | 951–1,420 sq ft | 1,001–1,500 sq ft | 1,051–1,570 sq ft | 1,111–1,660 sq ft |
Ranges assume an 8 ft ceiling, average sun, windows and insulation, and up to two people. Zone 3 matches the coverage MRCOOL publishes for each single-zone size; hotter zones need more capacity per square foot and colder zones less.
Already know your size?
Skip the calculator: tell us the project and go straight to the systems that fit.
Step 1 of 3
Preliminary guidance: Room area alone does not account for climate, insulation, windows, occupancy, or other loads. Confirm final sizing with a Manual J load calculation.
Sizing tips
An oversized system cools the air quickly and shuts off before it has pulled the humidity out, so the room feels clammy and the system cycles on and off more often — costing efficiency and wear.
Choose the size that meets the load, not the next one up to be safe.
The capacity is usually coded into the outdoor unit's model number, not its serial number. On MRCOOL DIY units the number after DIY- is thousands of BTU: DIY-12-… is 12,000 BTU. Central equipment uses the same code:
| Code | Size | BTU |
|---|---|---|
| 18 | 1.5 tons | 18,000 |
| 24 | 2 tons | 24,000 |
| 30 | 2.5 tons | 30,000 |
| 36 | 3 tons | 36,000 |
| 42 | 3.5 tons | 42,000 |
| 48 | 4 tons | 48,000 |
| 60 | 5 tons | 60,000 |
Matching the old size is a starting point. If the old system never kept up, or the home has been insulated or extended since, size it fresh.
A Manual J load calculation works from your actual walls, windows, orientation and local design temperatures rather than floor area.
One room or several, and how many rooms you want to condition.
Match every room to one of the published nominal capacities.
Pick the listed line-set lengths and confirm the supply voltage.
We show the one factory-approved bundle that matches. No nearest match is substituted.
You end with one exact system, its price, and its documents. Sizing here is a starting point; confirm with a Manual J load calculation.
