AC Size for a 300 Sq Ft Master Bedroom
Worked AC sizing for a 300 square foot space — recommended tonnage, equipment class, and the full BTU calculation.
Reviewed May 22, 2026
Recommended equipment
0.5 tons(7,000 BTU/hr)
Window AC unit (smallest standard equipment class)
Acceptable range: 5,940–7,920 BTU/hr
Your home or room
Enter the space characteristics, then click Calculate to see the recommended AC size, the Manual S tolerance band, sensible vs latent cooling for your climate, equipment recommendations, operating cost by efficiency tier, and the math step-by-step.
+600 BTU per person above 2
Recommended AC size
0.5
tons
(7,000 BTU/hr)
0.5 tons is the cooling capacity an AC must deliver at the local design condition — about 95°F outdoor for most of the US, the temperature exceeded only 1% of typical-year hours per ASHRAE Handbook of Fundamentals 2021. At any milder condition the AC modulates down (variable-speed) or cycles off (single-stage); at the design hour it should hit setpoint with the airflow you sized for, not run continuously.
Equipment class
Window air conditioner
Manual S range
6,300–8,400
BTU/hr (−10% to +20% of Manual J load)
Estimated SHR
0.78
Sensible heat ratio for zone 4
Raw calc
6,600
BTU/hr before rounding to standard size
Manual S tolerance band
ACCA Manual S allows the installed AC's nominal cooling capacity to exceed the Manual J cooling load by up to 15% for single-stage equipment and up to 25% for two-stage or variable-speed equipment. The band below shows the range of valid equipment sizes relative to your calculated load.
Window air conditioner — what fits at 0.5 tons
Single-room cooling at the lowest equipment cost
| Best fit | A bedroom or single small living space with a working double-hung window |
| Alternative | Rental units where structural changes are not permitted |
| Install cost | $200–$500 retail, DIY install |
| Efficiency tier | CEER 11–13 typical (12 = ENERGY STAR threshold for most sizes) |
| Skip this class when | You need multi-room cooling, the window cannot support the unit weight, or the unit would obstruct egress. |
Sensible vs latent cooling at your climate
Cooling work splits into two categories: sensible cooling lowers temperature, latent cooling removes water vapor. In zone 4, the load SHR is typically around 0.78 — meaning roughly 78% of the cooling work is sensible and 22% is latent (dehumidification). Oversized AC in humid climates cools to setpoint quickly without removing enough moisture, leaving the house "cool but sticky" at 65%+ relative humidity.
| Component | BTU/hr | Share | Purpose |
|---|---|---|---|
| Sensible cooling | 5,460 | 78% | Drops air dry-bulb temperature |
| Latent cooling | 1,540 | 22% | Condenses water vapor out of indoor air |
| Total recommended | 7,000 | 100% | At AHRI 95°F outdoor / 80°F indoor / 67°F WB |
SHR by climate is an estimate; the actual equipment SHR at your indoor design condition comes from the manufacturer's expanded performance data. Manual S equipment selection compares both the AHRI total capacity AND the SHR-adjusted sensible capacity against the load components — see /manual-s/ for the methodology.
Estimated annual operating cost
Operating cost depends on cooling-season hours (about 1200 hours of equivalent full-load operation per year in zone 4), the equipment's seasonal efficiency (SEER2), and the local electricity rate. The table below shows annual cost at five common efficiency tiers using the US average residential electricity rate of $0.163/kWh.
| Efficiency tier (SEER2) | Annual kWh | Annual cost | Description |
|---|---|---|---|
| 13.4 | 470 | $77 | Federal minimum (north) |
| 14.3 | 441 | $72 | Federal minimum (south) |
| 15.2 | 414 | $67 | ENERGY STAR |
| 18.0 | 350 | $57 | Higher-tier variable-speed |
| 22.0 | 286 | $47 | Premium variable-speed |
Estimates assume 75% average load factor across the cooling season. Local electricity rates vary significantly: Pacific Northwest averages $0.10/kWh while California averages $0.30/kWh — multiply costs in the table by (your-rate / 0.163) for a regional estimate.
