TryBuildCalcAC & Furnace Size Calculator (Cooling Tonnage & Heating BTU)
Find your AC tonnage and furnace BTU size 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.
What Is an AC & Furnace Size Calculator?
An AC & furnace size calculator estimates the cooling tonnage and heating BTU capacity your home needs, from square footage, climate zone, insulation, ceiling height, sun exposure, and occupancy — a single planning-stage estimate for both systems, since most US homes need both in the same climate.
This calculator computes a simplified square-footage planning estimate only. It is not an ACCA Manual J load calculation and should not be used as the final basis for purchasing HVAC equipment — always get a Manual J from a licensed HVAC contractor first.
Why this needs its own calculator, not just a flat sq-ft-per-ton guess:
- Climate zone changes the BTU/sqft multiplier dramatically — a hot-climate home needs roughly double the cooling capacity per sqft of a cold-climate home, and the reverse is true for heating
- Ceiling height, insulation quality, sun exposure, and occupancy all meaningfully shift the result beyond a flat sq-ft rule
- AC equipment is sold in standard nominal tonnage only — a raw calculated number isn't purchasable until it's rounded up to a real size, while furnace sizing isn't standardized the same way and needs its required BTU matched against actual manufacturer data instead
- A furnace's INPUT BTU (on the spec sheet) differs from its OUTPUT BTU (actual heat delivered) by its AFUE efficiency rating — conflating the two leads to a mismatched purchase
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.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Home & Climate Inputs+-
- Square footage entered is CONDITIONED (heated/cooled) floor area only — not garages, unfinished attics/basements, or porches
- Climate zone selection confirmed against your actual location, not just guessed from general region
✓Before Purchasing Equipment+-
- An ACCA Manual J load calculation obtained from a licensed HVAC contractor before purchasing equipment
- Confirmed the selected equipment size isn't being oversized "to be safe"
✓Installation & Safety+-
- Electrical circuit/breaker sized for the NEW equipment's actual amperage draw, not just reused from the old unit's circuit
- Gas line sizing and venting confirmed adequate for the new furnace's actual BTU input, especially when upsizing
- Carbon monoxide detector installed and tested near a new or replaced gas furnace
Full QC Checklist+−
Verification checklist for an AC/furnace sizing estimate — covering home/climate inputs, the equipment-purchasing decision, efficiency ratings, installation safety, and post-installation verification. Use the Essential Checklist for critical checks before finalizing, expand to Full QC Checklist for complete verification.
✓Home & Climate Inputs+-
- Square footage entered is CONDITIONED (heated/cooled) floor area only — not garages, unfinished attics/basements, or porches
- Climate zone selection confirmed against your actual location, not just guessed from general region
- Insulation quality assessed honestly (attic, walls, windows) rather than defaulting to Average without checking
- Sun/window exposure selection checked against actual window count, size, and orientation, not just a guess
- Ceiling height measured for the actual room(s), not assumed at the standard 8 ft default — especially for vaulted/cathedral ceilings
✓Before Purchasing Equipment+-
- An ACCA Manual J load calculation obtained from a licensed HVAC contractor before purchasing equipment
- Confirmed the selected equipment size isn't being oversized "to be safe"
- If ducted: existing ductwork confirmed adequate for the new equipment's actual airflow (CFM), not just reused by default
- Local permit requirements and electrical/gas line capacity checked before scheduling installation
- Contractor's HVAC license and liability insurance verified before signing a contract or making a deposit
✓Efficiency & Equipment Ratings+-
- Furnace AFUE selection confirmed against the actual unit being considered, not left at an assumed default
- Difference between the furnace's OUTPUT BTU (what heats the home) and INPUT BTU (what's on the spec sheet) understood before comparing units
- Contractor quotes cross-checked against each other for the SAME sizing basis (Manual J, not a rule of thumb)
- AC SEER/SEER2 rating confirmed against the current DOE regional minimum efficiency standard for your area
✓Installation & Safety+-
- Electrical circuit/breaker sized for the NEW equipment's actual amperage draw, not just reused from the old unit's circuit
- Gas line sizing and venting confirmed adequate for the new furnace's actual BTU input, especially when upsizing
- Carbon monoxide detector installed and tested near a new or replaced gas furnace
- AC evaporator coil's condensate drain line confirmed sloped and clear, with a secondary drain/float switch installed where required
- Outdoor condenser unit placement confirmed with adequate clearance (commonly 24-30 in on the service side) for airflow and future service access
✓After Installation+-
- System startup/commissioning performed by the installer, verifying refrigerant charge and airflow against manufacturer spec — not just powered on
- Thermostat programmed and tested through a full heating AND cooling call before considering the job complete
- Final permit inspection scheduled/passed where required, and manufacturer warranty registration completed
AC & Furnace Reference Tables
BTU/sqft figures by climate zone, and the input/output BTU relationship by AFUE.
