What this calculator does: You enter your area’s annual heating degree days (or, if you do not know them, your USDA hardiness zone), your home’s heated square footage and how well it is insulated. The calculator estimates how much heat your home loses over a winter, in BTUs, and converts that into the amount of each common heating fuel needed to supply it, using an efficiency for each heating system that you can change.
Who it’s for: Off-grid and rural homeowners deciding which fuel to plan around, or working out how much wood, propane or oil to stock for winter.
The answer it produces: The heat your home needs per year in BTUs, plus the amount of nine fuels needed to deliver it at the efficiency shown for each: heating oil, propane, natural gas, firewood, wood pellets, wood chips, kerosene, biogas and coal. The nine lines are alternatives to compare, not amounts to add together.
Important limitation: This is a planning estimate. It multiplies a whole-house heat-loss figure by the heating degree days of your climate, and it gets that heat-loss figure from your square footage and a four-step insulation choice unless you supply a measured number. It does not know your windows, air leakage, thermostat settings or sun exposure. The efficiencies are planning defaults, so replace them with the ratings of your own equipment. If you enter only a hardiness zone, the degree-day figure is a rough stand-in and can be well off.
Off-Grid Heating Fuel Calculator
For the best result, enter your town’s annual heating degree days (base 65°F). You can find a 30-year normal for a weather station near you in the NOAA U.S. Climate Normals, or ask your local weather service. If you leave that box blank, the tool will estimate degree days from your USDA hardiness zone (1 to 13), which you can look up by ZIP code on the USDA Plant Hardiness Zone Map. Do not enter a building-code climate zone, because those are numbered in the opposite direction.
Heating Fuel Calculator
How the Calculation Works
Every time you press Calculate, the tool checks your entries and then runs five steps. It is plain arithmetic, and the math runs in your browser.
- Heating degree days. The tool uses the degree days you enter. A heating degree day is a day’s average outdoor temperature below 65°F, counted in degrees, and the annual total measures how much heating your climate needs. If you leave the box blank and enter a hardiness zone instead, it looks up a rough figure for that zone (see the table below).
- Whole-house heat loss. Square footage × a heat-loss figure per square foot for your insulation choice, in BTU per hour per °F: 0.30 for Poor, 0.20 for Fair, 0.14 for Good and 0.09 for Very Good. If you know your own heat-loss figure from an energy audit, you can enter it under Adjust assumptions, and it replaces the square-footage estimate.
- Heat needed per year. Heat loss × heating degree days × 24 hours. This is heat delivered into your rooms, in BTU.
- Fuel burned. The heat needed ÷ the efficiency of the heating system for that fuel. The defaults are 80% for oil, propane, natural gas, kerosene, biogas and pellets, and 70% for firewood, wood chips and coal, and you can change any of them.
- Fuel conversion. The fuel BTUs ÷ the heat content of each fuel: 138,000 per gallon of heating oil, 91,500 per gallon of propane, 100,000 per therm of natural gas, 20,000,000 per cord of wood, 16,000,000 per ton of pellets, 12,000,000 per ton of wood chips, 135,000 per gallon of kerosene, 22,000 per cubic meter of biogas and 24,500,000 per ton of coal.
The heat content figures line up closely with the U.S. Energy Information Administration’s published values for oil, propane, natural gas and firewood. When you enter only a zone, the tool uses these rough annual heating degree days, rounded from city normals:
| Zone entered | Degree days used | Zone entered | Degree days used |
|---|---|---|---|
| 1 or 2 | 13,000 | 8 | 3,000 |
| 3 | 10,500 | 9 | 1,800 |
| 4 | 9,000 | 10 | 900 |
| 5 | 7,500 | 11 | 400 |
| 6 | 6,000 | 12 | 150 |
| 7 | 4,900 | 13 | 50 |
For scale, Golden Gate Weather Services lists 1981 to 2010 annual normals of about 7,580 degree days for Minneapolis, 6,340 for Chicago, 5,681 for Boston, 3,247 for Raleigh and 2,768 for Atlanta. Hardiness zones are based on winter minimum temperatures, not on how much heat a house uses, so a zone gives only an approximate degree-day figure. It is least reliable in zones 9 and above, where mild but cloudy or cool areas can need far more heat than warm areas in the same zone.