How the capacity was computed
The calculator multiplies a baseline (22 BTU per sq ft at zone 4, average insulation, 8-ft ceilings) by climate, ceiling, sun, insulation, and space-type factors, then adds occupancy and kitchen adjustments. Each step is shown below.
| Baseline | 300 sqft × 22 | = 6,600 BTU |
| × Climate (zone 4) | × 1 | |
| × Ceiling (8 ft) | × 1 | |
| × Sun (mixed) | × 1 | |
| × Insulation (average) | × 1 | |
| × Space type (bedroom) | × 1 | |
| Subtotal | = 6,600 BTU | |
| Raw calculation, rounded to standard size | 6,600 → | 7,000 BTU/hr |
What this calculator does NOT capture
- Blower-door measured air leakage. The insulation input lumps insulation and infiltration. A 4 ACH50 house performs noticeably differently than a 12 ACH50 house at the same nominal R-values.
- Window orientation and SHGC. Sun exposure is a coarse input. A wall of single-pane south-facing glass produces 3-4× more solar gain than the same area of triple-pane north glass.
- Duct losses to unconditioned space. Central AC with leaky attic ducts loses 20-30% of supply air. The output here is room load; duct losses sit on top of that for whole-house sizing.
- Permit-grade Manual J requirements. Permit applications, HEEHRA rebate documentation, and many state energy programs require ACCA-approved software output. This calculator is planning-grade — appropriate for evaluating a contractor's tonnage proposal, not for replacing the contractor's Manual J.
What this calculation is
A 300 square foot master bedroom is at the upper edge of single window AC capacity. A 8,000 BTU window unit works, but a 9,000 BTU mini-split delivers quieter operation, better humidity control, and inverter efficiency — worth the extra cost on a unit you sleep next to nightly.
How this calculation was reached
The calculator starts with a baseline of 22 BTU per square foot, applies multiplicative adjustments for climate, ceiling, sun, insulation, and space type, then adds fixed amounts for extra occupants and kitchen heat gain. Final result rounds to the nearest standard AC equipment size.
- Baseline: 300 sqft × 22 BTU/sqft = 6,600 BTU
- × Climate factor (zone 4): 1
- × Ceiling factor (8 ft): 1
- × Sun factor: 1
- × Insulation factor: 1
- × Space-type factor: 1
- = Subtotal: 6,600 BTU
- = Final raw: 6,600 BTU
- Rounded to nearest standard size: 7,000 BTU (≈ 0.5 tons)
Right-sizing matters
An AC unit sized at the recommended capacity runs efficiently and controls humidity. An oversized AC reaches setpoint too fast, short-cycles, and leaves the air clammy. An undersized unit runs continuously and never quite cools. For deeper discussion, see the AC short cycling article. Variable-speed (inverter) equipment tolerates moderate oversizing better than single-stage.
Adjust the inputs
The calculator above is interactive. Change any input — square footage, climate zone, ceiling, insulation, sun, occupants, space type — and the result updates live. Reset to defaults restores the values for this example.
Methodology
This calculation follows the ENERGY STAR room AC sizing guide and Manual J 8th Edition methodology, simplified for whole-room or whole-house cooling estimates. Full reference in the AC BTU chart article. For permit-grade central AC sizing on new construction, full Manual J with room-by-room load distribution is the correct tool — see the Manual J methodology article.
Try other AC sizing examples
Compare to nearby sizes or different scenarios.
- AC Size for a 200 Sq Ft Bedroom
0.5 tons (5,000 BTU)
- AC Size for a 100 Sq Ft Bedroom
0.5 tons (5,000 BTU)
- AC Size for a 400 Sq Ft Living Room
0.75 tons (10,000 BTU)
- AC Size for a 200 Sq Ft Kitchen
0.75 tons (10,000 BTU)
- AC Size for a 500 Sq Ft Studio
1 ton (12,000 BTU)
Reviewed May 22, 2026