hvacloadcalc.org

BTU Calculator

Calculate the AC BTU you need for any room or home. Adjusts for climate zone, ceiling height, insulation, sun exposure, occupancy, and space type.

Jonathan Stowe

Reviewed June 4, 2026

Your room or space

Enter the room characteristics, then click Calculate to see the recommended BTU, the equipment options that fit, the math step-by-step, and what the calculator does not account for.

+600 BTU per person above 2

Enter your inputs above, then click Calculate

Result will appear here with the recommended capacity, equipment options, full math breakdown, and a chart showing where your size lands on the equipment scale.

Find your climate zone first

Climate zone is the single most important input in any HVAC sizing decision — it drives both heating and cooling design temperatures and the equipment-class recommendation. The reference card below covers all eight US climate zones with sample cities and design temperatures.

Find your IECC climate zone — design temperatures and HVAC implicationsReference table of the eight IECC climate zones with sample US cities, the 99 percent heating design temperature, the 1 percent cooling design temperature, and the practical HVAC implication for each zone. Zone 1 (south Florida, Hawaii) is purely cooling-dominant. Zone 8 (interior Alaska) is heating-extreme and requires cold-climate equipment plus dual-fuel architecture.Find your IECC climate zoneDesign temperatures and HVAC implication for each US climate zone. Source: ASHRAE Standard 169-2021.ZONESAMPLE CITIESHEAT °F / COOL °FHVAC IMPLICATION1Miami, Honolulu, San Juan+47°F / +91°FCooling-dominant. AC essential, aux heat rarely fires.2Houston, New Orleans, Tampa+30°F / +95°FCooling-dominant, mild winter. Standard heat pump sufficient.3Atlanta, Memphis, Charlotte+22°F / +93°FMostly cooling. Low aux runtime on heat pumps.4DC, Cincinnati, St. Louis+15°F / +90°FBalanced. Heat pump or gas furnace both economical.5Chicago, Boston, Denver+5°F / +88°FHeating-dominant. CCASHP recommended for heat pumps.6Minneapolis, Buffalo-2°F / +86°FCold. CCASHP strongly recommended; aux heat sized for design.7Duluth MN, mountain west-10°F / +84°FVery cold. CCASHP required; dual-fuel often economical.8Interior Alaska-20°F / +80°FExtreme cold. CCASHP + dual-fuel typical architecture.
IECC climate zones are defined by Heating Degree Days and Cooling Degree Days per ASHRAE Standard 169-2021. Heating design temperature is the 99% winter outdoor temperature (the temperature exceeded by 99% of winter hours); cooling design temperature is the 1% summer outdoor temperature. Your county-level zone is on the IECC climate zone map at codes.iccsafe.org.

Why climate zone matters so much

The single biggest factor in the BTU number is your climate zone. A 1,500 sq ft home in Miami needs roughly 60% more cooling capacity than the same home in Minneapolis because both the cooling design temperature (95°F vs 88°F) and the latent (moisture-removal) load are dramatically higher in the south. The chart below shows the planning-grade BTU per square foot range across all eight IECC climate zones.

Cooling BTU per square foot by IECC climate zoneHorizontal bar chart showing planning-grade BTU per square foot ranges for residential cooling across IECC climate zones 1 through 8. Zone 1 (Miami) ranges from 25 to 35 BTU per square foot, decreasing through colder zones to zone 8 (interior Alaska) at 8 to 18 BTU per square foot.Cooling BTU per square foot — by IECC climate zone101520253035Zone 1 — Miami, S. Florida25–35Zone 2 — Houston, Gulf Coast22–32Zone 3 — Atlanta, Mid-South18–28Zone 4 — St. Louis, Mid-Atlantic16–26Zone 5 — Chicago, Boston14–24Zone 6 — Minneapolis, Denver12–22Zone 7 — Duluth, Anchorage10–20Zone 8 — Interior Alaska8–18BTU per square foot (cooling, planning-grade range)
Planning-grade ranges. Lower end = tight envelope (R-49 attic, low-E windows, 3 ACH50); upper end = leaky pre-1980 envelope (R-13 attic, single-pane, 10+ ACH50). Source: ACCA Manual J 8th + ASHRAE Fundamentals 2021 + IECC 2021 climate zone definitions.

The range within each zone reflects envelope quality: the lower end is a tight modern envelope (R-49 attic, low-E windows, 3 ACH50 air leakage) while the upper end is a leaky pre-1980 envelope (R-13 attic, single-pane windows, 10+ ACH50). The calculator captures this variation via the climate, insulation, and sun-exposure inputs. For a detailed walkthrough of the climate-zone-by-zone math see the AC BTU chart article.

Worked example: 1,500 sq ft home, zone 5

The default state above shows the calculator's answer for a typical 1,500 square foot home in IECC climate zone 5 (most of the northern US), with 8-foot ceilings, average insulation, mixed sun exposure, four occupants, and the cooled space being a living room.

