How Long Will My Heating Oil Last?
Heating oil carries about 138,500 BTU per gallon, and your burner's nozzle tells you the burn rate: a typical residential nozzle is 0.75–1.0 gallons per hour while firing. The burner doesn't fire constantly — duty cycle is what turns nozzle size into days.
Quick estimate: in mild weather (around 40°F) a typical home burns 2–4 gallons a day; in a hard freeze (around 15°F or below) 6–9 gallons a day is normal, more for large or leaky houses. So 100 gallons is roughly a month of shoulder-season heat or two cold-snap weeks.
Your own number beats any rule of thumb: stick the tank (or read a working gauge) on the same day two weeks apart, convert both readings with your tank chart, and divide. That gallons-per-day figure, taken in similar weather, is the most accurate forecast you can get without a flow meter.
Then set a reorder trigger: days remaining = gallons left ÷ your daily burn, and call when that drops near your supplier's delivery lead time plus a storm buffer. At a quarter tank on a 275 (about 12 inches), you have roughly 64 gallons — one to three weeks depending on the weather.
Three ways to answer, worst to best
- A rule of thumb by temperature. Quick, and it cannot know your house. Two buildings on the same street can differ by a factor of three.
- Nozzle size × running hours. Correct physics, but you have to know the duty cycle, and nobody watches their burner for a week.
- Your own K-factor from two stick readings. Derived from your house, your burner, your habits and your weather. This is what fuel dealers use to schedule automatic deliveries, and it is the method worth learning.
The energy behind a gallon
EIA's thermal conversion factor for distillate fuel oil in the 15–500 ppm sulfur band — the classic heating-oil grade — is 5.817 million Btu per barrel, which is 138,500 Btu per gallon. Ultra-low-sulfur distillate is slightly leaner at 137,400 and high-sulfur slightly richer at 138,700, so the honest figure is a band of about ±0.5 %, not a constant.
Your burner's nozzle sets the burn rate while it is firing. A nozzle marked 0.85 means 0.85 US gallons per hour at its rated pressure:
| Nozzle | Gallons per hour firing | Btu/hr input | Btu/hr output at 85 % AFUE |
|---|---|---|---|
| 0.65 | 0.65 | 90,000 | 76,500 |
| 0.75 | 0.75 | 103,900 | 88,300 |
| 0.85 | 0.85 | 117,700 | 100,100 |
| 1.00 | 1.00 | 138,500 | 117,700 |
| 1.25 | 1.25 | 173,100 | 147,200 |
Two things follow. First, the nozzle is stamped on the burner — go and read it rather than guessing. Second, the nozzle tells you the rate while firing, not consumption: a burner firing 40 % of the time on a 0.85 nozzle uses 0.34 gallons an hour averaged over the day, or about 8 gallons.
The degree-day method, and your own K-factor
A heating degree day is the amount by which a day's mean temperature falls below a base, conventionally 65 °F. EIA defines it as the base minus the average of the day's high and low, with negatives set to zero. It is the standard proxy for how hard your heating had to work.
HDD for the day = 65 − (day's high + day's low) / 2 (never below 0)
K = degree days accumulated ÷ gallons burned over the same period
gallons per day = HDD per day ÷ K
days remaining = gallons in tank × K ÷ HDD per dayK is a single number that encodes your house's insulation, your burner's efficiency, your thermostat habits and your hot-water load. It is roughly stable through a season, which is exactly what makes it useful.Fuel dealers compute K for every automatic-delivery customer and use it to decide when to send a truck. You can compute your own from two stick readings and a weather history, and it will beat any table on this page.
Finding your K-factor
- Nov 3 stick: 31 inches on a 275 vertical → 197 gal
- Nov 17 stick: 22 inches → 134 gal
- Used: 63 gal over 14 days
- Mean temperature over that fortnight: 41 °F → 24 HDD/day → 336 HDD total
- K = 336 ÷ 63 = 5.3
K ≈ 5.3 degree days per gallon
Use your own local weather history for the mean temperature — most national weather services publish daily highs and lows for the nearest station. One fortnight gives a usable K; a second confirms it.
Using K to answer the actual question
- Tank now reads 16 inches → 92 gal
- Cold week forecast: mean 20 °F → 45 HDD/day
- Burn rate = 45 ÷ 5.3 = 8.5 gal/day
- 92 ÷ 8.5 = 10.8 days
About 11 days — order now, not next week
Notice what changed. The same 92 gallons that lasted three weeks in November lasts eleven days in January. A fraction-of-tank rule cannot express that; a K-factor does it in one line.
Gallons per day, by K and by weather
Arithmetic, not estimation: this table is simply HDD ÷ K. Find your K from the worked example above, then read across.
| Mean outdoor temp | HDD/day | K = 4 | K = 6 | K = 8 | K = 10 |
|---|---|---|---|---|---|
| 50 °F | 15 | 3.8 | 2.5 | 1.9 | 1.5 |
| 40 °F | 25 | 6.3 | 4.2 | 3.1 | 2.5 |
| 30 °F | 35 | 8.8 | 5.8 | 4.4 | 3.5 |
| 20 °F | 45 | 11.3 | 7.5 | 5.6 | 4.5 |
| 10 °F | 55 | 13.8 | 9.2 | 6.9 | 5.5 |
| 0 °F | 65 | 16.3 | 10.8 | 8.1 | 6.5 |
A low K means a thirsty house. If yours comes out below about 4, that is worth knowing for its own sake — it usually points at insulation, air sealing or a burner overdue for service rather than at the weather.
