BTU calculator: what size mini split or AC does a room need?

By the Hewer team · · 2 min read

The BTU Calculator on a phone: a cooling load of 3,600 BTU per hour for a 180 square foot room, a 6,000 BTU unit to buy, and a plan of the room.
A 15 by 12 foot room: the load, and the unit size to buy.

Customers ask it on every bonus room, garage conversion and sunroom: what size unit do I need? The BTU Calculator gives a quick answer from the room size, plus the stock unit size to buy, so you can talk about it on the spot. Add a room for each zone and it sizes a multi-head mini split too.

What the BTU Calculator works out

  • The heating or cooling load in BTU per hour.
  • The unit size to buy, rounded up to a stock size, in BTU and tons.
  • The electric equivalent in kW.
  • The floor area, with a plan of the room.
  • For a multi-head system: a head size for each room, the total load and the outdoor unit to cover it.

How to use it

  1. Pick Cooling or Heating.
  2. Enter the room's length, width and ceiling height. For an odd-shaped room, enter the floor area under More options.
  3. For cooling, set the people in the room, the sun and whether it's a kitchen. For heating, pick the climate.
  4. Tap + Add a room for each extra zone on a multi-head system.
  5. Read the unit size at the top.

Open the BTU Calculator (free) →

The BTU Calculator with two rooms on a phone: a 180 square foot room with a 6,000 BTU head and a 120 square foot kitchen with a 9,000 BTU head, a total cooling load of 15,000 BTU per hour and a 1.25 ton outdoor unit.
Two rooms on one system: a head for each, and the outdoor unit to cover both.

Tips from the field

  • This is a quick sizing estimate, and the app says so. A whole-house system needs a proper load calculation (a Manual J) by the HVAC contractor.
  • Oversizing isn't safer: a unit that's too big cycles on and off and leaves a room clammy.
  • Save the room to a job folder and send the customer the PDF with your estimate.

Free, and no account needed

The Hewer Trade Toolkit is free. It runs in any phone browser, saves your last inputs, works in feet and inches or metric, and keeps layouts in job folders you can export as a PDF. If you run a crew, the same calculators are inside Hewer's crew app, next to the jobs, the schedule and the clock-in.

Questions

How many BTUs do I need per square foot?

It depends on the room, the climate and the sun. For cooling, the calculator follows ENERGY STAR's room air conditioner sizing chart, which goes from floor area to BTU per hour, and adjusts it for ceiling height, shade or sun, the people in the room and a kitchen. Heating has no official per-square-foot figure, so it uses a rough rule of thumb by climate and says so. Either way it rounds up to a stock unit size.

Is this a Manual J calculation?

No. It's a quick sizing estimate for a single room. Whole-house systems need a full load calculation by an HVAC professional.

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