The math:

  • Baseline: 1,500 sqft × 22 BTU/sqft = 33,000 BTU
  • × Climate factor (zone 5): 0.9
  • × Ceiling factor (8 ft): 1
  • × Sun factor (mixed): 1
  • × Insulation factor (average): 1
  • × Space-type factor (living room): 1.1
  • = Subtotal: 32,670 BTU
  • + Occupancy (2 extra): 1,200 BTU
  • = Final raw: 33,870 BTU
  • Rounded to standard equipment size: 36,000 BTU (≈ 3 tons)

How to use this calculator

  1. Measure the square footage of the space you want to cool
  2. Pick the IECC climate zone for your location (zone 1 is tropical, zone 8 is interior Alaska)
  3. Set ceiling height; 8 ft is the baseline. Higher ceilings need more BTU
  4. Pick insulation quality vs current code (most homes are average)
  5. Pick sun exposure and space type (kitchen, sun room, basement all matter)
  6. Enter the number of regular occupants and whether the space is a kitchen
  7. Read the recommended BTU and equipment class on the right

What the calculator does

The calculator multiplies a baseline of 22 BTU per square foot by the relevant adjustment factors (climate, ceiling, sun, insulation, space type), then adds fixed amounts for extra occupants and kitchen heat gain. The raw result is rounded to the nearest standard equipment size: 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, 24,000, 30,000, 36,000, 42,000, 48,000, or 60,000 BTU.

The methodology is documented in our AC BTU chart article and verified against ACCA reference cases per our verification methodology.

Equipment options by BTU range

The recommended BTU number maps to one of four equipment classes. Knowing the class tells you the approximate retail or installed cost range and what type of contractor (if any) needs to be involved.

Window AC

5,000 – 14,000 BTU/hr

Best for
Single room, low capital cost, no ductwork
Typical cost
$200 – $500 retail

Fits standard double-hung window opening. CEER 11+ is the federal minimum for modern units. Visible from outside; protrudes from window frame. 110V plug for units under 12,000 BTU; 220V required above.

Portable AC

8,000 – 14,000 BTU/hr

Best for
Rented apartments, casement windows, historic preservation
Typical cost
$300 – $700 retail

Wheel-mounted indoor unit with flexible exhaust hose to a window. Less efficient (CEER ~9) than window units because the exhaust hose pulls already-cooled room air outside as a side effect. Size up one tier vs an equivalent window unit.

Mini-split (ductless)

9,000 – 36,000 BTU/hr per indoor head

Best for
Quiet operation, native zone control, no existing ducts
Typical cost
$3,000 – $5,000 per zone installed

Wall-mounted indoor head + outdoor compressor connected by refrigerant lineset. SEER2 18-22 typical; modulating inverter compressor handles part-load well. Heat pump variants reverse for heating. Requires licensed contractor for refrigerant work.

Central AC (ducted)

18,000 – 60,000 BTU/hr (1.5 – 5 tons)

Best for
Whole-house cooling with existing ducts
Typical cost
$4,000 – $12,000 installed (replacement)

Indoor evaporator coil and air handler distribute conditioned air through ducts. Federal minimum SEER2 13.4 (north) / 14.3 (south). Sized via Manual J + Manual S; oversized central systems short-cycle and fail to dehumidify.

Five common BTU sizing mistakes

Most residential AC sizing errors fall into one of five categories. Avoiding these puts the BTU number within Manual S tolerance of the actual load.

1

Sizing by square footage alone

A 1,500 sq ft home in Miami (zone 1) needs about 38,000 BTU/hr of cooling; the same home in Minneapolis (zone 6) needs about 24,000 BTU/hr — a 60% difference at identical floor area. The "X BTU per sq ft" rule of thumb hides this factor and is one of the top three sources of residential AC sizing errors per DOE Building America research.

2

Oversizing "to be safe"

An AC sized 30%+ above the actual cooling load cools to setpoint in 5-7 minutes during mild weather, then shuts off for 12-15 minutes before restarting. The unit never runs long enough to remove moisture, so the room reads cool but feels sticky at 60% RH. The compressor cycles 8-12 times per hour, wearing out 2-3 years earlier than design life. Manual S caps oversizing at 15% for single-stage equipment for this reason.

3

Ignoring sun exposure

A 200 sq ft room with two 4×6 west-facing windows receives roughly 5,000 BTU/hr of peak solar gain on a clear summer afternoon — equivalent to adding two extra rule-of-thumb occupants worth of latent and sensible load. Solar gain through unshaded glass dominates afternoon cooling load in cooling-dominated climates and is the largest source of room-to-room load variation in a typical home.

4

Forgetting kitchens, sun rooms, and attic spaces

A kitchen adds about 4,000 BTU/hr of cooking-appliance heat gain; the calculator handles this via the kitchen flag. Sun rooms with extensive glazing on three sides need 75% more BTU per sq ft than equivalent interior rooms. Attic-converted bonus rooms with hot-attic ceilings need 30% more. These space-type factors are required inputs to land at the right BTU number.

