BTU Calculator

How to use this BTU calculator

1. Pick a room preset (or skip it). Bedroom, living room, kitchen, garage/shop, sunroom, or basement — each preset loads the typical adjustments for that space (a garage gets poor insulation and a 10-foot ceiling; a kitchen gets the +4,000 BTU cooking adder). Everything stays editable after.
2. Enter the room size. Length × width in feet, or type the square footage directly. Set the ceiling height if it isn't the standard 8 feet — a 10-foot garage holds 25% more air, and the calculator scales for it.
3. Pick your state. The calculator sets your DOE climate zone automatically (or set the zone yourself). Zone drives both sides: a Zone 6 Minnesota room needs almost twice the heating BTU per square foot of a Zone 2 Texas one.
4. Set the conditions. Insulation quality, sun exposure (ENERGY STAR's ±10% cooling adjustment), how many people are regularly in the room (+600 BTU each past two), and whether it's a kitchen (+4,000 BTU).
5. Read both loads. The result card shows the cooling and heating BTU/h side by side, recomputed on every keystroke — no Calculate button. Below them, the equipment grid translates the numbers into what you'd actually buy: window-AC class, mini-split head size, central tonnage, furnace input at 80% and 96% AFUE, and electric-heater watts (with the 1,500 W plug-in cutoff called out).
6. Save or share it. The PDF button downloads a branded, dated sizing sheet. The share button copies a link that rebuilds your exact inputs — handy for the store, a contractor, or the family group chat.

Why this BTU calculator is different

Most BTU calculators do one side of the problem — a cooling-only widget on an AC retailer's blog, or a heating-only one on a heater manufacturer's site — and stop at a raw number. This one is built for how the question actually comes up in a house:

• Heating and cooling in one pass. The same room needs both answers (a garage you're insulating, a sunroom you're finishing, a bedroom over the garage). One set of inputs, both loads, no re-entering everything on a second site.
• It tells you what to buy, not just a number. 13,300 BTU means nothing at the store. "14,000 BTU window unit, 18,000 BTU (1.5-ton) mini-split head, or 1.5-ton central" does. The equipment grid maps your load to the smallest standard size class of five equipment types — including whether a plain 1,500 W plug-in space heater covers your heating or you're in hardwired territory.
• State → climate zone built in. Pick your state and the DOE climate zone is set for you — the same state-to-zone mapping our R-value calculator uses. Zone changes the heating answer by up to 2× and nudges cooling ±10%.
• Room presets that set the right adjustments. "Garage/shop" isn't just a label — it loads poor insulation and a taller ceiling, because that's what a garage is. Kitchen loads ENERGY STAR's +4,000 BTU. Sunroom loads the +10% sun factor.
• Watts ↔ BTU converter on the same page. Space heaters are sold in watts, ACs in BTU. The quick converter (1 W = 3.412 BTU/h) settles the "is a 1,500 W heater enough?" question without leaving the page.
• Embeddable and free. If you run an HVAC blog, an energy-efficiency resource, or a homestead/off-grid forum, the iframe at the bottom hosts the same calculator with attribution.

How it works (the math behind the numbers)

Cooling starts from the ENERGY STAR room-air-conditioner sizing table — the same square-footage-to-BTU table printed on the box of every window unit — then adjusts it for your room's reality. Heating uses the contractor rule-of-thumb factor map: 30 to 60 BTU/h per square foot depending on climate zone.

Cooling  = ENERGY STAR table (sq ft → BTU)
           × (ceiling ÷ 8 ft)  × insulation (±10%)
           × climate zone (±10%) × sun exposure (±10%)
           + 600 BTU × each person past two
           + 4,000 BTU if it's a kitchen

Heating  = sq ft × zone factor (30–60 BTU/sq ft)
           × (ceiling ÷ 8 ft) × insulation (−10% / +20%)

Equipment = smallest standard class ≥ the load
Furnace input = heating ÷ AFUE (80% or 96%)
Heater watts  = heating ÷ 3.412  (1,500 W = plug-in max)

The sun (±10%), extra-person (+600 BTU), and kitchen (+4,000 BTU) adjustments are ENERGY STAR's published room-AC rules. The climate-zone cooling band is the regional design-temperature adjustment — Gulf-coast summers run hotter design temps than Maine ones. The heating factors are the standard 30–60 BTU/sq ft contractor map by DOE climate zone. A fixture file with thirteen input → output pairs runs as a build-time gate, so the math on this page can never silently drift.