Inputs and Assumptions
These are the fields you fill in:
| Input | What to enter | Notes |
|---|---|---|
| Annual Heating Degree Days (base 65°F) | Your location’s yearly total | 0 to 20,000. Overrides the zone if both are filled in. |
| Climate Zone (1 to 13) | Your USDA hardiness zone, as a whole number | Used only if the degree-day box is blank. A number outside 1 to 13 shows an error, and if both boxes are blank the tool asks for one of them. |
| Home Square Footage | Heated floor area | Count only the space you actually heat. Must be more than zero unless you enter a heat-loss figure. |
| Insulation Quality | Poor, Fair, Good or Very Good | Starts on Fair. A judgment call: poor means little insulation and a drafty house, and very good means high-R insulation with careful air sealing. |
| Known Whole-House Heat Loss (BTU per hour per °F) | Optional, under Adjust assumptions | From an energy audit or heat-loss calculation. If you enter it, it replaces the square-footage and insulation estimate. |
| Heating system efficiency by fuel (%) | Optional, under Adjust assumptions | 1 to 100 for each of the nine fuels, starting at the defaults below. |
And these are the assumptions built into the tool:
| Assumption | Value used |
|---|---|
| Winter severity | The degree days you enter, or a rough figure from your zone |
| Heat loss per square foot | 0.30, 0.20, 0.14 and 0.09 BTU per hour per °F for Poor, Fair, Good and Very Good, unless you enter your own |
| Heat delivered | Heat loss × degree days × 24, which counts heat only on days and hours when it is colder outside than the 65°F degree-day base |
| Default efficiencies | 80% for oil, propane, natural gas, kerosene, biogas and pellets, and 70% for firewood, wood chips and coal, all editable |
| Fuel heat content | Fixed values per unit, listed above |
| Not included | Air leakage beyond the insulation choice, windows, ceiling height, thermostat settings, sun exposure, fuel moisture and any heat sources other than the fuel |
The heat-loss figures per square foot are planning values, not measurements. They were chosen so that a typical home lands near the season heat requirement the previous zone-based version of the tool produced for a mid-cold climate. For example, a 1,500 square foot home with Poor insulation in zone 6 comes out at 64.8 million BTU in both versions. If you have an audit figure, use it instead.
What the Result Means
- The basis line. The first line tells you which degree days and heat-loss figure the tool used and where they came from (entered by you, or estimated from your zone or square footage). Check it before you read the rest.
- Heat your home needs per year. The estimated heat your home needs over the year, in BTU. This is heat delivered into your rooms, not fuel burned.
- The fuel lines. Each row shows how much of that one fuel it takes to supply the full heat requirement at the efficiency in the last column. Pick the fuel you plan to use and ignore the rest.
- The efficiency column. The efficiency the tool used for that row. Change it in the assumptions section to match your equipment, then calculate again.
- Error messages. A degree-day figure outside 0 to 20,000, a zone outside 1 to 13, missing square footage without a heat-loss figure, or an efficiency outside 1 to 100 stops the calculation and names the problem.