| Climate Zone | Cooling BTU/sqft | Heating BTU/sqft |
|---|---|---|
| Zone 1-2 — Hot | 24 | 18 |
| Zone 3 — Warm | 20 | 25 |
| Zone 4 — Mixed | 18 | 32 |
| Zone 5 — Cool | 15 | 38 |
| Zone 6 — Cold | 13 | 45 |
| Zone 7-8 — Very Cold | 11 | 55 |
AFUE: Output vs. Input BTU (80,000 BTU output example)
| AFUE | Output BTU (heats the home) | Input BTU (spec sheet rating) |
|---|---|---|
| 80% | 80,000 | 100,000 |
| 90% | 80,000 | 89,000 |
| 95% | 80,000 | 84,000 |
| 98% | 80,000 | 82,000 |
Standard AC Sizes vs. Furnace Sizing
| Equipment | How It's Sized |
|---|---|
| AC (tons) | Standardized across manufacturers: 1.5, 2, 2.5, 3, 3.5, 4, 5, 6 tons — this calculator rounds your load up to one of these. |
| Furnace (BTU) | NOT standardized the same way — input-BTU tiers vary by manufacturer/product line (e.g. one major manufacturer's lineup runs 30k/44k/66k/88k/110k/132k input, not round 40k/60k/80k steps). This calculator reports your required output/input only; match it against an actual model's rated performance data. |
When should you use this AC & furnace size calculator?
- Getting a ballpark AC/furnace size before requesting contractor quotes, so you can sanity-check what they propose.
- Budgeting for an HVAC replacement before committing to a full Manual J calculation.
- Comparing how climate zone, insulation upgrades, or ceiling height changes affect equipment sizing.
- Understanding the difference between a furnace's input and output BTU rating before shopping.
- Cross-checking whether an existing or proposed system size seems roughly reasonable for your home.
Quick AC & Furnace Sizing Tips
- Always get a Manual J from a licensed HVAC contractor before purchasing — this calculator is a planning estimate, not a final sizing number.
- Resist the urge to round up "to be safe" beyond what this calculator already rounds to — an oversized AC short-cycles and struggles with humidity control.
- If comparing contractor quotes, ask whether each one is based on an actual Manual J calculation or a rule of thumb — they aren't a fair comparison otherwise.
- Note whether a quoted furnace BTU is an input or output rating before comparing it against this calculator's output requirement.
- Insulation upgrades (attic, walls, windows) can meaningfully reduce the equipment size — and cost — you actually need; consider them before sizing new equipment for an older home.
Common Mistakes
- Treating this square-footage estimate as final and skipping a Manual J calculation before purchasing equipment.
- Assuming bigger is always better — an oversized AC short-cycles, cooling quickly but failing to remove humidity properly.
- Comparing a furnace's spec-sheet INPUT BTU directly against this calculator's OUTPUT requirement without converting via AFUE.
- Including unconditioned space (garage, unfinished attic/basement, porch) in the square footage entered.
- Matching a replacement system's size to the old unit without checking whether the old unit was itself correctly sized.
- Ignoring ductwork condition/sizing when replacing equipment — correctly-sized equipment on undersized or leaky ducts still won't perform well.
Limitations
- This is a simplified square-footage planning estimate, not an ACCA Manual J load calculation — it does not account for window count/type, air infiltration, duct losses, or your local design temperatures the way Manual J does.
- Furnace sizing is NOT standardized across manufacturers the way AC tonnage is — the BTU figures shown are the required output/input, not a guaranteed purchasable model size. Final equipment selection should follow ACCA Manual S using an actual manufacturer's rated performance data.
- Does not size ductwork (CFM/duct diameter) — a correctly-sized system on undersized or leaky ducts still won't deliver even comfort.
- Not intended for ductless mini-split sizing — mini-splits are commonly sized per-room using much smaller BTU tiers than this calculator's whole-home central-equipment model; using this calculator's output for a mini-split risks a significantly oversized, short-cycling system.
- When a load exceeds a single residential system's typical capacity (6 tons cooling, 150,000 BTU/hr heating), no equipment size is recommended — multiple systems or commercial-grade design needs a licensed contractor's professional design.
- Occupancy and sun-exposure adjustments affect the cooling estimate only, not heating.
- Cost estimation, if enabled, covers equipment only, priced per ton (AC) and per 1,000 BTU (furnace) at prices you supply — installation labor, ductwork, and electrical/gas line work are not included.
- Does not verify local permit requirements, electrical/gas line capacity, or a specific manufacturer's actual product lineup.
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.