Heating BTU per square foot, by climate zone

These are the heating factors the calculator uses at average insulation and 8-foot ceilings. Find your zone on the map (or just pick your state in the tool — it knows):

• Zone 1 — Hot (South Florida, Hawaii): 30 BTU/sq ft. Heating is an afterthought; cooling drives everything.
• Zone 2 — Hot/humid (most of Florida, S. Texas, Gulf Coast): 35 BTU/sq ft.
• Zone 3 — Warm (Georgia, Carolinas, N. Texas, SoCal): 40 BTU/sq ft.
• Zone 4 — Mixed (Virginia, Tennessee, Oregon coast): 45 BTU/sq ft. The national middle.
• Zone 5 — Cool (Midwest, Northeast, Colorado): 50 BTU/sq ft.
• Zone 6 — Cold (Minnesota, Maine, Montana, Wisconsin): 55 BTU/sq ft.
• Zone 7 — Very cold (Alaska, northern Minnesota): 60 BTU/sq ft. Double the Zone 1 factor — insulation pays for itself fastest here.

Poor insulation adds 20% to the heating side; good insulation takes 10% off. If you're not sure which you have, our R-value chart shows what your walls and attic should be for your zone, and the R-value calculator totals what you've actually got.

Three real-world sizings

A 300 sq ft bedroom in Virginia (Zone 4)

Average insulation, average sun, two people: the ENERGY STAR table puts 300 sq ft at 7,000 BTU of cooling, and Zone 4's 45 BTU factor puts heating at 13,500 BTU/h. At the store that's an 8,000 BTU window unit (the smallest class that covers it), a 9,000 BTU mini-split head, or — for heat — about 4,000 W, which is past any plug-in. A window unit plus the house furnace is the sane answer; if the room runs cold, a 1,500 W space heater closes a 5,000 BTU gap, not a 13,500 one.

A 600 sq ft garage shop in Minnesota (Zone 6)

Ten-foot ceiling, uninsulated: the poor-insulation and ceiling multipliers stack, and heating lands at 49,500 BTU/h — that's a 60,000–80,000 BTU furnace input or a dedicated gas garage heater, not a plug-in anything. This is the number that surprises people: an uninsulated Zone 6 garage needs more heating than a well-insulated 1,500 sq ft house. Insulate first (the R-value calculator will show you the wall assembly), then re-run this at "average" and watch the load drop by a sixth.

A sunny 450 sq ft living room in Texas (Zone 2)

Very sunny, four people: the sun adds 10%, the hot zone adds 10%, and two extra bodies add 1,200 BTU — cooling lands at 13,300 BTU/h, a 14,000 BTU window unit or an 18,000 BTU (1.5-ton) mini-split. Worth knowing: a good ceiling fan makes moving air feel about 4°F cooler, which is often the difference between the 14,000 class and the 12,000 one.

What affects how accurate this is

The arithmetic is fixture-tested and exact; the estimate is only as good as what a rule-of-thumb can see:

• This is shopping-grade, not permit-grade. The real standard is an ACCA Manual J load calculation, which counts every window's orientation, your actual wall assemblies, duct losses, and infiltration. For a window AC or a space heater, rule-of-thumb is exactly right. For a furnace, central AC, or heat pump, get the Manual J — most codes require one anyway.
• Windows and doors are averaged in. The table assumes typical window area. A wall of single-pane glass adds real load the calculator can't see — nudge insulation to "poor" as a proxy.
• Oversizing is a real failure, not a safety margin. An oversized AC short-cycles: it hits the thermostat before it dehumidifies, so the room is cold and clammy, and the compressor wears out cycling. In humid climates a right-sized unit plus a dehumidifier beats an oversized unit every time. Oversized furnaces do the same thing with temperature swings.
• Above 2,500 sq ft the table runs out. The calculator extrapolates and flags it — whole-house numbers at that scale are Manual J territory, full stop.
• Insulation and air sealing move the answer more than any input. The poor-to-good insulation swing is 30+ % on heating. If you're heating a leaky space, sealing it is cheaper than the bigger heater.

Frequently asked questions

How do I figure out how many BTUs I need?

Multiply the room's length by its width for square footage, then look it up: for cooling, the ENERGY STAR table runs from 5,000 BTU (up to 150 sq ft) to 34,000 BTU (2,500 sq ft) — roughly 20 BTU per square foot. For heating, multiply square footage by your climate-zone factor: 30 BTU/sq ft in the hot South up to 60 in the far North. Then adjust for ceiling height, insulation, and sun. This calculator does all of it in one pass and shows the equipment class that covers the result.

