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HVAC BTU Sizing Calculator
Estimates the cooling capacity (BTU/h, tons, kW) and heating output a room needs from floor area, ceiling height, climate, sun exposure, insulation, occupants and kitchen heat, using the 20 BTU/h per ft² rule of thumb with the ENERGY STAR room air conditioner adjustments.
When to use
You need a quick size for a window, portable or split air conditioner or a room heater before shopping, or want to sanity-check a quoted unit size.
Do not use when: You need a code-grade load calculation for a whole house (ACCA Manual J / ASHRAE methods account for windows, orientation and infiltration), or the heat flow through one wall (use heat-loss).
Formula
area_ft2 = floor_area (× 10.76391 if m²); cooling_btu_per_hour = area_ft2 × 20 × (ceiling_height_ft / 8) × climate_factor × sun_factor × insulation_factor + 600 × max(0, occupants − 2) + (kitchen ? 4000 : 0); heating_btu_per_hour = area_ft2 × heating_rate(climate) × (ceiling_height_ft / 8) × insulation_factor; cooling_tons = BTU/h / 12000; kW = BTU/h × 0.29307 / 1000
Rule-of-thumb sizing (20 BTU/h per ft² base cooling load) with the ENERGY STAR adjustments: heavily shaded −10 %, very sunny +10 %, +600 BTU/h per person above two, +4,000 BTU/h for kitchens; heating uses 30–60 BTU/h per ft² depending on climate. Oversized air conditioners short-cycle and dehumidify poorly, so do not round up beyond the next available size.
Inputs
| Parameter | Type | Unit | Required | Description |
|---|---|---|---|---|
floor_area | number | yes | Floor area of the room or zone, in the unit given by area_unit. Range: > 0, ≤ 100000 | |
area_unit | enum: ft2 | m2 | default ft2 | Unit of floor_area (1 m² = 10.76391 ft²). | |
ceiling_height_ft | number | ft | default 8 | Ceiling height in feet; the base rates assume 8 ft and scale with ceiling_height_ft / 8. Range: ≥ 6, ≤ 30 |
climate | enum: cool | moderate | warm | hot | default moderate | Local climate: multiplies the cooling load (0.9 / 1.0 / 1.1 / 1.2) and sets the heating rate (60 / 45 / 35 / 30 BTU/h per ft²). | |
sun_exposure | enum: shaded | average | sunny | default average | Sunlight on the room: shaded −10 %, sunny +10 % cooling load (ENERGY STAR). | |
occupants | integer | default 2 | People regularly in the room; each person above 2 adds 600 BTU/h of cooling load. Range: ≥ 0, ≤ 500 | |
kitchen | boolean | default false | true when the room is a kitchen (adds 4,000 BTU/h of cooling load for cooking appliances). | |
insulation | enum: poor | average | good | default average | Quality of walls, roof and windows; multiplies both cooling and heating loads. |
Outputs
| Output | Type | Unit | Description |
|---|---|---|---|
cooling_btu_per_hour | number | BTU/h | Estimated cooling load: 20 BTU/h per ft² adjusted for ceiling height, climate, sun, insulation, occupants and kitchen. |
cooling_tons | number | ton | Cooling load in tons of refrigeration (1 ton = 12,000 BTU/h). |
cooling_kw | number | kW | Cooling load in kilowatts (1 BTU/h = 0.29307 W). |
heating_btu_per_hour | number | BTU/h | Estimated heating load: climate heating rate × area, adjusted for ceiling height and insulation. |
heating_kw | number | kW | Heating load in kilowatts. |
floor_area_ft2 | number | ft² | Floor area used, in square feet. |
recommended_unit_size_text | string | Cooling capacity rounded up to the next 1,000 BTU/h (window units are sold in such steps), heating rounded up likewise, with the assumptions stated. |
Example
400 ft² room, moderate climate, defaults: {"floor_area":400,"area_unit":"ft2","ceiling_height_ft":8,"climate":"moderate","sun_exposure":"average","occupants":2,"kitchen":false,"insulation":"average"} → {"cooling_btu_per_hour":8000,"cooling_tons":0.67,"cooling_kw":2.34,"heating_btu_per_hour":18000,"heating_kw":5.28}
30 m² sunny kitchen, hot climate, 9 ft ceiling, 4 people, poor insulation: {"floor_area":30,"area_unit":"m2","ceiling_height_ft":9,"climate":"hot","sun_exposure":"sunny","occupants":4,"kitchen":true,"insulation":"poor"} → {"floor_area_ft2":322.9,"cooling_btu_per_hour":16229,"cooling_tons":1.35,"cooling_kw":4.76,"heating_btu_per_hour":12533}
GET https://tttkmbb.com/api/v1/calculate/hvac-btu-sizing?floor_area=400&area_unit=ft2&ceiling_height_ft=8&climate=moderate&sun_exposure=average&occupants=2&kitchen=false&insulation=average
Machine access
- API:
GET https://tttkmbb.com/api/v1/calculate/hvac-btu-sizing(query parameters) orPOSTwith a JSON body{"inputs": {...}} - Schema: https://tttkmbb.com/api/v1/calculators/hvac-btu-sizing · Markdown: https://tttkmbb.com/energy/hvac-btu-sizing.md · JSON definition: https://tttkmbb.com/energy/hvac-btu-sizing.json
- MCP: server
https://tttkmbb.com/mcp, toolrun_calculator with calculator_id="hvac-btu-sizing" - OpenAPI operationId:
estimate_hvac_btu_sizing - Freshness:
static. Authentication: none. Rate limit: fair use (see rate limits).
Sources
- ENERGY STAR – Room Air Conditioners (sizing chart and adjustments) (government)
- ASHRAE Handbook – Fundamentals (standard)
FAQ
Why does ENERGY STAR's chart give a different number for my area?
The chart lists capacities in 1,000 BTU/h steps for area bands (e.g. 350–400 ft² → 9,000 BTU/h) and averages to roughly 20–25 BTU/h per ft²; this calculator uses the continuous 20 BTU/h per ft² rule with the same shade, sun, occupant and kitchen adjustments.
Is the heating figure the furnace or heat-pump size?
It is the required heat output at the design temperature. A heat pump's rated heating capacity falls at low outdoor temperatures, so compare with its capacity at your design temperature, not the nameplate.
What are BTU/h, tons and kW?
All three are rates of heat flow: 12,000 BTU/h = 1 ton of refrigeration = 3.517 kW. Air conditioner electrical input is lower than the cooling output (divide by the EER or COP).
Related calculators
- Heat Loss Calculator — Heat flow through one wall, roof or window from its U-value.
- Air Changes per Hour Calculator — Ventilation rate of the same room.
- Electricity Cost Calculator — Running cost of the air conditioner from its wattage and hours.