Thermal expansion & ullage calculator
Liquids grow when they warm. A tank brim-filled on a freezing morning can weep by afternoon — this is the arithmetic behind every fill rule on this site: ΔV = V × β × ΔT, with sourced coefficients per liquid. Physics estimate only; codes and supplier practice govern.
| Liquid | β /°F | per 1,000 gal per 1°F | 250 gal over 40°F | Source / note |
|---|---|---|---|---|
| Gasoline | 0.00068 | 0.68 gal | 6.8 gal | API tank-testing average; grade spread ~0.0005–0.0007/°F |
| Diesel | 0.00046 | 0.46 gal | 4.6 gal | Chevron published typical (their example: 1.000 gal at 20°F → 1.037 at 100°F) |
| Heating oil #2 | 0.00046 | 0.46 gal | 4.6 gal | Same distillate family as diesel |
| Kerosene / Jet | 0.0005 | 0.50 gal | 5.0 gal | Mid-distillate typical |
| Ethanol | 0.00061 | 0.61 gal | 6.1 gal | Standard constant (0.00109/°C) |
| Hydraulic / lube oil | 0.0004 | 0.40 gal | 4.0 gal | Mineral oils, manufacturer data |
| Vegetable oil | 0.00039 | 0.39 gal | 3.9 gal | Edible oils, typical |
| Water | 0.00012 | 0.12 gal | 1.2 gal | Near room temperature — strongly nonlinear, near zero at 39°F |
| Seawater | 0.00013 | 0.13 gal | 1.3 gal | Near room temperature |
| Milk | 0.00012 | 0.12 gal | 1.2 gal | Water-like |
Why the fill rules exist
Every fill limit on this site is this physics plus deliberate margin. The 275-gallon oil tank's ~250 gallon practical fill leaves ~9% ullage — several times the worst single-day swing, covering gauge error and vent geometry too. Propane's 80% rule (NFPA 58) is the extreme case: liquid propane expands roughly 15× more than water per degree and sits in a sealed pressure vessel, so the code reserves a fifth of the tank and the float gauge is calibrated to it. When a code or supplier limit exists, it wins over this calculator — the rules embed conditions the bare formula doesn't.
The delivery "shortage" that isn't
Commercial fuel meters correct to 60°F reference gallons (API/ASTM volume correction). Fuel delivered warm into a cold tank contracts as it equalizes — so the stick legitimately reads fewer gallons than the ticket hours later, by V·β·ΔT. Run your delivery's numbers above before assuming a short measure; the honest discrepancy is usually under one percent.
Related: what ullage is · the 80% rule · tank charts for converting ullage inches to gallons.
FAQ
How much does fuel expand with temperature?
ΔV = V × β × ΔT. Diesel and heating oil run β ≈ 0.00046/°F (Chevron's published figure), gasoline ≈ 0.00068/°F — so 250 gallons of heating oil grows about 6.9 gallons over a 60°F swing, and gasoline half again more.
Why is my 275 gallon tank only filled to 250?
Exactly this math plus margin: a cold-morning fill that warms 50–60°F needs 6–8 gallons of expansion room, and suppliers add margin for gauge error and vent geometry. The ~25 gallon ullage is the practice answer to a physics problem.
Why is propane not in the list?
Propane is a pressure vessel governed by NFPA 58's 80% fill rule, which already embeds its (much larger) expansion plus vapor-pressure safety. You never compute propane ullage yourself — the rule is the answer.
Did my fuel supplier short me? The stick shows less than the ticket.
Possibly neither — commercial meters deliver temperature-corrected gallons referenced to 60°F. Warm fuel pumped into a cold tank contracts as it cools: 200 ticket gallons at 75°F shrink about 1.4 gallons settling to 30°F in the tank. Check the math here before the phone call.
Is this calculator a compliance tool?
No — it computes the physics. Fill limits are set by codes and practice (NFPA 30/31, EPA overfill rules, your supplier's procedures), which are deliberately more conservative than β·ΔT alone. This tool always shows the larger of physics and a 5% working margin, and the applicable rule governs.