5

Skipping the humidity question in humid climates

In zones 1A, 2A, and along the Gulf Coast, the cooling load includes substantial latent (moisture-removal) work in addition to sensible (temperature-drop) work. A correctly-sized AC running long cycles condenses moisture on the cold evaporator coil; an oversized one cycles too fast for this to happen. The result is "70°F at 65% RH" comfort complaints. Variable-speed equipment is the right answer in humid climates because it runs long cycles at lower capacity, removing more moisture per BTU delivered.

When this calculator is enough — and when to upgrade to Manual J

For window AC, portable AC, or single-zone mini split sizing this calculator is sufficient. For central AC equipment specification, this calculator gives a planning estimate; the final equipment selection should come from a full Manual J load calculation that accounts for orientation, room-by-room loads, ductwork losses, and infiltration measured with a blower door. The Manual J load calculator on this site goes deeper into envelope characterization for closer-to-permit-grade planning estimates.

Permit applications, HEEHRA and state rebate documentation, manufacturer warranty claims, and post-retrofit equipment selection (after envelope upgrades change the load) all require a full Manual J performed by a credentialed contractor using ACCA-approved software (Wrightsoft Right-J, Cool Calc Manual J, Elite RHVAC, EnergyGauge USA). Output from any free planning-grade tool — including this one — is not eligible for those uses. The methodology page documents the accuracy bands the calculator claims against ACCA reference cases.

Common scenarios

Pre-computed worked examples for typical room sizes and space types. Each example shows the full math and lets you adjust the inputs from that starting point.

Frequently asked questions

How many BTU do I need per square foot for cooling?
The US average baseline is 22 BTU/hr per square foot, but the actual number varies from about 18 (cold climate, well-insulated, shaded) to 35+ (hot/humid climate, leaky envelope, sun-exposed). The BTU calculator multiplies the 22 baseline by climate, ceiling, sun, insulation, and space-type factors to land at the right number for your specific space. A "20 BTU per sq ft" rule of thumb misses the climate and envelope variables that move the result by 50%+ in either direction.
Is bigger BTU always better?
No. An AC sized more than about 20% above the actual load short-cycles in cooling mode: the unit cools to setpoint quickly and shuts off before removing enough humidity, so the space feels cool but sticky. Manual S tolerates equipment up to 15% above Manual J cooling load for single-stage units and 25% for two-stage / variable-speed. Beyond that, comfort suffers and the compressor wears out faster because it cycles more often.
What does a "ton" mean in AC sizing?
One ton of refrigeration equals 12,000 BTU/hr. The unit comes from the heat needed to melt one short ton (2,000 lb) of ice in 24 hours: 2,000 × 144 BTU/lb (latent heat of fusion of ice) = 288,000 BTU over 24 hours = 12,000 BTU/hr. A "3-ton AC" delivers 36,000 BTU/hr at the AHRI 95°F outdoor / 80°F indoor test condition.
How accurate is this calculator versus a real Manual J?
For typical residential single-family homes, the calculator lands within ±20-30% of a permit-grade Manual J performed by a credentialed contractor. Accuracy is best for tight, modern construction (±10-15%) and worst for older leaky housing (±20-30%) because simplified infiltration models break down on older envelopes. The accuracy is more than sufficient for evaluating a contractor quote or comparing equipment options; it is not sufficient for permit-grade equipment specification on a new install.
Why does the calculator round to standard equipment sizes?
Because manufactured AC equipment comes in fixed BTU/hr increments, not continuous sizes. The standard residential sizes are 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, 24,000, 30,000, 36,000, 42,000, 48,000, and 60,000 BTU/hr. The calculator rounds the raw computed BTU to the nearest standard size so the recommendation matches what you can actually buy. The acceptable range output shows the Manual S tolerance band around that recommendation.
Can I use this calculator for whole-house central AC sizing?
Yes for planning purposes (budgeting, comparing contractor quotes, deciding between tonnage options). No for permit-grade central AC sizing on a new install — that requires a full Manual J performed by a credentialed contractor using ACCA-approved software (Wrightsoft, Elite, Cool Calc, EnergyGauge). The Manual J load calculator on this site goes deeper into envelope details for closer-to-permit-grade planning estimates.
What about sensible vs latent cooling?
The calculator output is the total cooling capacity needed (sensible + latent combined). In dry climates (zones 2B, 3B — Phoenix, Las Vegas, Albuquerque), almost all of the cooling load is sensible (temperature-drop) work. In humid climates (zone 1A, 2A — Miami, Houston), 25-35% of the cooling work is latent (moisture-removal). The same total BTU/hr equipment performs differently in those two climates — humid climates need lower SHR (sensible heat ratio) equipment, which usually means selecting variable-speed inverter units that run long cycles.
Does ceiling height really matter that much?
Yes. Cooling load scales with the conditioned air volume, not just the floor area. A 200 sq ft room with 8-ft ceilings holds 1,600 cubic feet of air; the same room with 12-ft cathedral ceilings holds 2,400 cubic feet — 50% more air to cool to setpoint. The calculator applies a ~10% load increase per foot above 8 ft. Vaulted ceilings, two-story great rooms, and cathedral ceilings can shift cooling load by 20-30% versus their square-foot equivalent flat-ceilinged spaces.

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Jonathan Stowe

Reviewed June 4, 2026