Ask a fuel dealer and you'll hear "it depends," which is true but not much use when the gauge is heading for 1/4 and the forecast says cold. It depends on two things you can pin down: how much oil your house burns in a normal year, and how cold it's going to be. Here are the numbers.
Days 100 Gallons Lasts, by House and Temperature
Three typical oil-heated homes in a New England climate (about 5,700 heating degree days a year), each with the boiler also making hot water for three people. Temperatures are the average of day and night, not the daytime high.
| Outdoor average | Small or tight house ~500 gal/yr | Typical house ~800 gal/yr | Large or drafty house ~1,200 gal/yr |
|---|---|---|---|
| 45°F | 61 days (1.6 gal/day) | 37 days (2.7 gal/day) | 24 days (4.1 gal/day) |
| 35°F | 44 days (2.3 gal/day) | 26 days (3.8 gal/day) | 17 days (6.0 gal/day) |
| 25°F | 35 days (2.9 gal/day) | 20 days (5.0 gal/day) | 13 days (7.8 gal/day) |
| 15°F | 29 days (3.5 gal/day) | 16 days (6.1 gal/day) | 10 days (9.7 gal/day) |
So the honest answer for a typical house is about three to five weeks, dropping to around two weeks in a hard freeze. The spread between houses matters as much as the weather: the same 100 gallons lasts the large house only about a third as long as the small one.
For your own numbers — your annual usage, your tank and gauge reading, this week's forecast — use the heating oil and propane days-left calculator.
Why Cold Weather Burns Oil So Much Faster
Heat loss follows the gap between inside and outside, and the industry measures it in heating degree days: how far each day's average temperature falls below 65°F. A 45°F day is 20 degree days; a 25°F day is 40. Double the degree days, double the heating oil.
Hot water is the exception. If your boiler heats your water, it burns roughly a third to half a gallon a day for a family no matter the weather — which is why the tank still drops slowly in July, and why the cold-weather multiplier in the table is a little less than double.
Dealers turn this into a single number for your house, the K-factor: degree days per gallon. The typical house above has a K-factor of about 9; the small one about 16; the large one about 5. If your delivery ticket shows one, divide the degree days you expect by it to get gallons.
What Your Gauge Is Really Telling You
Before you count on 100 gallons, make sure you have them. A float gauge shows the oil's depth, not its volume, and the common 275-gallon tank has a rounded bottom that holds less per inch. On that tank:
- 1/2 is about 137 gallons
- 1/4 is about 58 gallons, not 69
- 1/8 is about 22 gallons
A typical house at 35°F burns through 58 gallons in about two weeks. That's why "order at 1/4" is the standard advice — it's less of a cushion than it looks, and cold snaps are exactly when delivery waits get longer.
How Long a 500-Gallon Propane Tank Lasts
Propane tanks are filled to about 80% to leave room for expansion, so a "full" 500-gallon tank holds about 400 gallons. Propane gauges read percent of volume directly — no tank-shape correction needed. For homes heating and making hot water with propane:
| Outdoor average | ~700 gal/yr | ~1,000 gal/yr | ~1,500 gal/yr |
|---|---|---|---|
| 45°F | 175 days | 120 days | 78 days |
| 35°F | 128 days | 85 days | 54 days |
| 25°F | 101 days | 66 days | 42 days |
| 15°F | 83 days | 54 days | 34 days |
That's 80% down to empty. Down to a 30% reorder point — where many suppliers ask will-call customers to order — take off about 40%: the 1,000-gallon home gets roughly 53 days at 35°F before it should call.
Don't Run It Dry
- The bottom isn't usable. Some oil sits below the outlet, and older tanks collect sludge at the bottom that can clog the filter if it's drawn in.
- Running out costs more than fuel. An oil burner that runs dry usually needs its line bled and a restart; a propane system that runs empty must be leak-checked before it's relit.
- Order earlier before a cold snap. Everyone orders at once, and delivery waits stretch.
If you're deciding whether to lock in a price for the winter, our pre-buy vs cap vs market price calculator shows what each plan costs if prices fall, hold or spike.