Quick answer: on a standard 275 gallon oval tank (27″ wide × 44″ tall × 60″ long), a dipstick reading of 10″ ≈ 50 gallons, 22″ ≈ 134 gallons (half full) and 35″ ≈ 225 gallons. The relationship is not linear — the tank is widest in the middle, so an inch near the midpoint holds roughly twice what an inch near the bottom does.
Why half the depth is not half the oil
This is the whole reason this calculator exists. People look at 22 inches on a 44-inch tank, conclude they are half full, and happen to be right — but then read 11 inches, conclude they are a quarter full, and are badly wrong. At 11 inches a standard 275 gallon tank holds about 57 gallons, which is 21%, not 25%.
The reason is the tank's shape. An oval or cylindrical tank is widest across its middle and narrows towards the top and bottom. Each inch of depth therefore contains a different amount of oil depending on where in the tank it sits:
| Depth band | Gallons in that band | Where in the tank |
|---|---|---|
| 0″ – 4″ | 13.7 gal | Curved bottom — narrow |
| 4″ – 8″ | 23.1 gal | Widening |
| 8″ – 12″ | 27.0 gal | Approaching full width |
| 20″ – 24″ | 28.1 gal | Straight sides — maximum per inch |
| 32″ – 36″ | 27.0 gal | Narrowing again |
| 36″ – 40″ | 23.1 gal | Curved top |
| 40″ – 44″ | 13.7 gal | Curved top — narrow |
The first four inches and the last four inches each hold 13.7 gallons. The four inches either side of the middle hold 28 gallons — more than double for exactly the same movement of the stick. That is why a linear estimate is worst at the extremes, and why the low-level readings that matter most for ordering are precisely where the error is largest.
Dipstick chart — standard 275 gallon oval tank
For a tank measuring 27″ wide × 44″ tall × 60″ long. This is the most common residential heating oil tank in North America.
| Inches | Gallons | % full | Note |
|---|---|---|---|
| 2″ | 5.0 | 1.9% | Near empty — risk of drawing sludge |
| 4″ | 13.7 | 5.1% | Order urgently |
| 6″ | 24.6 | 9.2% | |
| 8″ | 36.9 | 13.8% | |
| 10″ | 50.1 | 18.7% | Typical reorder point |
| 12″ | 63.9 | 23.8% | Just under a quarter full |
| 14″ | 77.9 | 29.1% | |
| 16″ | 91.9 | 34.3% | |
| 18″ | 105.9 | 39.5% | |
| 20″ | 119.9 | 44.8% | |
| 22″ | 134.0 | 50.0% | Exactly half |
| 24″ | 148.0 | 55.2% | |
| 26″ | 162.0 | 60.5% | |
| 28″ | 176.0 | 65.7% | |
| 30″ | 190.1 | 70.9% | |
| 32″ | 204.1 | 76.2% | About three-quarters full |
| 34″ | 217.8 | 81.3% | |
| 36″ | 231.1 | 86.2% | |
| 38″ | 243.3 | 90.8% | Typical maximum delivery fill |
| 40″ | 254.2 | 94.9% | |
| 44″ | 267.9 | 100% | Geometric full — never reached in practice |
Figures are computed from exact geometry for a 27 × 44 × 60 inch oval tank. Your tank may differ by an inch or two in any dimension — measure it and use the calculator above for your actual numbers.
"275 gallon" tanks do not hold 275 gallons
Two separate things reduce a nominal 275 gallon tank below its label:
- Geometry. A 27 × 44 × 60 inch oval tank has a true internal volume of about 268 gallons when completely, absolutely full. The "275" is a nominal size class, not a measurement.
- Ullage. Delivery drivers never fill to the top — oil expands with temperature and the tank needs an air space. A "full" delivery typically stops around 240–250 gallons.
So if your tank reads empty and you order 275 gallons, roughly 30 gallons will not fit. Order around 240 for a genuinely empty standard tank, or simply ask for a "fill".
| Nominal size | Dimensions (W × H × L) | Geometric capacity | Typical usable fill |
|---|---|---|---|
| 275 gal vertical oval | 27″ × 44″ × 60″ | 268 gal | ~240 gal |
| 275 gal horizontal oval | 44″ × 27″ × 60″ | 268 gal | ~240 gal |
| 330 gal vertical oval | 27″ × 44″ × 72″ | 322 gal | ~290 gal |
| 340 gal vertical oval | 27″ × 48″ × 72″ | 341 gal | ~305 gal |
| 500 gal horizontal cylinder | 48″ dia × 65″ long | 509 gal | ~460 gal |
| 1,000 gal horizontal cylinder | 48″ dia × 130″ long | 1,018 gal | ~915 gal |
The same shape logic applies to fuel tanks generally and to any horizontal cylindrical tank.
The geometry, for anyone who wants it
The volume in a partially filled horizontal cylinder is length × the area of a circular segment:
V = Asegment × L
r = radius · h = depth of liquid · L = tank length
That arccos term is why no simple percentage rule works. An oval (obround) tank is handled by splitting it into three zones — a bottom half-circle, a straight-sided middle, and a top half-circle — and applying the segment formula only to the curved parts:
r < h ≤ r + s: A = ½πr² + W·(h − r)
h > r + s: A = Atotal − segment(r, H − h)
W = width · r = W/2 · H = total height · s = H − W (straight section)
The calculator uses these exactly. A useful check on any tank tool: at exactly half depth it must return exactly 50%, because every one of these cross-sections is symmetrical about its horizontal midline. This one does, to four decimal places.
