Air Change Rate (ACH) Calculator
Calculate the air changes per hour (ACH) for a room or building. Determine ventilation rate from airflow volume and room size.
Heat Loss & Degree Day Calculator
Calculate home heat loss using heating degree days. Estimate seasonal heating energy consumption and costs based on HDD and building thermal performance.
Total building heat loss rate
BTU/hr/deg F
Annual HDD for your location (base 65 deg F)
sq ft
AFUE or COP x 100
%
$/therm
Annual Heating Load⧉
66.0M BTU
Total energy needed
Fuel Required⧉
717 therms
At 92% efficiency
Annual Heating Cost⧉
$933.00
Total estimated yearly cost
Monthly Average⧉
$78.00
Averaged across 12 months
Cost per sq ft⧉
$0.47
2,000 sq ft home
Daily Loss per HDD⧉
12,000 BTU/day
Heat escaping per degree-day
BTU per sq ft⧉
33,000
Annual load intensity
Peak Day Cost⧉
$11.02
Design day at 0 deg F outdoor
Annual Cost Scale
$933.00
Heating Cost by City (Your UA + Efficiency)
| City | HDD | Load (M BTU) | Fuel Units | Annual Cost |
|---|---|---|---|---|
| Phoenix, AZ | 1,125 | 13.5 | 147 | $191.00 |
| Houston, TX | 1,525 | 18.3 | 199 | $259.00 |
| Atlanta, GA | 2,990 | 35.9 | 390 | $507.00 |
| Washington, DC | 4,200 | 50.4 | 548 | $712.00 |
| New York, NY | 4,900 | 58.8 | 639 | $831.00 |
| Chicago, IL | 6,500 | 78 | 848 | $1,102.00 |
| Minneapolis, MN | 7,900 | 94.8 | 1,030 | $1,340.00 |
| Anchorage, AK | 10,500 | 126 | 1,370 | $1,780.00 |
Cost by Heating Efficiency
| Efficiency | Fuel Units | Annual Cost | vs Current |
|---|---|---|---|
| 70% | 943 | $1,226.00 | +$293.00 |
| 80% | 825 | $1,073.00 | +$140.00 |
| 85% | 776 | $1,009.00 | +$76.00 |
| 90% | 733 | $953.00 | +$20.00 |
| 92% | 717 | $933.00 | $0.00 |
| 95% | 695 | $903.00 | -$30.00 |
| 96% | 688 | $894.00 | -$39.00 |
| 98% | 673 | $876.00 | -$57.00 |
Planning notes, formulas, and examples
About the Heat Loss & Degree Day Calculator
Heating degree days (HDD) quantify how cold a location is over a season, and when combined with a building's heat loss rate, they predict total heating energy consumption. The heat loss rate (in BTU/hr per °F) depends on insulation, air leakage, window area, and building size.
The formula is straightforward: Total Heating Energy = Heat Loss Coefficient × Heating Degree Days × 24 hours/day. This gives you the seasonal BTU requirement, which can then be divided by furnace efficiency to get fuel consumption and cost.
This calculator combines your building's heat loss coefficient with local HDD data to estimate annual heating energy, fuel use, and cost. It's an essential tool for energy auditors, HVAC designers, and homeowners evaluating efficiency improvements.
This analytical approach supports both immediate cost reduction and long-term sustainability goals, helping organizations balance economic and environmental priorities in their energy management. By calculating this metric accurately, energy analysts gain actionable insights that inform equipment selection, system design, and operational strategies for maximum efficiency and savings.
When This Page Helps
Understanding how degree days translate to heating costs helps you evaluate insulation upgrades, window replacements, and air sealing projects. It turns abstract energy concepts into concrete dollar figures. Regular monitoring of this value helps energy teams detect usage anomalies early and address equipment malfunctions or operational issues before they drive utility costs higher.
How to Use the Inputs
- Enter your building's heat loss coefficient (BTU/hr/°F).
- Enter the heating degree days (HDD) for your location.
- Enter your heating system efficiency (AFUE or COP).
- Enter your fuel cost.
- Review seasonal heating energy and cost estimates.
Formula used
Annual Heating BTU = UA × HDD × 24
Fuel Required = Heating BTU / (Efficiency × Fuel BTU Content)
Annual Cost = Fuel Required × Fuel PriceExample Calculation
Result: $920/year heating cost
A home with UA=500 BTU/hr/°F in a 5,500 HDD climate: Annual BTU = 500 × 5,500 × 24 = 66,000,000 BTU. At 92% AFUE gas furnace: 66M / (0.92 × 100,000) = 717 therms. At $1.30/therm = $932/year.
Tips & Best Practices
- Typical UA values: well-insulated home 250–400, average home 400–600, poorly insulated 600–1,000+ BTU/hr/°F.
- HDD data is available from NOAA or degreedays.net for your location.
- The 65°F base temperature is standard for HDD calculations.
- Solar gains and internal heat reduce actual heating needs by 10–25%.
- Use this calculation to compare before/after scenarios for insulation upgrades.
- Energy auditors can estimate UA from blower door results and insulation inspection.
The Degree Day Method Explained
Degree days quantify the severity and duration of outdoor temperatures relative to a base temperature (usually 65°F). If the average daily temperature is 35°F, that day has 30 HDD. Summing all HDD over a heating season gives the total heating requirement proportional to climate severity.
From UA to Energy Cost
The path from building physics to dollar cost: (1) Determine UA from building components, (2) Multiply by local HDD × 24 to get seasonal BTU, (3) Divide by heating equipment efficiency, (4) Multiply by fuel price. Each step adds practical reality to the theoretical heat loss.
Using Degree Days for Upgrade Analysis
To evaluate an insulation upgrade: Calculate current UA, determine the new UA after the upgrade, and compare the annual costs. The difference is your annual savings. For example, reducing UA from 600 to 400 in a 6,000 HDD climate saves about (200 × 6,000 × 24) / (0.92 × 100,000) = 313 therms × $1.30 = $407/year.
Sources & Methodology
Last updated:
Frequently Asked Questions
UA represents total heat loss rate in BTU per hour per degree Fahrenheit of temperature difference between inside and outside. It combines all heat loss paths: walls, ceiling, floor, windows, doors, and air infiltration. Lower UA means better insulation.
NOAA provides historical HDD data by zip code. Common values: Minneapolis 7,800 HDD, Chicago 6,500, New York 4,800, Atlanta 2,800, Miami 200. You can also use degreedays.net for custom base temperatures.
Divide your annual heating fuel consumption by (HDD × 24 / (efficiency × fuel BTU content)). Or estimate from building components: UA = Σ(Area × U-value) for each surface plus infiltration losses. Energy auditors calculate UA professionally.
HDD uses 65°F as the base because internal heat gains (appliances, people, lights) typically provide 5–10°F of warming. So heating isn't needed until the outdoor temperature drops below about 65°F, even with a 70°F thermostat setpoint.
For seasonal totals, the degree day method is within 10–20% of actual energy use for typical buildings. It doesn't account for solar gains, varying thermostat settings, or wind effects, but it's excellent for comparing scenarios.
Yes, apply the same formula using cooling degree days (CDD) and your AC efficiency (SEER). Annual Cooling BTU = UA × CDD × 24. However, cooling loads are also affected by solar gain and humidity, making cooling predictions less precise.
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