TryBuildCalcWindow/Portable AC BTU Calculator (Room Air Conditioner Sizing)
Size your window or portable air conditioner 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
7,000 BTU/hr
Match against the unit's nameplate/CEER BTU rating
Room & Conditions
Unit type: Window / Through-the-Wall
Room area: 300 sq ft
Sun exposure: Average / Typical
Kitchen: No
Occupants: 2
Sizing Breakdown
Base capacity (area only): 7,000 BTU/hr
After adjustments: 7,000 BTU/hr
Recommended nominal size: 7,000 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).
What Is a Window/Portable AC Calculator?
A window/portable AC calculator sizes a standalone room air conditioner — a window, through-the-wall, or portable unit — using Energy Star's official area-to-BTU sizing chart, then adjusts for sun exposure, kitchen use, and occupancy. Unlike whole-house central AC or ductless mini-split sizing (which model an entire home's or multi-room zone's heat gain), this calculator is built specifically for a single, enclosed room served by one self-contained unit.
This calculator is built for homeowners and renters shopping for a room air conditioner. It applies Energy Star's published sizing chart and adjustment factors — it does not replace a full ACCA Manual J load calculation for unusual rooms (very high ceilings, poor insulation, large glazed window walls, or open floor plans), which a licensed HVAC contractor can perform for a precise result.
Why window/wall and portable units get different guidance here:
- A window or wall unit has a single BTU/hr rating that reflects real delivered cooling reasonably directly — match this calculator's recommendation straight against the unit's nameplate rating
- A portable unit is dual-rated: an older ASHRAE BTU figure often used in marketing, and a DOE-mandated SACC (Seasonally Adjusted Cooling Capacity) rating that reflects real delivered cooling after accounting for the exhaust hose's infiltration effect — this calculator's recommendation should be matched against SACC, and shows an estimated ASHRAE-label range so you aren't misled by the larger marketed number
- Both unit types share the same underlying room sizing math (Energy Star's chart plus sun/kitchen/occupancy adjustments) — only the final matching step against the shopping specification differs
- Large units of either type commonly need a dedicated 230V circuit — this calculator flags that but doesn't size the circuit itself; use the MCA & MOCP Calculator for that separate step
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 | 300 sq ft |
| Ceiling height | 8 ft |
| Sun exposure | Average / Typical |
| Kitchen / occupants | No / 2 |
Step-by-Step Calculation
| Step | Calculation | Result |
|---|---|---|
| Base capacity (300 sq ft) | Energy Star chart lookup | 7,000 BTU/hr |
| Sun adjustment (Average / Typical) | 7,000 × 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 | 7,000 BTU/hr |
| Recommended nominal size | Round up to next chart size | 7,000 BTU/hr |
Therefore, this 300 sq ft room needs a 7,000 BTU/hr window / through-the-wall air conditioner.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Inputs & Sizing Method+-
- Room floor area measured directly (length × width), not estimated or guessed
- Confirmed the space is a single, enclosed room rather than part of an open floor plan or multi-room area
✓Unit Type & Rating Selection+-
- Unit type (window/wall vs. portable) matches the room's actual physical constraints, not just personal preference
- For a portable unit, confirmed checking the model's SACC (Seasonally Adjusted Cooling Capacity) rating on the EnergyGuide/DOE label, not just its marketed ASHRAE BTU number
- Selected unit's nameplate voltage and amperage checked against the outlet/circuit actually available in the room
✓Installation Planning+-
- For a window unit, window sill/frame confirmed able to support the unit's weight, with security brackets or a support bracket installed if required
✓After Installation+-
- Circuit breaker confirmed not tripping under normal startup and running load, especially on shared circuits with other appliances
Full QC Checklist+−
Verification checklist for a window/portable air conditioner BTU sizing estimate — covering input/sizing method confirmation, unit type and rating selection, installation planning, and post-installation verification. Use the Essential Checklist for critical checks before finalizing, expand to Full QC Checklist for complete verification.
