TryBuildCalcBTU Calculator (Cooling & Heating Load)
Estimate your home's BTU load instantly.
🕒 Last updated: September 10, 2026
Inputs
ℹ️Heated/cooled floor area only — not garages, unfinished attics/basements, or porches.
ℹ️IECC climate zone — hotter zones need more cooling capacity per sqft, colder zones need more heating capacity per sqft.
ℹ️A taller ceiling means more air volume to condition — adds roughly 10% per ft above the standard 8 ft baseline.
ℹ️Poor insulation increases both loads; good/newer insulation reduces both.
ℹ️Affects cooling load only — large south/west-facing glazing with little shade adds solar heat gain.
ℹ️Affects cooling load only — adds about 400 BTU/hr per person beyond a baseline of 2 occupants.
ℹ️Converts the required heating OUTPUT into the INPUT BTU rating printed on a furnace spec sheet/nameplate.
Cost
Recommended AC Size
3 Tons
36,000 BTU/hr cooling
Estimated Furnace Size
64,000 BTU
Output — ~67,000 BTU input at 95% AFUE
Planning estimate only — not a Manual J load calculation.This square-footage rule of thumb does not account for window count/type, air infiltration, duct losses, or your local design temperatures the way an ACCA Manual J calculation does. Final equipment selection should follow ACCA Manual S using an actual manufacturer's rated performance data. Get a Manual J (and Manual S equipment selection) from a licensed HVAC contractor before purchasing equipment.
Home & Climate
Square footage: 2,000 sq ft
Climate: Zone 4 — Mixed (Mid-Atlantic, TN, OK, N. CA)
Ceiling height: 8 ft
Cooling (AC)
Raw required load: 36,000 BTU/hr
Raw tonnage: 3 tons
Nominal size to buy: 3 tons
Heating (Furnace)
Raw required output: 64,000 BTU/hr
Approx. output to shop for: 64,000 BTU
Approx. input at 95% AFUE: 67,000 BTU
Assumptions Used
Cooling and heating loads are estimated from your square footage x a BTU/sqft figure for your IECC climate zone (hotter zones need more cooling capacity per sqft, colder zones need more heating capacity per sqft), then adjusted for ceiling height (+10% per ft above the 8 ft baseline), insulation quality (poor +15%, good -12%), sun/window exposure (cooling only: high +8%, low -5%), and occupancy beyond 2 people (cooling only: +400 BTU/additional person). AC tonnage is rounded UP to the nearest standard nominal size (1.5, 2, 2.5, 3, 3.5, 4, 5, or 6 tons) since AC equipment isn't sold in arbitrary fractional tonnage — the same reasoning this site applies when rounding up to whole bags of concrete elsewhere. Furnace sizing is NOT similarly standardized across manufacturers — input-BTU product tiers vary by manufacturer and product line, so the furnace numbers shown are the REQUIRED output/input, rounded only for readability, not snapped to a real product's size. The furnace's INPUT BTU (what's printed on a spec sheet/nameplate) is higher than its OUTPUT BTU (the actual heat delivered) — Input = Output / AFUE%, using your selected efficiency tier. If either load exceeds what a single residential system typically covers (6 tons cooling, 150,000 BTU/hr heating), no size is recommended — see the warning above instead. This is a square-footage planning estimate, not an ACCA Manual J load calculation— Manual J accounts for window count/type, air infiltration, duct losses, and local design temperatures this calculator does not model, and final equipment selection should follow ACCA Manual S using an actual manufacturer's rated performance data. Cost estimation, if enabled, covers equipment only (priced per ton / per 1,000 BTU) — installation labor, ductwork modifications, and electrical/gas line work are not included.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete AC & Furnace Size Calculator.
BTU load estimation
BTU (British Thermal Unit) measures the heating or cooling capacity a system needs to deliver — this calculator estimates both your cooling BTU/hr (converted to AC tonnage) and heating BTU/hr (furnace output) from the same home details.
This is a square-footage planning estimate, not an ACCA Manual J load calculation — treat the result as a starting point for budgeting and shopping, not a final purchase number.
