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What Size Furnace Do I Need? Heat Load Calculator from Your Utility Bills

Skip the $300 Manual J — Size Your Furnace or Heat Pump from Real Gas, Propane, or Electric Bills

Stop guessing at furnace size. Enter your actual heating bills and this calculator estimates your home's real heat load in BTU/hr — no tape measure, no room-by-room measurements, no expensive Manual J study. Just your fuel type, monthly consumption, and outdoor temperatures. See how your BTU/hr per square foot compares to homes of similar age in your ASHRAE climate zone, and get specific weatherization recommendations with estimated payback periods. The same approach HVAC contractors use for quick-sizing, now free in your browser.

Pro Tip: Use the burn rate at different outdoor temperatures to see how your home performs across the heating season, not just at design conditions. This reveals insulation weak spots that only show up in moderate cold.

Compare mini-split vs furnace costs using your heat load

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Heat Load from Bills Calculator
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How It Works

  1. Select Your Fuel Type

    Choose from natural gas (therms or CCF), propane (gallons), fuel oil (gallons), electric resistance (kWh), wood pellets (tons), or cord wood (cords). The calculator applies the correct BTU content and typical equipment efficiency for each fuel.

  2. Enter Your Bill Data

    Enter monthly fuel consumption and the corresponding average outdoor temperature for each billing period. Use 3-6 months of heating season data for best accuracy. Your utility company can provide historical usage and weather data.

  3. Set Your Climate Zone

    Select your ASHRAE climate zone (1 through 7, covering the entire US and Canada). This determines your local 99% design temperature - the coldest temperature your heating system needs to handle.

  4. Enter House Details

    Provide your home's square footage, year built, and number of stories. The calculator uses these to generate BTU/hr per square foot benchmarks and compare your home against typical performance for its era.

  5. View Results & Benchmarks

    See your estimated heat load in BTU/hr at design temperature, BTU/hr per square foot, and a tier classification (Excellent, Good, Average, Poor). A gauge visualization shows where your home falls relative to benchmarks for its age.

  6. Review ROI Recommendations

    Get up to 7 improvement measures ranked by estimated savings. See which upgrades - attic insulation, air sealing, window replacement - deliver the best return based on your specific heat load and fuel costs.

Built For

  • Homeowners sizing a new furnace or heat pump without paying for a Manual J calculation
  • Checking whether a contractor's proposed equipment size matches your actual heating needs
  • Comparing your home's energy performance against benchmarks for houses of similar age and size
  • Evaluating a home before purchase to understand potential heating costs and upgrade needs
  • Weatherization planning - identifying which improvements will deliver the most savings
  • DIY energy auditors estimating building heat loss from accessible utility bill data
  • Landlords and property managers benchmarking heating costs across a portfolio of buildings

Features & Capabilities

6 Fuel Types Supported

Natural gas (therms, CCF, cubic meters), propane (gallons), fuel oil (#2 heating oil in gallons), electric resistance (kWh), wood pellets (40-lb bags or tons), and cord wood. Each fuel type uses standard BTU content and assumes typical equipment efficiency.

24 ASHRAE Climate Zones

Covers all US and Canadian climate zones from hot-humid Zone 1 to subarctic Zone 7. Each zone provides the correct 99% design temperature for sizing calculations, so your results reflect real-world cold weather conditions in your area.

Automatic Benchmark Scaling

Benchmarks adjust based on your home's year built. Pre-1980 homes typically run 35-70 BTU/hr per square foot. Homes built 1980-2000 average 28-45. Modern code-built homes from 2010 and later should fall between 15-30 BTU/hr/ft2.

Tier Classification System

Your result is classified as Excellent, Good, Average, or Poor relative to homes of similar vintage. This tells you at a glance whether your home performs well or has significant air sealing and insulation opportunities.

Gauge Visualization

A visual gauge shows your BTU/hr/ft2 on a color-coded scale from green (well-insulated) to red (leaky). More intuitive than raw numbers for homeowners who want a quick read on their home's performance.

