Room AC BTU Calculator

Use this room AC BTU calculator to estimate the right size for a window, portable, through-wall, or single-room air conditioner. Account for ceiling height, sun, kitchens, people, gaming PCs, humidity risk, and the AC size you are thinking about buying.

Reviewed and updated July 8, 2026

Use it when you want to know

Pick a practical AC sizeStart from room dimensions or known square footage, then round to common BTU sizes.
Check if a unit is too bigCompare a 5k, 6k, 8k, 10k, or 12k unit against your room before you buy.
Avoid common comfort mistakesSee notes for portable AC ratings, high ceilings, short cycling, humidity, and airflow.

Calculator

Find the right room AC size

Use the room AC BTU calculator to start with the room size, then adjust for the real stuff people forget: ceiling height, sun, kitchens, people, electronics, portable AC ratings, and whether the unit you found is likely too small or oversized.

How do you want to enter the room?

Room dimensionsKnown areaCheck an AC size

Unit system

FeetMeters

Room length

Room width

Ceiling height

AC type

Sun exposure

Room details

Add the conditions that usually explain why basic BTU charts feel wrong.

Room useRegular occupants

PC / electronics heat

Use 300-700 W for a gaming PC while it is working hard.Room condition

Compare an AC you found

Useful when you are staring at a 6,000 or 8,000 BTU listing and wondering if it is a mistake.

Candidate AC size

BTU

For portable ACs, enter the DOE/SACC BTU if the product shows it.

Light running cost

Optional estimate. For exact appliance costs, use the full energy cost calculator.

Running watts

Leave 0 to estimate from BTU and CEER.CEER estimateElectricity rate

cents/kWh

Hours per dayDays per monthCurrency

Guide

How do you choose the right room AC size?

Room AC sizing starts with square footage, but the useful answer comes from adjusting that simple chart for the room you actually live in.

How does room AC BTU sizing work?

ENERGY STAR room AC guidance matches room square footage to cooling capacity, starting around 5,000 BTU for small rooms and rising as the area grows. That table assumes an 8-foot ceiling, so taller rooms need more capacity because there is more air volume to cool.

After the base size, adjust for the room. Shade can reduce the target by about 10%, strong sun can add about 10%, extra regular occupants add heat, and kitchens add a lot because cooking loads are real.

Useful heat conversion1 watt = 3.412 BTU per hour

That is why a gaming PC, TV setup, or always-on workstation can change the answer in a small bedroom or office.

Why can an oversized AC feel clammy?

A non-inverter AC that is much too large may cool the air quickly and then shut off before the coil has removed much moisture. The result can be a room that reaches the set temperature but still feels damp or uncomfortable.

Are window AC and portable AC BTU ratings the same?

Window and through-wall ACs are usually easier to compare by BTU. Portable ACs are trickier because some product pages show an older ASHRAE rating and a lower DOE/SACC rating. For portable units, use DOE/SACC when available and check the manufacturer coverage range.

What examples should you sanity-check?

A 12 by 12 ft bedroom with an 8-foot ceiling starts near 5,000 BTU. A 225 sq ft room starts near 6,000 BTU before adjustments. A 182 sq ft room with a 12-foot ceiling behaves like about 273 sq ft of standard-height room, which moves the starting point closer to 7,000 BTU.

When do high ceilings, sun, kitchens, and PCs matter?

A 180 sq ft room with a 12-foot ceiling behaves closer to a larger standard room than a plain 180 sq ft bedroom. A sunny west-facing room, open kitchen, or high-wattage computer can also explain why a basic chart feels too small.

Can one AC cool an adjacent room?

Buying a bigger unit for a bedroom does not automatically cool the living room through a doorway. Fans and open layouts can help, but walls, hallways, and stairs often limit how much cool air reaches the second space.

When should you get a load calculation?

This calculator is for room units. Whole-home central air, multi-zone mini-splits, major renovations, and unusual buildings need a professional load calculation instead of a quick room AC estimate.

Sources and sizing assumptions

The base BTU chart follows ENERGY STAR room air conditioner sizing guidance. The portable AC note reflects the difference between ASHRAE and DOE/SACC ratings described in FTC energy labeling updates and the DOE portable air conditioner test procedure.

FAQ

Room AC BTU questions

What size AC do I need for a 150 sq ft room?

For a standard 8-foot ceiling, a room around 150 square feet usually lands near 5,000 to 6,000 BTU depending on where the exact area falls, sun exposure, occupants, and heat sources. If the room is very sunny or has a hot computer setup, size up within the practical range.

Is 8,000 BTU too much for a bedroom?

It depends on the bedroom. For a small 120 to 150 sq ft bedroom with a standard ceiling, a traditional 8,000 BTU window unit is often oversized. For a larger bedroom, high ceiling, sunny room, top-floor room, or inverter unit, 8,000 BTU may make sense.

How many BTU do I need for 225 square feet?

ENERGY STAR sizing guidance puts 150 to 250 square feet at about 6,000 BTU before adjustments. Add capacity for high ceilings, strong sun, extra occupants, kitchens, poor insulation, or electronics that give off heat.

Should I size up for high ceilings?

Yes, usually. The ENERGY STAR room AC chart is based on an 8-foot ceiling. This calculator adjusts the room area by ceiling height, so a 12-foot ceiling acts like roughly 1.5 times the air volume of the same floor area with an 8-foot ceiling.

Do portable AC BTUs mean the same thing as window AC BTUs?

Not always. Portable ACs may show both ASHRAE and DOE/SACC ratings. The DOE/SACC number is usually lower and is better for judging real portable AC cooling performance. For portable units, also check the manufacturer room-coverage guidance.

Why can an oversized AC make a room feel humid?

An oversized non-inverter AC can cool the air quickly and shut off before it runs long enough to remove much moisture. That short cycling can leave the room cool but damp or clammy.

Does a PC or gaming setup change the AC size I need?

Yes. Electronics turn most of their power use into room heat. A 500 W gaming PC can add roughly 1,700 BTU per hour while it is working hard, which is enough to affect a small bedroom or office.

Can one window AC cool two rooms?

Sometimes, but airflow is usually the problem. If the rooms are separated by a doorway, wall, hallway, or stair opening, a larger AC in one room may not move cool air where you need it. Fans can help, but a second properly sized unit is often more comfortable.

How much does a room AC cost to run?

Running cost depends on watts, runtime, duty cycle, and your electricity rate. A rough estimate is watts multiplied by hours, divided by 1,000, then multiplied by your per-kWh rate. This calculator includes a light estimate and links to the appliance energy calculator for deeper cost planning.

Is this calculator for central air conditioning?

No. This is a practical room-unit calculator for window, through-wall, portable, and similar single-room ACs. Central HVAC and whole-home mini-split sizing should use a professional load calculation such as Manual J.