Free heating oil tool · Updated 5 August 2026
This 2,036 litre horizontal tank at a quarter of its depth holds about 398 litres, roughly 20% of capacity, not 25%. A lying-down cylinder is narrow at the bottom, so a dipstick reading of 25% depth is less oil than you would guess. Safe fill is about 1,731 litres.
Take a common horizontal cylinder tank, 180cm long with a 120cm diameter, so the radius is 60cm. Its full capacity is pi times 60 squared times 180, divided by 1000, which is 2,036 litres. At 85% that is a safe fill of about 1,731 litres.
Here is the part most tank calculators get wrong. If your dipstick shows the oil at a quarter of the tank height, a lying-down cylinder does not hold a quarter of its litres. Using the circular-segment formula, a 25% depth gives only about 398 litres, which is roughly 20% of capacity, not 25%. A naive linear reading would overstate this as 509 litres. The tank is narrow near the bottom, so there is less oil down there than the depth suggests.
At exactly half depth the two methods agree, giving 1,018 litres, but that is only because the cylinder is symmetrical about its middle. Everywhere else, the correct maths matters.
For the 2,036 litre example tank. Note how volume trails depth until the halfway point.
| Depth (% of height) | Actual litres | Actual % full | Linear (wrong) % full |
|---|---|---|---|
| 10% | 106 | 5% | 10% |
| 25% | 398 | 20% | 25% |
| 40% | 760 | 37% | 40% |
| 50% | 1,018 | 50% | 50% |
| 60% | 1,275 | 63% | 60% |
| 75% | 1,638 | 80% | 75% |
| 90% | 1,930 | 95% | 90% |
Rectangular: L x W x H (cm) / 1000 = litres Cylinder full: pi x r^2 x length (cm) / 1000 = litres Horizontal cylinder, partial fill by depth h (cm): segment_area = r^2 x acos((r - h) / r) - (r - h) x sqrt(2 x r x h - h^2) volume = segment_area x length / 1000 (litres) safe_fill = capacity x 0.85 (tank_safe_fill_pct, verified 2026-07-28) Worked example: r = 60cm, length = 180cm capacity = 2036 litres at 25% depth (h = 30cm): 398 litres = 20% of capacity linear (wrong) would say: 509 litres = 25% at 50% depth (h = 60cm): 1018 litres = 50% (true only by symmetry)
| Figure | Value | Source | Verified |
|---|---|---|---|
| tank safe fill pct | 0.85 ratio | OFTEC guidance: tanks are filled to 80-90% of stated capacity to allow expansion | 2026-07-28 |
Tank shape. Rectangular tanks fill linearly, so half the height is half the litres. Horizontal cylinders do not, which is why we use the circular-segment method. Vertical cylinders fill linearly like a rectangular tank.
Internal versus external measurements. Always measure the space the oil occupies. Wall thickness, the base and any bund reduce usable volume, so external dimensions overstate capacity.
Safe fill. Never order to the brim. Leaving 10 to 20 percent free gives room for expansion in warm weather and avoids the vent overflowing when the tanker delivers.
Sludge and the outlet. The usable amount can be a little below the calculated figure, because the outlet sits slightly above the very bottom and older tanks collect sludge that you do not want drawn into the boiler.
Know your safe fill? Compare quotes from local suppliers and order the right volume to top up to a sensible level without overfilling.
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