How much fuel oil does a heating season use?

Seasonal use scales with heating degree days and the building heat loss. This tool estimates delivered BTU as 24 times the degree days times the boiler output divided by the design temperature difference, then divides by AFUE and 138,500 BTU per gallon to get gallons. A typical cold-climate home lands between 600 and 1,000 gallons per winter.

Why 138,500 BTU per gallon?

That is the standard heat content of No. 2 fuel oil used across the heating-oil industry for fuel budgeting. It is a higher-heating-value figure; real delivered heat depends on the boiler's seasonal efficiency, which is why AFUE is applied separately.

What tank sizes does it recommend?

It rounds up to common residential and light-commercial steel and poly tanks: 138, 275, 330, 420, 550, and 1,000 gallon nominal sizes. The 275-gallon vertical tank is the most common basement size in North America.

Why add a reserve and only 85% usable?

You never run an oil tank to empty because sludge at the bottom can clog the line and a refill is needed before that point, so a 25% working reserve is added. Tanks are also never filled solid — about 85% of nominal is usable to leave vent and expansion space, which the deliveries-per-season figure accounts for.

Does this satisfy NFPA 31?

It sizes capacity only. NFPA 31 also governs tank construction, clearances, venting, fill and vent piping, and secondary containment. Always confirm the selected tank and its installation meet NFPA 31 and your local code before ordering or installing.

Fuel Oil Storage Tank Sizing Calculator

A fuel oil storage tank has to hold enough No. 2 oil that a single delivery comfortably covers the gap between fill-ups, even during the coldest stretch of the season. Size it too small and the homeowner risks running dry between deliveries; size it too large and you pay for steel and floor space that is never used. This calculator works backward from how much oil the heating system actually burns over a winter.

How it works

The seasonal heat the building demands is driven by heating degree days (HDD, base 65°F) and the home’s design heat loss. The boiler’s output at design conditions, divided by the design temperature difference, gives the heat loss per degree of difference. Multiplying by degree days and 24 hours gives the BTU delivered to the house across the season:

Seasonal output BTU = 24 x HDD x (boilerOutput / designDeltaT)
Fuel BTU            = Seasonal output BTU / AFUE
Seasonal gallons    = Fuel BTU / 138,500

No. 2 fuel oil carries 138,500 BTU per US gallon. Dividing the fuel BTU by that value gives gallons burned per season. To size the tank, the seasonal gallons are scaled down to the target delivery interval, a 25% working reserve is added, and the result is rounded up to the next standard tank size:

Per-interval gallons = Seasonal gallons x (intervalDays / seasonDays)
Minimum capacity     = Per-interval gallons x 1.25

Example and notes

Consider an 80,000 BTU/h boiler at 85% AFUE in a climate with 6,000 HDD, a 60°F design difference, a 21-day target between deliveries, and a 180-day season. The tool returns roughly 770 seasonal gallons, about 90 gallons per three-week interval, a 115-gallon minimum capacity, and recommends a standard 138-gallon tank — though most installers default up to a 275-gallon tank for longer fill intervals and price-shopping flexibility.

Use the largest tank that fits the space when oil prices are volatile: a bigger tank lets the homeowner buy on price dips rather than on a fixed schedule. Whatever size you pick, confirm the installation meets NFPA 31 for clearances, venting, and containment.