How to dipstick your tank

Use a clean stick — a wood dowel or metal rod, not your good tape measure (oil residue gums it up). Mark inches on it, or measure the wet part after.
Straight down the middle. Open the fill cap and push the stick straight to the bottom. A slanted stick reads high — you will think you have more oil than you do.
Wipe and read. Pull it out, wiping the excess as it comes, and measure the wet inches.
Cap it. Put the fill cap back on snug. No smoking or open flame near the tank while you measure.

Gauge not moving?

Float gauges gum up with sludge and stick — sometimes for months, which is how people run dry on a gauge that reads half. Test it: carefully lift the gauge's outer cover and press the float down gently. If it bobs back up to the same spot, it works; if it stays put, it's dead. Use the dipstick until it's replaced, and tell your supplier when you order.

What moves the number

Burn rate is the whole game. A 2,500 sq ft house at freezing burns roughly 5–8 gallons a day; shoulder season is closer to 3. Your thermostat, insulation, and domestic hot water move the days more than the tank math does.
The dead bottom. The last couple of inches are sludge and water the pickup should not drink, so we subtract them before computing days. A “quarter tank” on a 275 is about 55 burnable gallons, not 69.
Gauge precision. A float gauge in eighths is a blunt instrument — 1/8 of a 275 is 34 gallons, about a week of winter heat. The dipstick is meaningfully more accurate.
A full tank isn't 275 gallons. Drivers fill to about 90% (roughly 248 gallons in a 275) to leave expansion room, so the gauge rarely reads F.

Honest limits

Standard tanks only. These charts assume the standard aboveground oval tanks (275: 27x44x60 in; 330: 72x44x27 in) and the standard 500-gallon cylinder. Roth double-wall, vertical oval, buried, and fiberglass-ball tanks use different charts.
Burn presets are rules of thumb. They come from oil-dealer experience, not your burner's meter. Two gauge readings a week apart in cold weather, divided by seven, is the truest number you will get.
Days are burnable gallons, not total gallons. Running the tank to the last drop pulls sludge into the filter and nozzle and can air-lock the burner — that is exactly why the industry says order at a quarter.
No smoking or open flame near the tank while measuring. Heating oil is a flammable liquid; treat the fill area like it.

Frequently asked questions

Why does the tool subtract gallons before computing days?

The bottom ~2 inches of the tank hold sludge and water that settle out of the oil. The pickup tube should not drink it — dealers size the “order at a quarter” rule around exactly this. Subtracting it keeps the days honest.

My gauge hasn't moved in weeks. Is it broken?

Probably stuck. Gauges gum up with sludge and freeze in place, sometimes reading half a tank for months. Press the float as described above; if it does not bob back, the gauge is dead — dipstick until it is replaced and mention it when you order.

How many gallons does a “full” 275-gallon tank actually take?

About 240–250 gallons. Drivers fill to roughly 90% of nameplate to leave room for expansion, and they stop when the vent whistle stops. That is why a full tank's gauge reads about 7/8, not F.

What if my tank runs dry?

Call for a delivery and tell the supplier the tank is empty. An oil burner that runs dry needs to be reprimed and restarted — that is a service call on top of the delivery, and it is the expensive way to learn the quarter-tank rule.

My tank isn't 275, 330, or 500 gallons. What now?

The gauge-fraction math still works — a reading of 1/2 means roughly half the nameplate capacity, and the quarter-tank order rule still applies. The dipstick inches need your tank's own chart: check the nameplate for the manufacturer and model, or ask your supplier.

Sources

PickHVAC — Oil Tank Measuring Chart — the inch-to-gallon tables for 275H, 330H, and 500-gallon tanks, the 1/8-increment gauge fractions, and the 90%-fill rule; every number this tool computes with comes from these tables
Earl's residential fuel-oil gauging chart (AST Forum) — independent second printing of the 275H/330H/500 charts; agrees within 1 gallon at every checked point
Hocon Gas — How to Read Your Home's Oil Tank Gauge — gauge anatomy (float in a tube, F-to-E markings), the stuck-gauge float test, and the ~240-gallon usable figure for a 275
Globe Petroleum — How Do I Read a Heating Oil Tank Gauge? — 225–230 usable gallons of a 275, the 5–8 gal/day burn figure for a 2,500 sq ft house at ~32°F, and the run-out-means-a-service-call warning
Engineer Fix — How Low Can You Let Your Oil Tank Go? — the refill-at-a-quarter rule, the 5–8 gal/day colder-month range, and the sludge/air-lock risk of running low
EIA — Winter Fuels Outlook — ~400 gallons per heating season for the average oil-heated household (Nov–Mar), the basis for the ~3 gal/day shoulder-season preset

Deciding when to call is half the job — the other half is price. Run your last delivered price through our heating oil price checker, and read how home heating works for rural homes for the full fuel picture.