How to take a reading properly
- Use the right opening. The gauge port or fill pipe, not the vent. On many tanks the float gauge unscrews to expose a vertical opening.
- Use a stick longer than the tank is tall. A 5-foot wooden batten works. Losing a stick inside the tank is a genuinely expensive mistake.
- Lower it slowly and keep it vertical. An angled stick reads deeper than the true level.
- Read the wet line immediately. Heating oil evaporates slowly, but it will run and blur if you wait.
- Measure from the bottom of the tank, not the bottom of the pipe. If the fill pipe stops short of the tank floor, your reading is offset by that gap.
- Do not trust the float gauge. Mechanical float gauges stick — often at "½" — and are the most common reason people run out with a gauge showing plenty. If your gauge has read the same for weeks in winter, it is stuck.
How long will it last?
Rough consumption figures for an oil-fired boiler or furnace in a typical home:
| Conditions | Typical daily use | Days from 100 gal |
|---|---|---|
| Summer — hot water only | 0.5–1.5 gal | 65–200 days |
| Mild, around 50 °F / 10 °C | 2–3 gal | 33–50 days |
| Cool, around 40 °F / 4 °C | 3–4 gal | 25–33 days |
| Cold, around 30 °F / −1 °C | 4–6 gal | 17–25 days |
| Very cold, around 20 °F / −7 °C | 6–8 gal | 13–17 days |
| Severe, below 10 °F / −12 °C | 8–12 gal | 8–13 days |
Consumption depends heavily on house size, insulation, thermostat setting and burner efficiency. Track your own usage across two dipstick readings a week apart — that single measurement beats any table here.
Enter your daily usage in the calculator and it will tell you roughly how many days of oil are left at the current level.
Common mistakes & pro tips
- Assuming depth is proportional to volume. The core error. Wrong by the largest margin exactly when the tank is low.
- Trusting a stuck float gauge. Confirm with a stick before deciding you have plenty.
- Ordering the nominal capacity. A "275 gallon" tank takes about 240 when empty.
- Reading a horizontal oval tank against a vertical oval chart. The same 275 gallon tank exists in both orientations, and the charts are completely different. Check which way the long oval face runs.
- Running the tank very low. Water and sludge collect on the bottom of every oil tank. Drawing them into the line blocks the filter and nozzle, and restarting a burner that has run dry usually needs bleeding.
- Pro tip — mark your own stick. Once you know your tank's dimensions, mark the stick in gallons rather than inches. Ten seconds of work, and no arithmetic in the dark.
- Pro tip — check the tank bottom for water. Water sinks below oil. Water-finding paste on the stick shows a colour change at the oil–water interface, which tells you both that water is present and how deep it is.
How to use this calculator
- Pick your tank shape. Oval covers the standard residential tank and is pre-filled with 27 × 44 × 60 inches.
- Enter your tank's actual dimensions if they differ. Measure the outside and subtract a little for wall thickness if you want to be precise.
- Enter the dipstick reading — the depth of oil measured from the bottom of the tank.
- Optionally enter daily usage to get an estimate of days remaining.
Frequently asked questions
How do I tell how much oil is in my tank?
Lower a clean stick to the bottom of the tank through the gauge or fill port, withdraw it, and measure the wet mark in inches. That gives the depth of oil. Then convert depth to gallons using the tank's geometry. On a standard 275 gallon oval tank, 22 inches of oil is about 134 gallons, which is half full. Do not assume the conversion is linear — it is not for any tank with curved sides.
How many gallons is 10 inches of oil in a 275 gallon tank?
About 49 gallons in a standard 275 gallon oval tank measuring 27 × 44 × 60 inches. That is only 19% of capacity even though 10 inches is nearly a quarter of the tank's 44-inch height, because the bottom of the tank is curved and holds less per inch than the middle does. Ten inches is a reasonable point at which to order a delivery.
Why is a tank gauge not proportional to depth?
Because oval and cylindrical tanks are widest across the middle and narrow towards the top and bottom. An inch of depth near the middle contains far more oil than an inch near the bottom. On a standard 275 gallon tank the first 4 inches hold about 14 gallons while the 4 inches either side of the midpoint hold about 28 gallons — twice as much for the same depth. Only a rectangular tank gives a truly linear gauge.
How much does a 275 gallon oil tank actually hold?
A nominal 275 gallon tank measuring 27 × 44 × 60 inches has a true geometric capacity of about 268 gallons when completely full. In practice it is never filled to the very top: delivery drivers leave an air space for expansion, so a full tank usually reads around 240 to 250 gallons. This is why an order for 275 gallons on an empty tank will not fit.
What are the dimensions of a standard heating oil tank?
The most common residential tank is the 275 gallon vertical oval at 27″ wide, 44″ tall and 60″ long. The same 275 gallon tank is also made in a horizontal orientation at 44″ wide by 27″ tall. Other common sizes are 330 gallon at 27 × 44 × 72 inches, and 500 or 1,000 gallon horizontal cylinders for larger properties and buried installations. Always measure your own tank rather than assuming.
At what level should I order more heating oil?
A common rule is to order when the tank falls to about a quarter full, which is roughly 10 inches on a standard 275 gallon oval tank. Ordering earlier is sensible in cold weather, when delivery lead times lengthen and consumption rises. Running a tank very low risks drawing sediment and water from the bottom into the fuel line, which can block the filter and nozzle and require a service call to restart the burner.