What the result means
The supplied ENERGY STAR guideline selects a base capacity from the room’s square footage. Shading or strong sun adjusts that base; regular occupants above two and a kitchen add their stated amounts afterward.
This is a room and window unit estimate, not a whole-home design calculation. Above 2,500 square feet, the tool intentionally returns no capacity and directs users to a Manual J load calculation.
How it's calculated
- Start with the ENERGY STAR room-size band’s Btu/h value
- Apply −10% for heavily shaded or +10% for very sunny conditions
- Add 600 Btu/h for each regular occupant above two
- Add 4,000 Btu/h for a kitchen; tons = Btu/h ÷ 12,000
The area band includes its lower bound and excludes its upper bound; exactly 2,500 ft² uses the 34,000 Btu/h band before adjustments.
Below 100 ft², the base estimate is 5,000 Btu/h, labelled as the smallest common room-unit capacity.
Sizing guideline from ENERGY STAR.
Worked example
A 400 ft² very sunny room with four regular occupants and no kitchen.
- The 400–450 ft² band starts at 10,000 Btu/h.
- Very sunny adds 10%: 10,000 × 1.10 = 11,000 Btu/h.
- Two occupants above the included two add 2 × 600 = 1,200 Btu/h.
Estimated capacity = 12,200 Btu/h, or about 1.02 tons.
Reference table
The values below are computed from the same calculator functions used above. They let you compare common sizes and operating points without replacing the interactive calculation or its assumptions.
| Room area (ft²) | Base capacity (Btu/h) |
|---|---|
| 100–150 | 5,000 |
| 150–250 | 6,000 |
| 250–300 | 7,000 |
| 300–350 | 8,000 |
| 350–400 | 9,000 |
| 400–450 | 10,000 |
| 450–550 | 12,000 |
| 550–700 | 14,000 |
| 700–1,000 | 18,000 |
| 1,000–1,200 | 21,000 |
| 1,200–1,400 | 23,000 |
| 1,400–1,500 | 24,000 |
| 1,500–2,000 | 30,000 |
| 2,000–2,500 | 34,000 |
The normal upper bound is exclusive. The confirmed 2,500 ft² endpoint uses the final 34,000 Btu/h band before adjustments. For room and window units; central systems need Manual J. Source: ENERGY STAR (energystar.gov).
Sources and assumptions
The area band includes its lower bound and excludes its upper bound; exactly 2,500 ft² uses the 34,000 Btu/h band before adjustments. Below 100 ft², the base estimate is 5,000 Btu/h, labelled as the smallest common room-unit capacity. Sizing guideline from ENERGY STAR. Calculator formulas are published with their units so a technician can check a result instead of treating it as a black box.
Last reviewed . Spot an error? Email support@tiermenu.com.
FAQ
What does the room BTU calculator estimate?
It gives a room or window air conditioner capacity estimate from the supplied ENERGY STAR area bands and the listed sun, occupant and kitchen adjustments.
How does sun exposure change the result?
Heavily shaded uses 10% less than the chart value. Very sunny uses 10% more. Occupant and kitchen adjustments are added afterward.
What happens below 100 square feet?
The calculator uses 5,000 Btu/h as the smallest common room-unit capacity, then applies the selected adjustments.
Can I use this for a whole home?
No. Whole homes need a Manual J load calculation with building and local design details.
How is tonnage calculated?
The displayed tons are the adjusted Btu/h result divided by 12,000.
Using a reference result in the field
A calculator is most useful when the inputs match the job being checked. Confirm whether a value is nominal or measured, and whether readings were taken at design or actual operating conditions. The unit label beside the result is part of the result; do not carry a number into another calculation without checking its units.
Reference rows are provided for comparison, not as universal design limits. They use the values and conditions stated with each table. Elevation, air temperature, equipment and installation details can change a field result. Where the page calls out a code check, equipment rating or manufacturer table, that check remains part of the job.
Use the interactive inputs for the exact conditions at hand, then review the displayed formula and assumptions. If a value looks unexpected, change one input at a time and compare it with a known point in the reference table. Keep the measured readings and job-specific notes with the work record so another technician can understand how the result was obtained.
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