Heating oil K-factor
K-factor is the number your oil dealer keeps on your account to predict when your tank will need oil. It is one division problem, you can compute it yourself from two delivery slips, and it explains how automatic delivery knows to show up before you run out.
The formula
K-factor is the heating degree days elapsed between two deliveries divided by the gallons delivered. The units are degree days per gallon: how much cold weather your house wrings out of each gallon. A lower K means the house burns oil faster. Typical homes land between 3 and 8.
How dealers use it
Take the gallons of usable oil remaining in your tank and multiply by K: that is your degree-day budget, the amount of cold weather you can cover before running dry. The dealer's degree-day calendar, a running count of degree days as the season unfolds, turns that budget into a calendar date, and the truck is routed to arrive before the budget runs out. That is automatic delivery: no gauge watching, just arithmetic on weather data.
With the example K of 6.0, a tank holding 100 usable gallons has a 600 degree-day budget. In mild fall weather that lasts a long time; in a January cold snap the same 600 degree days disappear much faster, which is exactly why the schedule runs on degree days instead of days on the calendar.
Compute your own from two slips
- Find two consecutive delivery slips. The dates matter, and so do the gallons on the second slip: that is what you burned between fills.
- Get the degree days between the two dates. NOAA publishes heating degree days for your area, and many dealers print the running total on the ticket itself.
- Divide degree days by gallons. The result is your K.
Log two deliveries and you can do this math yourself. Repeat it across a few intervals and you will see your own number settle, which also tells you whether your dealer's automatic schedule is keeping a sane margin in your tank.
What moves your K
- Insulation and air sealing raise it. The house extracts more degree days from each gallon, so deliveries spread out.
- Thermostat setbacks raise it. Less heating for the same weather has the same effect as a tighter house.
- An oil-fired water heater lowers the summer effective K. Hot water burns oil on days with zero degree days, so warm-weather intervals look thirstier than the formula expects.
Common questions
Is a higher K-factor better?
Higher means each gallon covers more degree days, so the house burns slower. But K is a planning number, not a grade: a small tight house and a large drafty one can both be comfortable with very different Ks. Typical homes land between 3 and 8.
Why is my computed K different from what my dealer uses?
Dealers usually average across several deliveries and adjust for hot water use, while one interval is noisy: a vacation, a thermostat change, or guests can skew any single computation. Compute K over two or three intervals before comparing notes.
Does an oil-fired water heater break the math?
It bends it. Hot water burns oil at zero degree days, which drags the effective K down in warm weather, so a single year-round K will misfire in summer. Many dealers handle this with a separate summer usage rate on the account.