Your Gauge Is Showing Depth, Not Gallons
The float gauge on an oil tank rides on the surface of the oil, so its marks — F, 3/4, 1/2, 1/4 — are fractions of the tank's height. The common 275-gallon tank is an upright oval with rounded top and bottom, and the rounded bottom holds much less oil per inch than the straight middle. The result:
- 3/4 ≈ 216 gallons
- 1/2 ≈ 137 gallons
- 1/4 ≈ 58 gallons — not the 69 you'd assume
- 1/8 ≈ 22 gallons
That's why the needle seems to fall faster as it gets low: it does, per gallon burned. And "F" after a delivery usually means 240–250 gallons, not 275, because tanks are never filled to the brim. Round outdoor and underground tanks have the same effect, slightly stronger. Propane gauges are different — they're calibrated to read percent of volume directly, and propane tanks are filled to about 80% to leave room for the liquid to expand.
How Dealers Predict When You'll Run Out
Automatic-delivery customers rarely run out because the dealer tracks two numbers. Heating degree days measure how cold it's been: each day, take how far the day's average temperature falls below 65°F. A 35°F day is 30 degree days. Your K-factor is how many degree days your house gets through per gallon — worked out from the gallons in each delivery and the degree days since the one before. A house with a K-factor of 8 burns a gallon every 8 degree days, so a week averaging 35°F (210 degree days) burns about 26 gallons of heating oil.
Hot water doesn't depend on the weather, so it's handled separately as a fixed amount per day. That's why a tank still drains slowly in summer if the boiler makes your hot water. This calculator does the same: it splits your annual usage into a weather-driven part and a hot-water part, then runs both forward at the temperature you expect.
Why "Days Left" Changes So Much With the Weather
Heating fuel use tracks the gap between 65°F and the outdoor temperature, not the temperature itself. A week at 25°F has 40 degree days a day; a week at 45°F has 20. Same house, same thermostat, twice the oil. The table in your result shows how far the same tank goes at different temperatures — useful before a forecast cold snap, which is also when delivery waits are longest.
Don't Plan to Run It to Empty
- The last few gallons aren't usable. Some fuel sits below the outlet, and the bottom of an older tank collects sludge that can clog the filter or nozzle if it gets drawn in.
- Running dry costs more than the fuel. An oil burner that runs out often needs a service call to bleed air out of the line and restart it. A propane system that runs empty has to be leak-checked before it can be relit.
- Deliveries slow down when it's coldest. Will-call orders pile up in a cold snap. Ordering at 1/4 (oil) or 20–30% (propane) builds in that buffer.
Whether to lock in a price before winter is a separate question — our pre-buy vs cap vs market price calculator works out the break-even. And if you're wondering how much a typical tank lasts before you have your own numbers, see how long 100 gallons of heating oil lasts.