7 ROI Improvement Measures

Ranked recommendations including attic insulation, wall insulation, air sealing, window upgrades, basement/crawlspace insulation, duct sealing, and heat pump conversion. Each shows estimated BTU/hr reduction and simple payback period.

Diagnostics Panel

Shows the intermediate calculations - baseload subtraction, fuel-to-BTU conversion, temperature regression - so you can verify the math and understand exactly how the estimate was derived.

Comparison

House Era Typical BTU/hr/ft2 Tier Classification Common Issues
Pre-1950 45-70+ Often Poor No wall insulation, single-pane windows, major air leaks
1950-1979 35-55 Average to Poor Minimal insulation, original windows, uninsulated ducts
1980-1999 28-45 Average Some insulation, double-pane windows, better air sealing
2000-2009 22-35 Good to Average Code-minimum insulation, improved windows, some air sealing
2010-Present 15-30 Good to Excellent Modern code insulation, low-E windows, blower door tested
Passive House 5-12 Excellent Superinsulated, triple-pane, continuous air barrier, HRV

Frequently Asked Questions

It uses your real heating bills to figure out how much heat your house loses on a cold day, measured in BTU per hour. That number tells you what size furnace or heat pump you need. Instead of measuring every wall, window, and door, it works backward from how much fuel your house actually burns.
For most homes, this gets within 10-20% of a full Manual J analysis. Manual J accounts for every window, wall orientation, and insulation detail. The bills-based method captures the same physics through actual measured fuel consumption. It is accurate enough for equipment sizing decisions and upgrade planning, but a professional Manual J is still recommended for new construction or major renovations.
Your utility company often provides this data on your bill or online account. You can also use Weather Underground or NOAA to look up monthly average temperatures for your ZIP code. Use the average temperature for the billing period, not the coldest day.
Yes. The calculator identifies your baseload - the non-heating energy usage like hot water, cooking, and dryer - by looking at your summer bills (when no heating is needed) and subtracts that from winter bills. This isolates the heating-only energy consumption.
For a reasonably insulated home built after 2000 in a cold climate (Zone 5-6), 20-30 BTU/hr/ft2 is typical. Under 20 is excellent. Over 40 means your home is losing a lot of heat and likely has significant insulation or air sealing opportunities. The benchmarks in the results table are scaled to your home's year built, so you can compare apples to apples.
Yes, that is the primary use case. Your estimated heat load at design temperature tells you the minimum BTU/hr output your equipment needs. For furnaces, divide by efficiency (e.g., 80,000 BTU load / 0.95 AFUE = 84,200 BTU input). For heat pumps, match the capacity at your design temperature, not just the rated capacity at 47 degrees F.
Natural gas: 100,000 BTU per therm (1,000 BTU per cubic foot). Propane: 91,500 BTU per gallon. Fuel oil (#2): 138,500 BTU per gallon. Electricity: 3,412 BTU per kWh. Wood pellets: 16.5 million BTU per ton. These are standard industry values.
It depends heavily on your home's age, insulation, and climate zone. Pre-1980 homes typically need 35-70 BTU/hr per square foot. Homes built 1980-2000 average 28-45 BTU/hr/ft2. Modern code-built homes from 2010 and later should fall between 15-30 BTU/hr/ft2. A well-insulated home in a moderate climate (Zone 4-5) might need only 20 BTU/hr/ft2, while a leaky old farmhouse in Zone 6-7 could need 50+ BTU/hr/ft2. This calculator tells you your specific number based on your actual heating bills.
For equipment sizing and upgrade planning, this bills-based method gets within 10-20% of a full Manual J analysis and is adequate for most decisions. Manual J is still the gold standard for new construction, permit applications, and situations where HVAC equipment must be precisely matched to the load. The advantage of the bills-based method is that it captures your home's actual performance - including real air leakage, occupant behavior, and existing insulation - rather than relying on assumptions from a software model.
Disclaimer: This calculator provides planning-level estimates based on utility bill data. It is not a substitute for a professional ASHRAE Manual J heat load calculation. Equipment sizing, permit applications, and system design should be verified by a licensed HVAC contractor. Actual building performance varies with construction quality, occupant behavior, and weather patterns.

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