BTU calculator
Sizing an air conditioner or heater starts with the load: how much heat, in BTU per hour, the space gains in summer and loses in winter. Buy too small and it never keeps up; buy too big and it short-cycles, cools unevenly and dehumidifies poorly.
This calculator turns a room's floor area, ceiling height, sun exposure, occupants and kitchen use — plus a project-wide climate zone and insulation quality — into a cooling load in BTU/h, the matching AC tonnage and kW, a recommended unit size, and a heating load. Everything runs in your browser.
How many BTU do you need?
A rough starting point for cooling is about 20 BTU/h per square foot of floor area at an 8 ft ceiling, then adjusted for the things that change the real load: taller ceilings add volume, sunny or shaded rooms shift the gain, extra occupants and a kitchen add heat, and insulation quality moves it either way.
This is a rule-of-thumb estimate, not a Manual-J load calculation. A full Manual-J accounts for window area and orientation, air leakage, duct losses and local design temperatures. Treat the number here as a sensible starting point and verify with an HVAC professional before buying equipment.
A rough cooling load at 20 BTU/h per ft², next to the published Energy Star room-AC chart. The chart tapers below the linear formula for larger rooms.
| Floor area | Linear (20 BTU/ft²) | Energy Star chart |
|---|---|---|
| 150 ft² | 3,000 | 5,000 |
| 250 ft² | 5,000 | 6,000 |
| 400 ft² | 8,000 | 9,000 |
| 550 ft² | 11,000 | 12,000 |
| 700 ft² | 14,000 | 14,000 |
| 1,000 ft² | 20,000 | 18,000 |
| 1,500 ft² | 30,000 | 24,000 |
The cooling-load formula
The cooling load is built additively so every factor is a visible part of the total: base area load, plus a ceiling-volume delta above 8 ft, plus sun and insulation percentages on the base, plus flat adders for extra occupants and a kitchen.
Every load here is built from a handful of transparent formulas — no black box.
Cooling load
cooling BTU/h = 20 × area(ft²) + ceiling + sun + insulation + occupants + kitchen- Base is 20 BTU/h per ft² at an 8 ft ceiling.
- Ceiling adds base × (height ÷ 8 − 1) above 8 ft; short ceilings add nothing.
- Sun is base × ±10%; insulation is base × (−10% good … +15% poor).
- Occupants add 600 BTU/h each beyond two; a kitchen adds 4,000 BTU/h.
Tonnage and kW
tons = BTU/h ÷ 12,000 · kW = BTU/h ÷ 3,412- One ton of cooling is 12,000 BTU/h; one kilowatt is 3,412 BTU/h.
- A 24,000 BTU/h load is 2.0 tons or about 7.0 kW.
Heating load
heating BTU/h = rate(zone) × area(ft²) + ceiling + insulation- The rate comes from your IECC climate zone: 32.5 (hot) … 55 (cold) BTU/h per ft².
- The same ceiling and insulation deltas as cooling are applied for consistency.
Furnace input (optional)
furnace input BTU/h = heating output ÷ AFUE- AFUE is the furnace's seasonal efficiency as a fraction (90% = 0.90).
- A 40,000 BTU/h output at 90% AFUE draws about 44,400 BTU/h of gas.
What the live diagrams show
The tool draws two figures for the room you are editing: a stacked contribution bar that splits the load into its factors, and a sizing ladder that snaps the load up to a real unit. Here they are for the sunny-living-room example above.
- base area
- 8,000 BTU/h
- ceiling
- +1,000 BTU/h
- sun
- +800 BTU/h
- insulation
- 0 BTU/h
- occupants
- +600 BTU/h
- kitchen
- 0 BTU/h
- base area
- 22,000 BTU/h
- ceiling
- +2,750 BTU/h
- insulation
- 0 BTU/h
BTU, tonnage and kW
One ton of cooling is 12,000 BTU/h, and one kilowatt is 3,412 BTU/h, so a 24,000 BTU/h load is 2.0 tons or about 7.0 kW. Metric users read kW first; imperial users read BTU/h and tonnage.
| Tons | BTU/h | kW |
|---|---|---|
| 1.0 | 12,000 | 3.52 |
| 1.5 | 18,000 | 5.28 |
| 2.0 | 24,000 | 7.03 |
| 2.5 | 30,000 | 8.79 |
| 3.0 | 36,000 | 10.55 |
| 3.5 | 42,000 | 12.31 |
| 4.0 | 48,000 | 14.07 |
| 5.0 | 60,000 | 17.58 |
Choosing a unit size
Air conditioners come in fixed capacities, so the calculator snaps the load up to the nearest available size on three ladders — window/portable, mini-split and central tonnage — never below the load, because undersizing is the worse failure. It then flags how far above the load that size sits, with a guard tighter than ACCA Manual S.
| Unit type | Available sizes (BTU/h) |
|---|---|
| Window / portable | 5,000 · 6,000 · 8,000 · 10,000 · 12,000 · 14,000 · 18,000 · 25,000 |
| Mini-split | 9,000 · 12,000 · 18,000 · 24,000 · 30,000 · 36,000 |
| Central (tonnage) | 12,000 · 18,000 · 24,000 · 30,000 · 36,000 · 42,000 · 48,000 · 60,000 |
Sizing a heater
Heating is estimated from the climate zone: colder zones lose more heat, so they call for more BTU/ft². Pick your IECC zone and the calculator crosswalks it to a heating band and rate. An optional AFUE step converts the heating output into the gas input a furnace draws.