How long a full tank lasts
A 275 is filled to roughly 250 gallons and a 330 to about 300, because the supplier leaves ullage for thermal expansion. Those are the numbers to divide.
| Daily burn | 275 tank (250 gal) | 330 tank (300 gal) | 550 tank (500 gal) |
|---|---|---|---|
| 2 gal/day | 125 days | 150 days | 250 days |
| 4 gal/day | 63 days | 75 days | 125 days |
| 6 gal/day | 42 days | 50 days | 83 days |
| 8 gal/day | 31 days | 38 days | 63 days |
| 10 gal/day | 25 days | 30 days | 50 days |
| 12 gal/day | 21 days | 25 days | 42 days |
What throws the estimate off
- Domestic hot water. If the same boiler makes your hot water, it burns through the summer too and your K is not purely a heating number.
- Wind. Degree days measure temperature, not infiltration. A windy cold day burns more than a still one at the same mean.
- A water bottom in the tank. Your stick reads total depth. Two inches of water in a 275 vertical is five gallons you counted and cannot burn — see water in your oil tank.
- Thermal contraction. Oil delivered warm and gauged cold has shrunk. On 250 gallons a 15 °F drop is roughly two gallons of apparent loss with nothing missing.
- An unexplained drop that none of the above covers. That is a leak question, not a consumption question.
FAQ
How long will 100 gallons of heating oil last?
Divide by your daily burn rate. At 4 gallons a day it is 25 days; at 10 it is ten. Get the rate from your own K-factor — degree days divided by gallons used between two stick readings — rather than from a table, because houses differ by a factor of three.
How many BTU are in a gallon of heating oil?
About 138,500 Btu for the 15–500 ppm sulfur grade, per EIA's thermal conversion factors. Ultra-low-sulfur distillate is nearer 137,400 and high-sulfur 138,700, so treat it as a band of roughly ±0.5 %.
What is a K-factor for heating oil?
Degree days per gallon: the degree days accumulated over a period divided by the gallons burned in it. It encodes your house, burner and habits in one number, and fuel dealers use it to schedule automatic deliveries. Compute yours from two stick readings and a local weather history.
How much oil does a house use per day in winter?
It depends far more on the house than on the weather. The arithmetic is degree days divided by your K-factor: at a mean of 20 °F that is 45 degree days, which is 11.3 gallons a day at K = 4 and 4.5 at K = 10.
Does nozzle size tell me my consumption?
It tells you the rate while the burner is firing, not consumption. A 0.85 nozzle burns 0.85 gallons an hour when running; at a 40 % duty cycle that is about eight gallons a day.
Why did my oil last less time than last winter?
Usually weather — compare degree days, not calendar days. If the degree days match and the consumption does not, look at the burner service, a water bottom in the tank, or a leak.
Sources and standards
Where a standard genuinely governs what you should do here, it is cited and linked.
- Thermal Conversion Factors — Monthly Energy Review, Appendix A (source documentation)U.S. Energy Information Administrationhttps://www.eia.gov/totalenergy/data/monthly/pdf/mer_a_doc.pdfWhere the Btu-per-gallon figures come from and how they were derived. Distillate fuel oil is 5.770 million Btu/bbl at 15 ppm sulfur and under, 5.817 at 15–500 ppm and 5.825 above 500 ppm — 137,400, 138,500 and 138,700 Btu per gallon respectively. Propane is 3.841 million Btu/bbl, or 91,452 Btu per gallon, estimated by EIA from NIST Chemistry WebBook enthalpy-of-combustion data and liquid density at 60 °F.
- Glossary — Heating Degree DaysU.S. Energy Information Administrationhttps://www.eia.gov/tools/glossary/index.php?id=heating_degree_daysThe definition the whole degree-day method rests on: subtract the average of the day’s high and low from a base temperature, most commonly 65 °F, set negatives to zero, and sum over the period. It is the standard indicator of space-heating energy demand, which is why fuel dealers schedule deliveries against it rather than against the calendar.
- ASTM D396 — Standard Specification for Fuel OilsASTM InternationalPaid standard — cited by designation; obtain from the publisher.Defines the grades of fuel oil, including No. 2 heating oil. It sets the density and viscosity ranges this site uses for weight and thermal-expansion figures — which is why a heating-oil weight is a range rather than a single number.
- ASTM D975 — Standard Specification for Diesel FuelASTM InternationalPaid standard — cited by designation; obtain from the publisher.Specifies the diesel grades that substitute for heating oil in many installations. Together with ASTM D396 it sets the density and sulfur ranges behind the Btu-per-gallon spread — the reason a heat content is a band, not a number.
- NFPA 31 — Standard for the Installation of Oil-Burning EquipmentNational Fire Protection AssociationPaid standard — cited by designation; obtain from the publisher.Governs how fuel-oil supply tanks are installed, vented, filled and located, including tank-to-appliance clearances and fill and vent piping. Adopted, often with amendments, by most US states.
- Methodology — the partial-fill formulas and liquid densities used on this sitetankvolumecalc.comhttps://tankvolumecalc.com/methodology/Every chart value here comes from a closed-form partial-fill formula, not interpolation: circular segments for cylinders, a piecewise obround model for 275/330 heating-oil tanks, and scaled spherical caps for dished heads. Densities and their bases are tabulated there.
- Handbook 44, Appendix C — General Tables of Units of MeasurementNational Institute of Standards and Technology, U.S. Department of Commercehttps://www.nist.gov/document/2026-nist-handbook-44-appendix-cThe legal-metrology reference for U.S. customary units. Fixes the U.S. liquid gallon at exactly 231 cubic inches and tabulates the full liquid-volume chain (minim → fluid dram → fluid ounce → gill → pint → quart → gallon).
Every linked source above was retrieved and checked on 2026-09-02. Factors quoted as exact are exact by definition in the cited document, not rounded by us.