BTU (AC) Calculator
Calculate the ideal BTU/h power for a room — based on area, people, electronics and sun exposure. Everything in your browser.
Sizing BTU/h for residential air conditioning
The residential rule of thumb runs 600 BTU/h per m², plus 600 per extra person beyond the 2nd, plus 600 per active electronic appliance in the room (TV, computer, fridge). Take a 20 m² living room with 4 people and 1 TV: 20·600 + 2·600 + 1·600 = 12,000 + 1,200 + 600 = 13,800 BTU/h, which rounds up to a commercial 12,000 BTU/h unit (or 18,000 if you want margin). Bump the figure up when you have high ceilings (> 2.8 m), west-facing sun exposure, or poor insulation. The standard capacities sold in Brazil are 7,500 / 9,000 / 12,000 / 18,000 / 24,000 / 30,000 BTU/h.
Applications and standards
People use this to spec units at Casas Bahia, Magazine Luiza or Fast Shop, and to pre-size a residential install. There are three types worth knowing: split is efficient and quiet, window is simple and cheap, and portable is a last resort with lower efficiency. Efficiency itself shows up as EER/COP, and inverter compressors draw less energy. Check for the Procel seal (INMETRO), where class A is the most efficient. An undersized unit runs nonstop, while an oversized one cycles too fast and barely dehumidifies.
How many BTUs for your room
An air conditioner that's too weak won't cool the room. One that's too strong burns energy for nothing and dehumidifies poorly on top of that. The balance lies in getting the BTU/h power right, and that's what this calculator estimates for your room, based on a few details you provide.
The math doesn't stop at floor area. It weighs how many people usually stay in the space, the electronics heating the place up, and sun exposure. That last detail shifts the result a lot: picture a bedroom always in the shade versus a living room that catches sun all afternoon. Add it all up and the BTU estimate reflects what the room needs far better.
The calculation happens in your own browser. Take it as a practical reference when picking a unit, before you pay for power that's more or less than you need.
Frequently asked questions
Is it better to oversize for safety?
Does ceiling height matter?
Split vs window — which to choose?
Related Tools
Air Conditioner BTU Calculator by Room Area
Sizes cooling at 600 BTU/h per square meter plus 600 per extra person and per appliance: 15 m² with 2 people and 2 devices asks for 10,800.
Axle Load Equivalency Factor
Computes how many passes of the standard axle are equivalent to one pass of the real axle, using the power law of pavement design: factor = (axle load ÷ standard axle load) raised to the damage exponent. This factor is what converts a traffic count into the number N of standard axle repetitions, which in Brazil is the 8.2 tf, or 80 kN, single axle with dual wheels. The exponent amplifies overload brutally: an axle 20% heavier than the standard does not consume 20% more pavement but 2.07 times as much, which is why a single overloaded truck weighs more on the life of the road than thousands of cars, whose factor is practically zero. The exponent is an input rather than fixed at 4, the AASHTO value known as the fourth power law, because rigid pavement and fatigue cracking models work with exponents between 3 and 5 and the result shifts by a whole level depending on the choice. Enter the axle load, the standard axle load and the damage exponent.
Wave Power
Estimate the power flux of an ocean wave per meter of wave front, P ≈ 0.5 × H² × T, from the wave height H (m) and the period T (s), in deep water (seawater). The result, in kW/m, is the power available per meter of wave front width — a key indicator of wave energy potential for converters (WECs). Coasts exposed to ocean swells (European Atlantic, Pacific) reach 30-70 kW/m, a significant renewable resource. The power grows with the square of the height and linearly with the period. Enter the wave height and period.
Propeller (Propulsive) Efficiency
Compute a propeller's propulsive efficiency, η = (T·V/P)·100%, the ratio of useful propulsion power (thrust × speed) to the power delivered to the shaft. It measures how much of the engine power the propeller converts into forward thrust — well-designed propellers reach 80–88% in cruise. It drops sharply at low speed (takeoff) and near the speed of sound at the blade tips. Enter the thrust, the speed and the shaft power.
Inductor Current Ripple
Calculate the inductor current ripple of a buck converter, ΔI_L = (V_in × D) ÷ (L × f), from the input voltage V_in, the duty cycle D, the inductance L and the switching frequency f. The result, in amperes, is the peak-to-peak variation of the inductor current each cycle. It is a central design parameter: a typical ripple of 20-40% of the average current is a good compromise. Higher inductance and frequency reduce the ripple (larger inductor, more costly; higher frequency, more switching losses). It also sets the boundary between continuous and discontinuous conduction. Enter the voltage, duty cycle, inductance and frequency.
Square-Wave RMS Current
Calculate the RMS value of a pulsing square-wave current, I_rms = I_p × √D, from the peak current I_p and the duty cycle D (fraction of the period the current flows). The result, in amperes, is the RMS current that determines the actual heating (I²R losses) of a component that conducts in pulses — such as a transistor or winding in a switching converter. Unlike the average value, the RMS is what matters for sizing conductors, resistances and dissipation. The smaller the duty cycle, the lower the RMS for the same peak current. Enter the peak current and the duty cycle.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.