How many BTU do I need to cool a room?

A common rule of thumb is about 20 BTU per hour for every square foot of floor area at a standard 8 ft ceiling. A 4 m by 3.5 m room is roughly 150 sq ft, so it needs in the region of 3,000 BTU/h before adjusting for insulation, sun, windows and occupants. This calculator applies all of those factors for you.

What is the difference between heating and cooling BTU?

Both are measured in BTU per hour, but heating usually needs more capacity because the indoor-to-outdoor temperature gap in winter is larger than in summer. This tool uses a higher base for heating, then adjusts for climate and sun, which provide free heat gain in summer but help your heater in winter.

How do BTU convert to AC tons?

One refrigeration ton equals 12,000 BTU per hour. So a 24,000 BTU/h unit is a 2-ton air conditioner, and an 18,000 BTU/h unit is 1.5 tons. The calculator rounds your cooling load up to the nearest common off-the-shelf size and shows the ton rating.

Why does insulation change the BTU figure?

Poorly insulated rooms leak heat in winter and gain it in summer, so they need more capacity to hold a set temperature. The tool multiplies the base load by about 1.15 for poor insulation, 1.0 for average and 0.85 for good, modern insulation.

Does ceiling height matter for BTU sizing?

Yes. The per-square-foot rule assumes an 8 ft (about 2.4 m) ceiling. A taller ceiling means more air volume to condition, so the calculator scales the load in proportion to the ceiling height you enter.

How is the running cost estimated?

An air conditioner does not draw its full BTU rating as electricity. Modern units have a SEER or COP near 3, so the electrical input is roughly the thermal output divided by 3. The cost equals that input in kW multiplied by your daily run hours and your price per kWh. It is an estimate, not a bill.

BTU Calculator — Gera Tools

A BTU calculator that sizes both the heating and cooling capacity a room needs, recommends a real off-the-shelf air-conditioner size in tons, and estimates the running cost from your electricity price. It is built for anyone shopping for a portable AC, a split system or a space heater who wants a defensible number instead of a guess — homeowners, renters, landlords, and small-office managers alike.

How it works

BTU stands for British Thermal Unit, the amount of energy needed to raise one pound of water by one degree Fahrenheit. HVAC capacity is rated in BTU per hour — how fast a unit can add or remove heat. The starting point is floor area: a widely used rule of thumb is about 20 BTU/h per square foot of cooling at a standard 8 ft ceiling, with heating set higher because winter temperature gaps are larger.

From that base the calculator applies a chain of multipliers. Ceiling height scales the air volume you must condition. Insulation quality raises the load for draughty, single-glazed rooms and lowers it for modern, well-sealed ones. Sun exposure cuts both ways — a south-facing room needs more cooling but less heating, because the sun does some of the heating work for free. A climate selector tilts the balance toward heating in cold regions and toward cooling in hot ones.

On top of those proportional factors come fixed internal gains: roughly 1,000 BTU/h per window, 600 BTU/h for each person beyond the first, and a 4,000 BTU/h allowance if the room is a kitchen, where the oven, hob and fridge dump heat into the space. The cooling total is rounded up to the nearest common appliance size so you can match it to what is actually on the shelf, and the ton rating is shown alongside.

Worked example

Take a 4 m by 3.5 m lounge with a 2.4 m ceiling — about 150 sq ft — with average insulation, average sun, a temperate climate, two windows and two people. The base cooling load is roughly 150 × 20 = 3,000 BTU/h. Windows add about 2,000 and the second person adds 600, giving close to 5,600 BTU/h, which rounds up to a 6,000 BTU/h unit — a 0.5-ton air conditioner. Heating, on its heavier base, lands near 6,500 BTU/h. At a price of 0.28 per kWh running 6 hours a day, the estimated cooling cost is only a few units of currency per day, because the unit draws far less electricity than its thermal rating.

Change one input and watch the chain react: switch the climate to hot summers and the cooling load jumps about 20%, often pushing the recommendation up to the next size.

Reference and formula

The core relationships used here:

Cooling base: area_sqft × 20 × (ceiling_ft ÷ 8), then × insulation × sun × climate
Heating base: area_sqft × 30 × (ceiling_ft ÷ 8), then × insulation × sun × climate
Fixed adders: windows × 1000 + max(0, people − 1) × 600 + (kitchen ? 4000 : 0)
AC tons: cooling_BTU ÷ 12000 · Thermal kW: BTU × 0.293071 ÷ 1000
Running cost: (cooling_kW ÷ COP) × hours_per_day × price_per_kWh, with COP ≈ 3

These are sizing heuristics. A professional Manual J load calculation models exact wall construction, orientation, infiltration and local design temperatures, and is the right tool before a permanent installation. Use this estimate to narrow the shortlist and sanity-check a quote.