Furnace Sizing Calculator

Heating BTU load from square footage, climate zone, insulation, and ceiling height — then the furnace input rating you actually shop for

This is the heating-side counterpart to the BTU / tonnage cooling load calculator. Climate-zone BTU/sqft ranges, insulation and ceiling adjustments, AFUE-to-input conversion, and a round-up to a standard furnace size. Walkthrough: furnace sizing guide. Wrong-size symptoms: troubleshooting. When to stop guessing: rule of thumb vs Manual J heat loss. Cooling-side twin: rule of thumb vs Manual J cooling load. HVAC index: HVAC calculators hub.

Starting point only — not a substitute for ACCA Manual J heat-loss or a contractor’s load calculation. This tool uses published climate-zone BTU-per-square-foot ranges (average insulation, 8 ft ceilings) plus coarse insulation and ceiling-height knobs. It does not account for window area, orientation, infiltration, design outdoor temperature, or room-by-room losses the way Manual J does. Oversizing short-cycles and wears the heat exchanger; undersizing never reaches set point. Use this to ballpark a quote — then pay for a real heat-loss calc before you buy.

Embed this calculator Free · no API key · keep the attribution link

Copy the snippet below to embed this calculator on your site. The attribution link must stay intact and dofollow (no rel="nofollow"). Browse all embeddable calculators

Theme Width

<div class="ttq-embed" style="max-width:100%;margin:0 auto;">
  <iframe src="https://testtalkhq.com/embed/furnace-sizing-calculator/?theme=light&utm_source=embed" width="100%" height="980" frameborder="0" loading="lazy" scrolling="no" title="Furnace Sizing Calculator — TestTalkHQ" style="border:0;max-width:100%;width:100%;"></iframe>
  <p style="font:400 13px/1.4 system-ui,sans-serif;text-align:center;margin:6px 0 0;">Powered by the <a href="https://testtalkhq.com/furnace-sizing-calculator/?utm_source=embed&utm_medium=referral&utm_campaign=calculator_embed&utm_content=furnace-sizing-calculator" target="_blank" rel="noopener">Free Furnace Sizing</a> from <a href="https://testtalkhq.com/?utm_source=embed&utm_medium=referral&utm_campaign=calculator_embed&utm_content=furnace-sizing-calculator" target="_blank" rel="noopener">TestTalkHQ</a></p>
</div>

<div class="ttq-embed" style="max-width:100%;margin:0 auto;">
  <iframe src="https://testtalkhq.com/embed/furnace-sizing-calculator/?theme=light&utm_source=embed" width="100%" height="980" frameborder="0" loading="lazy" scrolling="no" title="Furnace Sizing Calculator — TestTalkHQ" style="border:0;max-width:100%;width:100%;"></iframe>
  <p style="font:400 13px/1.4 system-ui,sans-serif;text-align:center;margin:6px 0 0;">Powered by the <a href="https://testtalkhq.com/furnace-sizing-calculator/?utm_source=embed&utm_medium=referral&utm_campaign=calculator_embed&utm_content=furnace-sizing-calculator" target="_blank" rel="noopener">Free Furnace Sizing</a> from <a href="https://testtalkhq.com/?utm_source=embed&utm_medium=referral&utm_campaign=calculator_embed&utm_content=furnace-sizing-calculator" target="_blank" rel="noopener">TestTalkHQ</a></p>
</div>
<script>(function(){function r(e){if(!e||!e.data||e.data.type!=="ttq-embed-height")return;var h=+e.data.height;if(!h)return;document.querySelectorAll("iframe[src*='testtalkhq.com/embed/']").forEach(function(f){if(!e.source||f.contentWindow===e.source)f.style.height=h+"px"})}window.addEventListener("message",r)})();</script>

How to Use This Furnace Sizing Calculator

Enter heated square footage. Pick the climate zone that matches your region. Set insulation quality, average ceiling height, and the furnace AFUE you are quoting or replacing. Read the output range, the midpoint recommended output, the input rating (output ÷ AFUE), and the standard size to shop.

Output BTU = sq ft × climate BTU/sqft × (1 + insulation %) × (1 + 12.5% per ft above 8 ft)
Furnace input BTU = output BTU ÷ AFUE
Shop size = round input up to 40k / 60k / 80k / 100k / 120k

Worked example — 2,000 sq ft, Zone 4, average, 8 ft, 80% AFUE Base output = 2,000 × 45 to 50 = 90,000–100,000 BTU/hr. Midpoint 95,000.
No insulation or ceiling adder.
Input = 95,000 ÷ 0.80 = 118,750 BTU/hr.
Round up to the next standard size: 120,000 BTU furnace.

Walk through climate zones, AFUE, and why you shop input not output: furnace sizing guide. If the existing unit short-cycles or never catches up: troubleshooting. When this ROT is not enough: rule of thumb vs Manual J heat loss.

What This Number Is (and Is Not)

The number you buy on a furnace is the input rating — gas the burner consumes. The house needs output. AFUE is the ratio. An 80% AFUE 100,000 BTU furnace delivers about 80,000 BTU/hr to the ductwork. Size the output to the heating load, then divide by AFUE to get the catalog input.

Climate-zone BTU/sqft is a published rule of thumb, not a room-by-room heat-loss takeoff. It does not know your windows, wind exposure, or design outdoor temperature. That is why the result is a range plus a midpoint, matching how the cooling-load calculator presents sq-ft-per-ton bands.

Rounding up is not a license to oversize. Standard cabinets jump 20,000 BTU at a time. A 105,000 input lands on a 120,000 unit. That gap is exactly why Manual J exists for the final purchase. Cooling-side twin: rule of thumb vs Manual J cooling load.

HVAC

HVAC Refrigerant Charging Card

$3.99

Buy

Complete HVAC Charging Guide

$12.99

Buy

HVAC Job Estimator

$29.00

Buy

BEST VALUE

HVAC Complete Bundle

$39.00

Buy

See all HVAC products

Checkout opens here — you stay on this page.

Furnace Comfort & Diagnosis Gear

A BTU estimate is only a starting point. After install (or when a furnace misbehaves), confirm runtime, air-side split, and static before you chase the wrong size. These products are from the live TestTalkHQ affiliate CSV — thermostats, manometers, and infrared thermometers. Furnace filters and duct sealant were not in the CSV.

ecobee Smart Thermostat Premium

Cycle history and remote sensors show short-cycling versus a furnace that never reaches set point — the two classic sizing failure modes.

View on Amazon →

Honeywell T6 Pro programmable thermostat

Honeywell T6 Pro Thermostat

A stable setpoint and cycle count beat guessing whether the new cabinet is too big or the old one was too small.

View on Amazon →

Fieldpiece SDMN5 Dual-Port Manometer

External static is not furnace BTU, but high TESP starves airflow and looks like an undersized heat exchanger. Measure before you upsize.

View on Amazon →

Dwyer Series 475 Mark III digital manometer

Dwyer 475 Mark III Manometer

Handheld differential pressure for filter drop, coil drop, and total external static versus the blower nameplate.

View on Amazon →

Klein Tools IR5 dual laser infrared thermometer

Klein Tools IR5 Infrared Thermometer

Non-contact supply and return temps for a heating-mode delta-T check after the furnace runs. Pair with the delta-T calculator.

View on Amazon →

As an Amazon Associate, TestTalkHQ earns from qualifying purchases. Product links are from the live TestTalkHQ affiliate CSV only.

Furnace Sizing Calculator

How to Use This Furnace Sizing Calculator

What This Number Is (and Is Not)