Example Calculation
Take a 1,500 square foot home in hardiness zone 6, entering only the zone. The tool uses about 6,000 degree days. With Fair insulation the heat loss is 1,500 × 0.20 = 300 BTU per hour per °F, so the heat needed is 300 × 6,000 × 24 = 43,200,000 BTU a year. Insulation changes the result:
| Insulation | Heat loss per sq ft | Whole-house heat loss | Heat needed per year |
|---|---|---|---|
| Poor | 0.30 | 450 | 64,800,000 BTU |
| Fair | 0.20 | 300 | 43,200,000 BTU |
| Good | 0.14 | 210 | 30,240,000 BTU |
| Very Good | 0.09 | 135 | 19,440,000 BTU |
With Fair insulation (43,200,000 BTU) and the default efficiencies, the fuel lines read:
| Fuel | Calculation | Amount |
|---|---|---|
| Heating oil | 43,200,000 ÷ 0.80 ÷ 138,000 | 391.30 gallons |
| Propane | 43,200,000 ÷ 0.80 ÷ 91,500 | 590.16 gallons |
| Natural gas | 43,200,000 ÷ 0.80 ÷ 100,000 | 540.00 therms |
| Firewood | 43,200,000 ÷ 0.70 ÷ 20,000,000 | 3.09 cords |
| Wood pellets | 43,200,000 ÷ 0.80 ÷ 16,000,000 | 3.38 tons |
| Coal | 43,200,000 ÷ 0.70 ÷ 24,500,000 | 2.52 tons |
The tool also lists wood chips, kerosene and biogas in the same way. Read down the column for the fuel you plan to use.
Adjusting the example for the real world. Efficiency is now part of the answer, and you can change it. A wood stove at 60% instead of the default 70% turns 3.09 cords into 3.60 cords. Propane in a 95% condensing furnace needs 43,200,000 ÷ 0.95 ÷ 91,500 = 497 gallons instead of 590 at 80%. The Department of Energy lists advanced-combustion wood stoves at roughly 65% to 75% efficiency and EPA-certified pellet stoves at roughly 70% to 83%, and its furnace guidance shows ratings from 80% for standard gas furnaces to 90% to 95% for ENERGY STAR condensing models.
Degree days matter more than the zone. If you enter 6,300 degree days instead of using the zone estimate, the same house needs 45,360,000 BTU. Chicago’s 1981 to 2010 normal of about 6,340 would give about 45,650,000 BTU, while a Raleigh-like 3,247 would give about 23,380,000 BTU and a Minneapolis-like 7,580 about 54,580,000 BTU, all for the same 300 BTU per hour per °F house. Doubling the square footage in the tool exactly doubles every number, and entering a measured heat-loss figure replaces the whole square-footage estimate.
For comparison, the U.S. Energy Information Administration reports from its 2020 Residential Energy Consumption Survey an average of about 45 million BTU of natural gas per household for space heating, and about 52 million and 63 million for the Northeast and Midwest. Those figures cover homes of all sizes and measure fuel input, not delivered heat, so use them only as a rough sense of scale. In this example, 540 therms of natural gas at 80% efficiency is 54 million BTU of fuel.
Why electric resistance heat does not belong on a small solar system. One kilowatt-hour equals 3,412 BTU, so 43,200,000 BTU is about 12,660 kWh a year. Even spread over all 365 days, that is roughly 35 kWh a day, far beyond the 2.9 kWh a day of the small cabin sized on our off-grid solar calculator page, and heating demand is concentrated in the coldest weeks. Plan heat from fuel or a heat pump instead.
How to Use the Result in a Real System
- Use degree days if you can. Look up the annual heating degree days for your nearest weather station. A zone estimate is a rough substitute, and it can be off by a large margin.
- Be honest about insulation. Moving one step from Poor to Very Good changes the result by a lot, so if you are between two choices, pick the worse one. Better still, get an energy audit and enter its heat-loss figure.
- Enter your equipment’s real efficiency. The Department of Energy lists advanced-combustion wood stoves at roughly 65% to 75% efficiency and EPA-certified pellet stoves at roughly 70% to 83%. Its furnace guidance shows ratings from 80% for standard gas furnaces to 90% to 95% for ENERGY STAR condensing models. Use the rating of your own unit.
- Add a cushion. Degree days are a 30-year average, so a colder-than-average winter needs more. Plan a reserve, and for wood, remember it has to be seasoned to about 20% moisture or less before it delivers its rated heat.