| Band | IECC zones | BTU/h per ft² (range) | Used |
|---|---|---|---|
| Hot | 1, 2 | 30–35 | 32.5 |
| Warm | 3 | 35–40 | 37.5 |
| Moderate | 4 | 40–45 | 42.5 |
| Cool | 5 | 45–50 | 47.5 |
| Cold | 6, 7, 8 | 50–60 | 55.0 |
The adjustment factors
Sun, ceiling height, occupants, kitchen use, insulation and climate each move the load. Change any one and the matching segment of the contribution bar and the number update immediately, so you can see what drives the size.
| Factor | Effect on the load |
|---|---|
| Base area | 20 BTU/h per ft² of floor at an 8 ft ceiling — the starting point. |
| Ceiling height | Above 8 ft, adds base × (height ÷ 8 − 1) for the extra volume; short ceilings add nothing. |
| Sun exposure | A sunny room adds 10% to the base; a shaded room subtracts 10%. |
| Insulation | Good insulation subtracts 10%; poor insulation adds 15%. Applies to both loads. |
| Occupants | Each person beyond two adds a flat 600 BTU/h of body heat. |
| Kitchen | A kitchen adds a flat 4,000 BTU/h for appliance heat gain. |
| Climate zone | Sets the heating BTU/h per ft²; cooling stays climate-invariant in this tool. |
Worked examples
A few quick estimates at the default constants. Your own numbers will differ once you set the climate zone, insulation and factors to your space.
Two worked estimates at the default constants — a single room, then a whole home. Your own numbers will differ once you set the zone, insulation and factors to your space.
Single room — sunny living room
Cooling- Given
- Floor area 400 ft², ceiling 9 ft
- Sunny exposure, 3 occupants, no kitchen, average insulation
- Formula
20 × 400 + ceiling + sun + occupants- Steps
- Base: 20 × 400 = 8,000 BTU/h
- Ceiling: 8,000 × (9 ÷ 8 − 1) = +1,000
- Sun (sunny +10%): 8,000 × 0.10 = +800
- Occupants: (3 − 2) × 600 = +600
- Total: 8,000 + 1,000 + 800 + 600 = 10,400 BTU/h
- Result
- 10,400 BTU/h ≈ 0.87 ton ≈ 3.0 kW → snaps up to a 12,000 BTU/h mini-split (comfortable)
Whole home — three rooms
Cooling- Given
- Living room 400 ft² (sunny, 3 occupants)
- Bedroom 200 ft² (neutral, 2 occupants)
- Kitchen 150 ft² (kitchen adder)
- Formula
Σ per-room cooling loads- Steps
- Living room: 10,400 BTU/h (from above)
- Bedroom: 20 × 200 = 4,000 BTU/h
- Kitchen: 20 × 150 + 4,000 = 7,000 BTU/h
- Home total: 10,400 + 4,000 + 7,000 = 21,400 BTU/h
- Result
- 21,400 BTU/h ≈ 1.78 ton ≈ 6.3 kW → a 2.0-ton (24,000 BTU/h) central unit
How to size an AC or heater with this calculator
- Choose units and modePick metric or imperial and whether you are sizing cooling, heating or both.
- Add your roomsEnter each room's floor area, ceiling height, sun exposure, occupants and whether it is a kitchen. Add a row per room for a whole-home total.
- Set climate zone and insulationChoose your IECC climate zone and insulation quality — these apply to the whole project and drive the heating rate and both loads.
- Read the load and unit sizeRead the cooling load in BTU/h, tonnage and kW, the recommended unit on each ladder with its oversizing guard, and the heating load.
What a full Manual-J adds
This calculator is a rule-of-thumb estimate. A professional ACCA Manual-J load calculation goes further: it accounts for window area, glazing type and orientation, wall and roof assemblies, measured air leakage and infiltration, duct gains and losses, and your local 99% design temperatures.
Use this number to sanity-check a quote or narrow a unit size, then have an HVAC professional run a Manual-J and Manual-S selection before you buy equipment.
Frequently asked questions
How many BTU do I need per square foot?
As a rule of thumb, about 20 BTU/h per square foot for cooling at an 8 ft ceiling, before adjustments. Sun, ceiling height, occupants, a kitchen and insulation all move it, which is why this calculator adjusts the base rate rather than using a flat number. It is guidance, not a Manual-J calculation.
What size AC (tonnage) do I need?
Divide the cooling load by 12,000 to get tons — a 24,000 BTU/h load is 2.0 tons. The calculator snaps that up to the nearest real unit on the window, mini-split and central ladders and warns when a size is far above the load, since an oversized unit short-cycles and dehumidifies poorly.
Is this a Manual-J load calculation?
No. This is a rule-of-thumb estimate for a starting point. A full ACCA Manual-J accounts for window area and orientation, air leakage, duct losses and local design temperatures. Verify with an HVAC professional before purchasing equipment.
Does it work in metric?
Yes. Switch units at the top and the tool reads kW first, with BTU/h and tonnage alongside, and accepts floor area in square metres and ceiling height in metres. The physics is the same either way.