AC & Furnace Size Calculator Formula: How Is It Determined?
Cooling and heating loads are estimated separately from the same home inputs. Cooling is then rounded up to a standard AC tonnage (up to 6 tons); heating is reported as the required output/input BTU, since furnace equipment isn't standardized into round sizes the same way. Either load beyond a single residential system's typical capacity is flagged instead of rounded to a number that would understate it.
Step 1 — Base Load by Climate Zone
Cooling BTU (raw) = Cooling BTU/sqft(zone) x Square Footage
Heating BTU (raw) = Heating BTU/sqft(zone) x Square Footage
Each IECC climate zone has its own BTU/sqft figure for cooling and heating — hotter zones need more cooling capacity per sqft, colder zones need more heating capacity per sqft, roughly the inverse of each other.
Step 2 — Adjustments
Height Factor = 1 + max(0, Ceiling Height − 8 ft) x 10%
Cooling x= Height Factor x Insulation Factor x Sun Factor, + Occupancy Addend
Heating x= Height Factor x Insulation Factor
Ceiling height and insulation quality adjust both loads. Sun/window exposure and occupancy beyond 2 people (+400 BTU/person) adjust cooling only — solar gain and body heat matter far more to summer cooling load than winter heating load.
Step 3 — AC Sizing & Furnace Requirement
Nominal Tons = next standard AC size ≥ (Cooling BTU / 12,000), up to 6 tons
Furnace Output (approx.) = Heating BTU (raw), rounded to the nearest 1,000 BTU
Furnace Input (approx.) = Furnace Output / AFUE%
AC equipment IS sold in a standard set of nominal sizes across manufacturers (1.5-6 tons) — the raw calculated cooling load is rounded UP to the next standard size, since a system must meet or exceed the load. Furnace equipment is NOT similarly standardized: input-BTU product tiers vary by manufacturer and product line, so the furnace numbers shown are the REQUIRED output/input only, rounded for readability — match them against an actual manufacturer's rated performance data (ACCA Manual S), not assumed to be a specific purchasable model size. A load beyond 6 tons cooling or 150,000 BTU/hr heating exceeds what a single residential system typically covers, and is flagged rather than capped to a number that would understate the true requirement.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Square footage / climate | 2,000 sq ft / Zone 4 — Mixed (Mid-Atlantic, TN, OK, N. CA) | Sets the base BTU/sqft multiplier for both loads |
| Ceiling height / insulation | 8 ft / average | Scales both loads by the height and insulation factors |
| AFUE | 95% | Converts furnace output requirement into an input BTU rating |
Step 1 — Base Load by Climate Zone
| Calculation | Result |
|---|---|
| Base cooling = 18 BTU/sqft x 2,000 sqft | 36,000 BTU/hr |
| Base heating = 32 BTU/sqft x 2,000 sqft | 64,000 BTU/hr |
Step 2 — Adjustments
| Calculation | Result |
|---|---|
| Height factor = 1 + max(0, 8 − 8) x 10% | x1 |
| Insulation factor (average) | x1 |
| Sun factor (average), cooling only | x1 |
| Occupancy addend (2 people), cooling only | +0 BTU/hr |
| Cooling raw = 36,000 x 1 x 1 x 1 + 0 | 36,000 BTU/hr |
| Heating raw = 64,000 x 1 x 1 | 64,000 BTU/hr |
| Raw tons = 36,000 / 12,000 | 3 tons |
Step 3 — AC Sizing & Furnace Requirement
| Calculation | Result |
|---|---|
| AC size, rounded up to standard | 3 tons (36,000 BTU/hr) |
| Furnace output required (approx.) | 64,000 BTU |
| Furnace input (approx.) = output / 95% AFUE | 67,000 BTU |
Therefore, a 2,000 sq ft home in Zone 4 — Mixed (Mid-Atlantic, TN, OK, N. CA) needs approximately 3 tons of AC capacity and approximately a 64,000 BTU output furnace (~67,000 BTU input at 95% AFUE). Get a Manual J (and Manual S equipment selection) from a licensed HVAC contractor before purchasing.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.