- Compare fuels by cost, not just quantity. Multiply the amount shown by your local price per unit. Firewood cut on your own land changes that math completely. Our firewood calculator goes deeper on wood.
- Cut the load before you buy fuel. Air sealing and insulation are usually the cheapest way to shrink every number on this page. A home energy audit can show where your house leaks.
- Have equipment sized properly. This tool estimates a season’s energy. It does not tell you how large a furnace or stove you need, which is a separate calculation for a heating professional.
Important Variables That Can Change the Result
- Local climate. Degree days capture how cold your winters are, so entering them removes most of the error a hardiness zone brings. A zone is based on the coldest night of the year, not the total heating demand.
- Air leakage. A drafty house can lose more heat through leaks than through its walls, and the tool only knows your insulation choice.
- Windows, doors and ceiling height. Lots of glass or tall ceilings raise heat loss beyond what square footage alone suggests.
- Equipment efficiency. It is now an input, and it is still one of the largest gaps between the estimate and reality if you leave the defaults on the wrong system.
- Thermostat habits and occupancy. Degree days assume a 65°F base. Heating to 72°F all day and heating to 62°F when you are out are very different loads.
- Fuel quality. Firewood species and moisture change the heat per cord, and wet wood wastes energy drying itself out.
- Sun exposure and passive solar. A well-oriented home with good southern glazing can gain useful heat that the tool ignores.
- Heating only part of the house. A single wood stove in a large home may warm only the rooms near it, so the tool’s whole-house figure will not match what you burn.
Common Mistakes
- Entering a building-code climate zone. Those zones number from hot to cold, the opposite of hardiness zones, and would push the estimate the wrong way.
- Entering both degree days and a zone and expecting a blend. The degree days win. The zone is only used when that box is blank.
- Leaving the efficiencies at the defaults. They are planning values. A 60% stove or a 95% furnace changes the fuel amount noticeably.
- Adding the fuel lines together. Each line is a full alternative for the whole house.
- Counting unheated space. Garages, unfinished basements and attics do not belong in the square footage, unless you heat them.
- Rating insulation too generously. Most people think their house is better insulated than it is.
- Burning unseasoned wood. Wood that is too wet delivers far less heat than the calculator assumes.
- Typing a half zone. The zone box takes a whole number, so enter 7 for 7a or 7b. Fractions are rounded.
When This Calculator Should NOT Be Used
- To size a furnace, boiler or stove. It gives seasonal energy, not the hourly heat output a system needs at the coldest moment.
- For heat pump or electric heating estimates. It has no electric option, and heat pumps deliver far more heat per kWh than resistance heating.
- For unusual buildings. Earth-sheltered homes, insulated concrete form homes, super-insulated builds and homes with large passive solar gains behave very differently from the average the tool assumes. If you know your building’s heat loss, enter it.
- In warm zones on a zone estimate alone. In zones 9 and above the hardiness zone is a weak guide to heating demand, so enter degree days or skip the tool.
- For commercial buildings, barns and greenhouses. The heat-loss values per square foot are for ordinary homes.
- When you need a precise number. A professional energy audit or heat-loss calculation will beat any square-footage estimate.
Related Honey Hen Calculators
- Firewood Calculator: a closer look at how much wood you need if wood is your fuel.
- Off-Grid Solar Calculator: size your electricity separately, and keep heat off the solar budget.
- Off-Grid Budget Calculator: estimate the total cost of your off-grid setup.
- Off-Grid Home Cooling: cooling options, including DC heat pumps that can also heat.
- All Homestead Calculators: the full set of planning tools in one place.
Related Honey Hen Articles
- Off-Grid Passive Solar Home Materials to Use
- Earth-Sheltered Homes Passive Solar Design
- ICF Home Construction Guide: Durable & Energy-Efficient
- Energy-Efficient Off-Grid Appliances: 10 Picks for Homesteads
- Energy Independence Off-Grid Living: Power Options
Authoritative Sources
- NOAA National Centers for Environmental Information: U.S. Climate Normals. Look up 30-year normals for a weather station near you, including heating degree days.
