Heating oil K-factor calculator
Your K-factor says how fast your house burns oil compared with the cold outside. Enter the gallons burned between two fills and the heating degree days over the same dates, and the calculator gives K both ways round. Add a degree day forecast and it gives the gallons you should expect to burn and the days until the tank reaches your reserve.
What a K-factor is
Fuel Oil News calls it "a number showing how rapidly a home owner uses their fuel, often also referred to as a burn rate" (Fuel Oil News, Degree Days). K-factor is built from degree days, which count how far each day's mean temperature sits below 65 °F. EIA's example: a day with a high of 33 and a low of 25 has a mean of 29 °F, so 65 − 29 = 36 heating degree days (EIA, degree days). EIA also says where to find the numbers: some utilities print them on bills, and the National Weather Service Climate Prediction Center publishes historical degree-day data.
The ratio can run either way, so check which one a number means:
K = degree days ÷ gallons burned (degree days per gallon)
gallons per degree day = gallons burned ÷ degree days (the inverse, 1 ÷ K)
gallons expected = forecast degree days ÷ K
Energo, a New York supplier, writes that "K-factor is often expressed as degree days per gallon" and uses the same form for forecasting: "fuel usage = degree days / K-factor". A higher K means a house that gets more heat from each gallon. Energo's example: a building that used 800 gallons over 400 degree days burned 2 gallons per degree day, so K = 0.5 degree days per gallon. With 100 degree days in a cold week it expects 100 ÷ 0.5 = 200 gallons (Energo, forecasting heating oil demand). Energo adds that for a new property with no history some dealers start from about 3 to 5 degree days per gallon for moderately insulated buildings, and that "the lower that number, the thirstier the building". Its 0.5 example is a building far thirstier than that range, so treat it as an illustration of the arithmetic.
Worked example
The numbers in the form are mine, for illustration. A house burns 150 gallons over 900 heating degree days. K = 900 ÷ 150 = 6.0 degree days per gallon, and the inverse is 150 ÷ 900 = 0.167 gallons per degree day. Say next week is forecast at 210 degree days (an average of 30 a day, a mean of 35 °F). Gallons expected = 210 ÷ 6.0 = 35. In a 275 gallon tank holding 150 gallons with a 10% reserve (27.5 gallons), that is 5 gallons a day, and (150 − 27.5) ÷ 5 = 24.5, so about 24 days to the reserve.
K both ways, and what it burns
A few K values from the thirsty end to the thrifty end, with gallons for 100 degree days and for one cold day of 30 degree days (a mean of 35 °F).
| K (degree days per gallon) | Gallons per degree day | Gallons per 100 degree days | Gallons on a 30 degree day |
|---|---|---|---|
| 2 | 0.500 | 50.0 | 15.0 |
| 3 | 0.333 | 33.3 | 10.0 |
| 4 | 0.250 | 25.0 | 7.5 |
| 5 | 0.200 | 20.0 | 6.0 |
| 6 | 0.167 | 16.7 | 5.0 |
| 8 | 0.125 | 12.5 | 3.8 |
| 10 | 0.100 | 10.0 | 3.0 |
Delivery reserves
Fuel Oil News says automatic deliveries are set with a usable reserve to avoid a run-out, based on the most common tank sizes, and gives these examples as approximate:
| Tank | Reserve | As a share of the tank |
|---|---|---|
| 275 gal | 75 gal | 27% |
| 550 gal | 150 gal | 27% |
| 1,000 gal | 250 gal | 25% |
Those work out to about a quarter of the tank, so a reserve of 25% in the form matches them. My default is 10%, the same as the run-out calculator. Energo suggests not going below about 1/8 of a tank, partly because very low levels can stir up sediment from the bottom.
What moves your K
- Energo lists new windows, tenant behavior and boiler efficiency changes as things that shift a K-factor, so recalculate after any of them.
- My suggestion: if the boiler also makes your hot water, that oil burns all year. A K worked out over a mild stretch then looks thirstier than the same house in January, so use fills from the heating season.
- My suggestion: a thermostat you turned down for a vacation, or up for a house guest, changes the gallons without changing the degree days. Leave those stretches out or note them.
- My suggestion: a fill that did not top the tank to the same level both times adds error. Use full-to-full fills, or two dips converted with the tank calculator.
- Energo notes the method assumes consistent use, and that thermostat controls shutting off in mild weather can make it over- or under-predict in the shoulder seasons.
Related
- Heating oil use per day: gallons a day at outdoor temperatures, from gallons per degree day.
- How long will heating oil last? Days left from two dated dips, no degree days needed.
- Heating oil use per month: a yearly total split by month.
- 275 gallon oil tank chart: inches on the stick to gallons.