How many square feet does a 12,000 BTU AC cool?

About 450–550 square feet under the ENERGY STAR sizing table, assuming 8-foot ceilings, average insulation, and average sun. Very sunny rooms knock that down roughly 10% (call it 400–500 sq ft), heavily shaded rooms stretch it about 10%. A 12,000 BTU unit is also exactly 1 ton of cooling — the same capacity as the smallest central-AC and mid-size mini-split classes.

How many BTUs do I need to cool a 20×20 room?

A 20×20 room is 400 square feet, which the ENERGY STAR table puts at 9,000 BTU. There's no common 9,000 BTU window unit, so you'd buy the 10,000 BTU class — or a 9,000 BTU mini-split head, which is a standard size. Add roughly 10% if the room is very sunny, 600 BTU per person past two, and 4,000 BTU if it's actually a kitchen.

How big a room will a 15,000 BTU heater heat?

Between about 250 and 500 square feet, depending almost entirely on climate zone and insulation. At Zone 4's 45 BTU/sq ft factor, 15,000 BTU covers ~330 sq ft of average-insulated space; in a Zone 2 southern winter it stretches to ~430 sq ft, and in a Zone 7 winter it shrinks to 250. Poor insulation cuts every one of those numbers by a sixth. Run it backwards in the calculator: enter your room and see whether the heating load lands under 15,000.

What's the difference between watts and BTUs?

They're the same thing in different units: 1 watt = 3.412 BTU per hour. Space heaters are labeled in watts (a standard plug-in maxes out at 1,500 W ≈ 5,100 BTU/h), while ACs and furnaces are labeled in BTU. The converter row in the calculator flips between them — useful for the "will one plug-in heater do it?" check, since anything over ~5,100 BTU of heating need means no.

Can I embed this BTU calculator on my site?

Yes — copy the iframe snippet at the bottom of this page. The embedded version is a streamlined variant built for HVAC and home-performance blogs, energy-efficiency nonprofits, off-grid and tiny-home resources, and DIY forums. Required attribution is built into the snippet. Free, no signup, no analytics attached to the embed.

Related tools

• R-Value Calculator — total the insulation in the wall or attic you're about to heat; dropping the load is cheaper than buying BTUs.
• R-Value Insulation Chart — the recommended insulation levels for your climate zone, the other half of the sizing equation.
• Lumber Calculator — framing out that garage or sunroom you just sized.
• Browse all free tools by Jake →

Recommended gear once you know your number

The BTU number is step one; here's the gear Jake points people to once they have it:

• Best Space Heaters — if your heating load came in under ~5,100 BTU (1,500 W), a good plug-in covers the whole room. The roundup is organized by exactly the wattage classes this calculator outputs.
• Best Smart Thermostats — right-sized equipment plus a learning schedule is the whole efficiency play. The cheapest upgrade with the biggest bill impact.
• Best Ceiling Fans — moving air feels ~4°F cooler, which regularly buys you one AC size class down.
• Best Dehumidifiers — in humid climates, half of what feels like "not enough AC" is humidity. A right-sized AC plus a dehumidifier beats an oversized AC.
• Best Air Purifiers — rooms that run AC or heat all season are closed rooms; a HEPA unit keeps the recirculated air clean. (Not sure HEPA is what you need? HEPA vs. ionic, explained.)

Sources & methodology

• ENERGY STAR — Room Air Conditioner Sizing — the base square-footage table and the sun (±10%), occupancy (+600 BTU/person), and kitchen (+4,000 BTU) adjustments.
• U.S. DOE / energy.gov — Climate Zones — the state → climate zone mapping (shared with our R-value tools).
• ACCA Manual J — the residential load-calculation standard this tool approximates; the 30–60 BTU/sq ft zone factors are the industry rule-of-thumb map, and Manual J is what to get for whole-house equipment.
• NIST — SI units — 1 watt = 3.412142 BTU/h, the conversion behind the watts row and heater sizing.

The sizing table, zone factors, and adjustments are reviewed annually against the ENERGY STAR and DOE sources; thirteen fixture tests gate every deploy. About Jake · Last reviewed July 17, 2026.

Embed this calculator on your site

Free for HVAC and home-performance blogs, energy-efficiency and weatherization nonprofits, off-grid and tiny-home resources, appliance-repair sites, and DIY forums. Required attribution is included in the snippet. No fee, no account, no analytics attached to the embed.

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  &middot; Reviewed by Jake Morrison, Licensed GC
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