✓Inputs & Sizing Method+-
- Room floor area measured directly (length × width), not estimated or guessed
- Confirmed the space is a single, enclosed room rather than part of an open floor plan or multi-room area
- Ceiling height confirmed and, if taller than 8 ft, treated as a reason to size up rather than ignored
- Sun exposure selected honestly for the room's actual orientation and window area, not left at the default
- Kitchen adjustment applied if the room includes a range, oven, or other major heat-generating appliances
- Occupant count reflects how many people regularly use the room, not just the default of 2
✓Unit Type & Rating Selection+-
- Unit type (window/wall vs. portable) matches the room's actual physical constraints, not just personal preference
- For a portable unit, confirmed checking the model's SACC (Seasonally Adjusted Cooling Capacity) rating on the EnergyGuide/DOE label, not just its marketed ASHRAE BTU number
- Single-hose vs. dual-hose portable design considered, understanding dual-hose typically delivers a higher share of its ASHRAE rating as real SACC cooling
- Selected unit's nameplate voltage and amperage checked against the outlet/circuit actually available in the room
- If the recommended size is large (commonly 18,000 BTU and up), circuit/breaker sizing cross-checked with the MCA & MOCP Calculator against the specific unit's actual nameplate MCA/MOCP
✓Installation Planning+-
- For a window unit, window sill/frame confirmed able to support the unit's weight, with security brackets or a support bracket installed if required
- Window/wall unit installation planned with a slight outward tilt (typically 1/4 to 1/2 in low on the outside) so condensate drains away from the building rather than into the room
- For a portable unit, exhaust hose routed as short and straight as possible to the window/wall port, avoiding sharp bends or excess coiled length
- Window/slider kit (for a portable unit) or surrounding window frame gaps (for a window unit) sealed with the included foam/weatherstripping
- Manufacturer-specified clearance around the unit (front intake, side/rear exhaust for window units; front and rear for portable units) confirmed clear of curtains, furniture, or other obstructions
- In a humid climate, condensate handling plan confirmed for a portable unit — many self-evaporate under moderate humidity but may need a gravity drain hose or internal reservoir emptied under high humidity
✓After Installation+-
- Cooling performance observed over the first full day of operation, confirming the room reaches and holds its target temperature during peak heat
- Circuit breaker confirmed not tripping under normal startup and running load, especially on shared circuits with other appliances
- No unusual rattling, vibration, or excessive noise checked for at both the unit itself and the window frame/wall sleeve
- Condensate draining outward (window/wall units) or evaporating/collecting as expected (portable units), with no dripping or pooling inside the room
- For a portable unit, exhaust hose and window kit connection re-checked as secure after the first few days of use, not just at initial installation
Window/Portable AC Reference Tables
Energy Star's official room air conditioner sizing chart, plus the hose-type SACC/ASHRAE ratio ranges used for portable units.
| Room Area | Base Capacity |
|---|---|
| 100 to 150 sq ft | 5,000 BTU/hr |
| 150 to 250 sq ft | 6,000 BTU/hr |
| 250 to 300 sq ft | 7,000 BTU/hr |
| 300 to 350 sq ft | 8,000 BTU/hr |
| 350 to 400 sq ft | 9,000 BTU/hr |
| 400 to 450 sq ft | 10,000 BTU/hr |
| 450 to 550 sq ft | 12,000 BTU/hr |
| 550 to 700 sq ft | 14,000 BTU/hr |
| 700 to 1000 sq ft | 18,000 BTU/hr |
| 1000 to 1200 sq ft | 21,000 BTU/hr |
| 1200 to 1400 sq ft | 23,000 BTU/hr |
| 1400 to 1500 sq ft | 24,000 BTU/hr |
| 1500 to 2000 sq ft | 30,000 BTU/hr |
| 2000 to 2500 sq ft | 34,000 BTU/hr |
Adjustment Factors
| Condition | Adjustment |
|---|---|
| Very sunny room | +10% of base capacity |
| Heavily shaded room | −10% of base capacity |
| Kitchen | +4,000 BTU flat |
| Each occupant beyond 2 | +600 BTU |
Portable AC: Estimated SACC/ASHRAE Ratio by Hose Type
Published estimate ranges only — DOE does not publish one fixed ratio, since it varies by specific model. Always check the EnergyGuide/DOE label for the actual SACC rating.
| Hose Type | SACC as % of ASHRAE |
|---|---|
| Single-Hose | 55-71% |
| Dual-Hose | 68-86% |
When should you use this window/portable AC calculator?
- Shopping for a window, through-the-wall, or portable air conditioner for a single bedroom, living room, home office, or similar enclosed room.
