TryBuildCalcAC Size Calculator (Tonnage by Square Footage & Climate)
Find your AC tonnage 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.
AC tonnage sizing
Central air conditioners are sold in standard nominal tonnage sizes (1.5 through 6 tons) — this calculator estimates your home's cooling load from square footage and climate zone, then rounds up to the next standard size, since equipment needs to meet or exceed the load.
This is a planning-stage rule of thumb, not a Manual J load calculation — get a Manual J from a licensed HVAC contractor before purchasing equipment.
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.