# 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.

- Calculator id: `hvac-btu-sizing` · Category: Home Energy & Electrical (`energy`) · Tool name: `estimate_hvac_btu_sizing`
- Canonical page: https://tttkmbb.com/energy/hvac-btu-sizing · This document: https://tttkmbb.com/energy/hvac-btu-sizing.md · JSON definition: https://tttkmbb.com/energy/hvac-btu-sizing.json

## Purpose

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.

**Use when:** 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).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `floor_area` | number |  | required | Floor area of the room or zone, in the unit given by area_unit. (> 0, max 100000) |
| `area_unit` | enum: ft2 \| m2 |  | optional, default "ft2" | Unit of floor_area (1 m² = 10.76391 ft²). |
| `ceiling_height_ft` | number | ft | optional, default 8 | Ceiling height in feet; the base rates assume 8 ft and scale with ceiling_height_ft / 8. (min 6, max 30) |
| `climate` | enum: cool \| moderate \| warm \| hot |  | optional, 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 |  | optional, default "average" | Sunlight on the room: shaded −10 %, sunny +10 % cooling load (ENERGY STAR). |
| `occupants` | integer |  | optional, default 2 | People regularly in the room; each person above 2 adds 600 BTU/h of cooling load. (min 0, max 500) |
| `kitchen` | boolean |  | optional, 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 |  | optional, default "average" | Quality of walls, roof and windows; multiplies both cooling and heating loads. |

## Output

| Field | 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. |

## 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.

## Data Sources

- ENERGY STAR – Room Air Conditioners (sizing chart and adjustments) — https://www.energystar.gov/products/room_air_conditioners (government, retrieved 2026-09-24)
- ASHRAE Handbook – Fundamentals — https://www.ashrae.org/technical-resources/ashrae-handbook (standard, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/hvac-btu-sizing?floor_area=…`
- `POST https://tttkmbb.com/api/v1/calculate/hvac-btu-sizing` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/hvac-btu-sizing · OpenAPI operationId `estimate_hvac_btu_sizing` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "hvac-btu-sizing", "inputs": {…}}`

## Example

- 400 ft² room, moderate climate, defaults: inputs `{"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: inputs `{"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
```

## Limitations

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). 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. All values are computed from the formula above; no measurement or live data is involved.

## 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

- [Heat Loss Calculator](https://tttkmbb.com/energy/heat-loss.md) — Heat flow through one wall, roof or window from its U-value.
- [Air Changes per Hour Calculator](https://tttkmbb.com/energy/air-changes-per-hour.md) — Ventilation rate of the same room.
- [Electricity Cost Calculator](https://tttkmbb.com/everyday/electricity-cost.md) — Running cost of the air conditioner from its wattage and hours.