- Golden Gate Weather Services: Normal Heating Degree Days. Annual heating degree day normals (base 65°F, 1981 to 2010) for U.S. cities, used for scale above.
- USDA Plant Hardiness Zone Map. Look up your zone by ZIP code. The map is based on the average annual extreme minimum winter temperature, which is why it is only a rough guide to heating demand.
- U.S. Energy Information Administration: British Thermal Units. Published heat content for heating oil, propane, natural gas and a cord of wood.
- U.S. Energy Information Administration: 2020 Residential Energy Consumption Survey, Average Space Heating Natural Gas Consumption. Household space-heating gas use by region, for scale.
- U.S. Department of Energy: Wood and Pellet Heating. Efficiency ranges for wood and pellet stoves, and guidance on seasoned wood.
- U.S. Department of Energy: Purchasing Energy-Efficient Residential Furnaces. Shows furnace efficiency (AFUE) ratings from standard to ENERGY STAR condensing models.
- U.S. Department of Energy: Insulation. Explains R-value, air sealing and why installation quality matters.
- U.S. Department of Energy: Home Comfort. A starting point for energy audits, weatherization and heating system options.
Off-Grid Heating Calculator FAQ
Should I enter degree days or my climate zone?
Enter degree days if you can find them. They measure how much heating your climate needs, and they are much more accurate than a zone. You can look up a 30-year normal for a weather station near you in the NOAA climate normals. The zone box is only a fallback, and the tool uses it only when the degree-day box is blank.
How do I find my climate zone?
Enter your USDA plant hardiness zone, which you can find by ZIP code on the USDA map linked above. Use the whole number, so 7 for zone 7a or 7b. Lower numbers are colder. Remember that the zone is based on winter minimum temperatures, not on heating demand.
Which heating fuel is cheapest for an off-grid home?
It depends on local fuel prices, availability and whether you have free or low-cost wood. The calculator converts your heat requirement into several fuels, each at the efficiency you set, so you can compare them. Multiply each amount by your local price.
How much does insulation quality actually matter?
A lot. In the calculator, moving from Poor to Very Good insulation cuts the estimated heat need by 70% (0.30 to 0.09 BTU per hour per °F per square foot), and moving from Fair to Good cuts it by 30%. Air sealing and insulation upgrades are often the most cost-effective way to reduce ongoing heating costs.
Can I use my own heat-loss figure?
Yes. Open Adjust assumptions and enter your whole-house heat loss in BTU per hour per °F, which an energy audit or heat-loss calculation can give you. It replaces the square-footage and insulation estimate.
Why is my real fuel use higher than the calculator says?
The most common reasons are equipment efficiency lower than the default, air leaks, wet wood, heating to a warmer temperature than the 65°F degree-day base, and a winter colder than the 30-year average. If you have last winter’s fuel bills, use them to check the numbers and adjust the efficiencies.
How many cords of wood do I need?
The firewood row already includes the efficiency you set for your stove, which starts at 70%. In the example above, 3.09 cords is the answer at 70%, and it rises to about 3.60 cords at 60%. Add a reserve for a cold winter, and see our firewood calculator for more detail.
Can I use this to size a furnace or wood stove?
No. It estimates a whole season’s heat, not the hourly output a heater needs on the coldest night. Equipment sizing should come from a heating professional or an energy audit.
Does this calculator account for a backup heat source?
No. It estimates the fuel for a single heat source. If you plan a primary and backup system, such as wood plus propane, run it once for your expected primary fuel and treat the backup fuel as a separate reserve.
This calculator provides general planning estimates, not an engineering heat-loss calculation. Actual fuel needs depend on your home’s specific construction, air sealing, appliance efficiency, and local weather variation — consult a qualified HVAC professional or energy auditor for a precise assessment.