- Comparing a portable unit's marketed ASHRAE BTU number against what it actually needs to deliver as SACC for your room.
- Deciding between a single-hose and dual-hose portable unit and understanding how that choice affects the ASHRAE-label figure you should shop for.
- Checking whether a room's sun exposure, kitchen use, or higher-than-typical occupancy calls for more capacity than the bare floor area alone would suggest.
- Flagging early whether a larger unit is likely to need a dedicated 230V circuit, before finalizing a purchase.
- Comparing two scenarios side by side (via Compare mode) — e.g. window vs. portable, or single-hose vs. dual-hose, for the same room.
Quick Sizing Tips
- For a portable unit, check the EnergyGuide/DOE label for the model's actual SACC rating rather than trusting the larger, marketed ASHRAE BTU figure alone — SACC is the number that reflects what you'll actually feel in the room.
- Round up to the next available nominal size, never down — a unit smaller than the recommendation runs continuously without properly reaching your target temperature during peak heat.
- Resist the urge to size up "just in case" beyond this calculator's recommendation — an oversized unit short-cycles, cooling the air quickly but shutting off before it properly removes humidity, leaving the room feeling cold and clammy.
- If your ceiling is noticeably taller than 8 ft, treat this calculator's recommendation as a floor and consider the next size up, since Energy Star's chart assumes a standard 8 ft ceiling.
- Prefer a dual-hose portable unit where the price and size difference is acceptable — it typically delivers more of its rated capacity as real SACC cooling than an equivalent single-hose model.
- Check your lease or building rules before installing a window unit — many buildings require approved support brackets or restrict window units entirely.
- For any unit in the upper size range, check its nameplate voltage/amperage before buying — some require a dedicated 230V circuit that a standard outlet can't provide.
Common Mistakes
- Shopping for a portable AC by its marketed ASHRAE BTU number instead of its DOE-mandated SACC rating — the ASHRAE figure can overstate real cooling capacity by 15-45% depending on the model and hose type.
- Ignoring the sun exposure or kitchen adjustments and sizing purely off floor area — a sunny room or an active kitchen genuinely needs more capacity than the bare area suggests.
- Using a single room unit to try to cool an open floor plan spanning multiple connected spaces — Energy Star's chart (and this calculator) is built for a single, enclosed room.
- Buying several sizes larger than the recommendation for "extra cooling power" — oversizing causes short-cycling, which cools quickly but leaves humidity poorly removed.
- Installing a window unit without a support bracket or a sill rated for its weight, risking the unit falling.
- Coiling or kinking a portable unit's exhaust hose to make it fit, which increases resistance and reduces real cooling performance below even its rated SACC figure.
- Ignoring nameplate voltage/amperage on a large unit and discovering only after purchase that it needs an electrical upgrade the room doesn't have.
- Assuming a taller-than-8-ft ceiling doesn't matter, when Energy Star's own chart is explicitly built around that baseline.
Limitations
- This is a planning estimate based on Energy Star's published sizing chart and adjustment factors, not a substitute for a full ACCA Manual J room-by-room load calculation, which a licensed HVAC contractor can perform for unusual rooms.
- Built for a single, enclosed room only — does not model a multi-room open floor plan, which has more effective air volume and heat gain than its combined floor area alone suggests.
- Energy Star's chart assumes an 8 ft ceiling and gives no official numeric adjustment for taller rooms — this calculator flags the condition qualitatively rather than inventing an unverified per-foot multiplier.
- The estimated ASHRAE-label range for portable units is a published estimate range (55-71% single-hose, 68-86% dual-hose), not a precise per-model conversion — DOE does not publish one fixed ratio, since it varies by specific model. Always check the EnergyGuide/DOE label for the model's actual SACC rating.
- Does not size the electrical circuit a large unit needs — use the MCA & MOCP Calculator with the specific model's actual nameplate MCA/MOCP for that separate calculation.
- Does not account for unusually poor insulation, large glazed window walls, high local humidity beyond typical assumptions, or other above-average heat-gain sources beyond the sun/kitchen/occupancy adjustments modeled here.
- Beyond the published chart's 2,500 sq ft / 34,000 BTU ceiling, a single room unit is no longer a practical solution — consider a central or ductless multi-zone system instead.
- Does not verify local building/lease restrictions on window unit installation or replace inspection by a qualified installer.
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.