TryBuildCalcWindow Air Conditioner Size Calculator (Energy Star Sizing Chart)
Find your window AC size instantly.
🕒 Last updated: September 18, 2026
Inputs
ℹ️Measured floor area of the single, enclosed room this unit will cool — not a multi-room open floor plan.
ℹ️Energy Star's sizing chart assumes an 8 ft ceiling. Taller rooms hold more air and may need a larger unit than shown.
ℹ️Very sunny rooms need +10% capacity; heavily shaded rooms need 10% less, per Energy Star's guidance.
ℹ️Adds 600 BTU for each person beyond a baseline of 2 who regularly uses this room.
Recommended Capacity
8,000 BTU/hr
Match against the unit's nameplate/CEER BTU rating
Room & Conditions
Unit type: Window / Through-the-Wall
Room area: 350 sq ft
Sun exposure: Average / Typical
Kitchen: No
Occupants: 2
Sizing Breakdown
Base capacity (area only): 8,000 BTU/hr
After adjustments: 8,000 BTU/hr
Recommended nominal size: 8,000 BTU/hr
Assumptions Used
Base capacity comes from Energy Star's official room air conditioner sizing chart (area to BTU/hr), which assumes an 8 ft ceiling. That base is then adjusted ±10% for very sunny/heavily shaded exposure, +4,000 BTU flat for kitchen use, and +600 BTU for each occupant beyond a baseline of 2 — all per Energy Star's own published sizing guidance. The adjusted figure is rounded up to the next real, purchasable nominal unit size on that same chart. For a portable unit, this recommended figure should be matched against the model's DOE-mandated SACC (Seasonally Adjusted Cooling Capacity) rating, not its older ASHRAE BTU rating — SACC reflects real delivered cooling after accounting for the exhaust hose's infiltration/heat leakback, while ASHRAE does not. The estimated ASHRAE-label range shown is a rough published estimate (SACC is typically 55-71% of ASHRAE for single-hose units, 68-86% for dual-hose), not a precise conversion — DOE does not publish one fixed ratio, since it varies by model. This is a planning estimate only, not a substitute for a room-by-room Manual J load calculation for unusual rooms (very high ceilings, poor insulation, large window walls, or open floor plans).
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Window/Portable AC BTU Calculator.
Window/wall air conditioner sizing
Window and through-the-wall units use a single BTU/hr rating that reflects real cooling capacity directly — no separate SACC/ASHRAE distinction the way portable units have. Energy Star's official sizing chart maps your room's floor area to a base capacity, then adjusts for sun exposure, kitchen use, and occupancy.
This page defaults to a 350 sq ft room with a window/wall unit — edit the inputs above to match your actual room.
Window/Portable AC Formula: How Is It Determined?
Sizing starts from Energy Star's official area-to-BTU chart, applies three independent adjustments, then rounds up to the next real, purchasable unit size — with an additional SACC/ASHRAE step for portable units only.
Base Capacity from Room Area
Area = Room Length × Room Width, or your entered known area
Base BTU/hr = Energy Star sizing chart lookup for your room's floor area (assumes an 8 ft ceiling)
Energy Star publishes an official chart mapping room floor area directly to a base cooling capacity in BTU/hr, ranging from 5,000 BTU for a 100-150 sq ft room up to 34,000 BTU for a 2,000-2,500 sq ft room. This chart explicitly assumes an 8 ft ceiling — Energy Star gives no official numeric adjustment for taller rooms, only a qualitative note that more capacity may be needed, which this calculator surfaces as a warning rather than an invented multiplier. You can enter a known area directly, or switch Calculation Mode to compute it from the room's length and width instead — both feed the exact same chart lookup.
Sun, Kitchen, and Occupancy Adjustments
Sun adjustment = Base BTU × (+10% very sunny, 0% average, −10% heavily shaded)
Kitchen adjustment = +4,000 BTU flat, if the room is a kitchen
Occupancy adjustment = +600 BTU × occupants beyond a baseline of 2
Adjusted BTU = Base BTU + Sun Adjustment + Kitchen Adjustment + Occupancy Adjustment
These three adjustments come directly from Energy Star's own published sizing guidance, applied on top of the area-based base capacity. Sun exposure and kitchen use both address real, sustained heat gain sources the floor area alone doesn't capture; the occupancy adjustment accounts for body heat from more than the typical 2 people the base chart assumes.
Recommended Nominal Size
Recommended BTU = next available nominal unit size on the sizing chart ≥ Adjusted BTU
The adjusted BTU figure is rounded UP to the next real, purchasable unit size on Energy Star's own chart — never down — since a unit smaller than the actual need runs continuously without ever comfortably reaching the target temperature during peak heat.
Portable Units Only: Estimated ASHRAE-Label Range
Estimated ASHRAE low = Recommended BTU ÷ hose type's maximum SACC/ASHRAE ratio
Estimated ASHRAE high = Recommended BTU ÷ hose type's minimum SACC/ASHRAE ratio
Portable units are dual-rated: SACC (Seasonally Adjusted Cooling Capacity, the DOE-mandated figure reflecting real delivered cooling) and the older ASHRAE BTU figure, which doesn't account for the exhaust hose's infiltration effect and is typically higher. Since SACC is typically 55-71% of ASHRAE for single-hose units and 68-86% for dual-hose units, this calculator shows the estimated ASHRAE-label range you should expect on a unit that actually delivers your recommended real (SACC) capacity — a published estimate range, not a precise per-model conversion, since DOE does not publish one fixed ratio. Always check the specific model's EnergyGuide/DOE label for its actual SACC rating.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Unit type | Window / Through-the-Wall |
| Room area | 350 sq ft |
| Ceiling height | 8 ft |
| Sun exposure | Average / Typical |
| Kitchen / occupants | No / 2 |
Step-by-Step Calculation
| Step | Calculation | Result |
|---|---|---|
| Base capacity (350 sq ft) | Energy Star chart lookup | 8,000 BTU/hr |
| Sun adjustment (Average / Typical) | 8,000 × 0% | +0 BTU/hr |
| Kitchen adjustment | Not a kitchen | +0 BTU/hr |
| Occupancy adjustment | 0 extra × 600 | +0 BTU/hr |
| Adjusted BTU | Sum of the above | 8,000 BTU/hr |
| Recommended nominal size | Round up to next chart size | 8,000 BTU/hr |
Therefore, this 350 sq ft room needs a 8,000 BTU/hr window / through-the-wall air conditioner.
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.