How to check your oil tank level
Most oil tanks have some kind of gauge, and most gauges are wrong some of the time. Here is how to read the common types, how to dip a tank yourself, and how to turn a depth into litres so you know how much to order.
Updated
What kind of gauge does my tank have?
Walk around the tank and look on top and down one end. You will usually find one of these:
- A sight gauge: a clear plastic tube running up the side of the tank, often with a small valve or button at the bottom.
- A float gauge: a round dial or a pointer on top of the tank, sometimes marked in fractions or litres.
- An electronic monitor: a small sensor screwed into the top of the tank, with a display indoors or an app on your phone.
- No gauge at all. That is common on older tanks, and on some bunded tanks where the gauge has broken or been removed.
How to read a sight gauge
On most sight gauges, the tube is kept empty by a valve. Press or pull the button at the bottom, wait a few seconds for the oil to rise into the tube, then read the level against the scale or against the tank's height. Let go and make sure the valve closes again.
A sight tube is a weak point. If the valve stays open and the tube cracks, oil can run out of the tank. If the tube is cracked, weeping or so stained you can't read it, or the valve won't close by itself, stop using it and ask an OFTEC-registered technician to replace it.
How to read a float gauge
A float sits on the oil inside the tank and moves a pointer on the dial. Read it straight on. Tap the dial gently: if the pointer jumps, it had stuck. Floats can snag on the side of the tank or get stuck in sludge, so a float gauge that hasn't moved in weeks is a warning sign, not good news.
Many dials show fractions such as 1/4, 1/2 and 3/4. On a tank with upright sides, a quarter on the dial is about a quarter of the oil. On a cylinder lying on its side it isn't, as the table further down shows.
What do tank monitors do?
An electronic tank monitor is a sensor that sits in an opening on top of the tank. Most measure the distance down to the oil, using sound or a beam, and work out the litres from the tank size you enter when you set it up. The reading goes to a small screen in the house or to an app.
The useful part is that you don't have to go out in the rain to check. Many can warn you when the level gets low, and some flag a sudden drop, which can mean a leak or theft. They are only as accurate as the setup: enter the wrong tank height or shape and every reading is off. Ask the seller how it handles your tank's shape, and check it against a dipstick after fitting.
How to dip an oil tank
If you have no gauge, or don't trust it, measure the depth yourself.
- Find a straight, clean, dry stick or rod that is longer than the tank is tall. A length of wooden dowel works well. Don't use anything dirty or wet, as you don't want water or grit in the oil.
- Put on gloves. Kerosene irritates skin, and the Health and Safety Authority notes its vapour may cause drowsiness or dizziness. Don't smoke or use a naked flame near the tank.
- Open the fill cap or inspection hatch on top. Don't lean your face over the opening.
- Lower the stick straight down until it touches the bottom. Keep it upright.
- Pull it out and read where the wet line ends. Measure from the bottom of the stick to that line in centimetres.
- Wipe the stick, close the cap firmly, and wash your hands.
Measure the inside height of the tank the same way. Lower the stick until it touches the bottom, mark it level with the top of the opening, and allow for the thickness of the tank wall. For a bunded tank, the stick goes into the inner tank, so measure that.
How to turn the depth into litres
You need the tank's capacity. Look for a label or plate on the tank, often near the fill point, or check the paperwork from whoever fitted it.
Tanks with straight, upright sides
This covers box-shaped tanks and round tanks that stand upright. The oil fills them evenly from bottom to top, so:
Litres in the tank = capacity × (oil depth ÷ inside height)
If you know the inside measurements instead, litres = length × width × oil depth ÷ 1,000, all in centimetres.
Example: a box tank 150 cm long, 80 cm wide and 100 cm high holds 1,200 litres. With 40 cm of oil it has 150 × 80 × 40 ÷ 1,000 = 480 litres.
Cylinder tanks lying on their side
These are trickier. The tank is narrow at the bottom and top and widest in the middle, so the first few centimetres of oil hold far less than the centimetres in the middle. Use this table, where depth is the oil depth as a share of the tank's inside diameter:
| Oil depth (share of diameter) | Share of tank full |
|---|---|
| 10% | 5% |
| 20% | 14% |
| 25% | 20% |
| 30% | 25% |
| 40% | 37% |
| 50% | 50% |
| 60% | 63% |
| 70% | 75% |
| 75% | 80% |
| 80% | 86% |
| 90% | 95% |
Example: a tank 180 cm long and 100 cm across inside holds about 1,415 litres. With 30 cm of oil, the depth is 30% of the diameter, so the tank is about 25% full, or 355 litres. A straight sum would have said 30%, which is too high.
If you want the exact formula, with r as the inside radius, h as the oil depth and L as the length, all in centimetres:
Litres = L × (r² × arccos((r − h) ÷ r) − (r − h) × √(2rh − h²)) ÷ 1,000
Use a calculator set to radians for the arccos.
Plastic bunded tanks and odd shapes
Many plastic tanks have curved corners, sloping tops or a waist. The sums above only give a rough answer for these. The maker often publishes a depth chart for each model, so search for the model name on the label. Otherwise, use a straight-sided sum and treat it as an estimate.
When gauges lie
- The float has stuck. Tap the dial, or compare with a dipstick reading.
- The sight tube is stained, or oil is sitting in it from an earlier reading. Read it only after opening the valve and letting the level settle.
- The tank isn't level. A tank on a sloping or sinking base reads differently at each end. A base that has cracked or sunk also needs checking by a technician.
- There is water and sludge in the bottom. Water is heavier than kerosene and sinks under it. A dipstick and most gauges count it as oil, so the tank holds less oil than it seems.
- A monitor was set up with the wrong tank height or shape, or its sensor has condensation on it.
- You read it right after a delivery. Give the oil time to settle before you take a reading you will rely on.
If the level falls faster than usual and the weather hasn't turned colder, look for a leak. Northern Ireland's environment agency advises that if you suddenly use more oil than expected, you should check the tank and pipework. The same advice applies here. See what to do if your oil tank is leaking, and if you smell oil indoors, read kerosene smell in the house.
How often should you check?
Once a month is plenty in summer. In winter, check every week or two, and more often in a cold spell. Write down the date and reading each time. After two or three readings you will know your own rate of use, which is the best guide to how long your oil will last. Order with a good margin left; running the tank too low can pull sludge into the boiler.
Common questions
How do I check how much oil is in my tank?
Read the gauge if your tank has one: a sight tube on the side, a dial on top, or an electronic monitor. If not, dip the tank with a long, clean, dry stick, read the wet mark, and convert that depth to litres using the tank's shape and capacity.
How do I convert oil tank depth to litres?
For a tank with straight, upright sides, divide the oil depth by the tank's internal height and multiply by its capacity. For a cylinder lying on its side, the tank holds less oil near the top and bottom, so use the percentage table in this guide or the formula for a horizontal cylinder.
Why is my oil tank gauge wrong?
Float gauges stick, sight tubes stain, and electronic monitors can be set up with the wrong tank height. A tank that isn't level, or water and sludge in the bottom, also throw readings off. Check against a dipstick now and then.
What should I do if the level drops suddenly?
Check the tank, pipes and ground around them for a leak or for signs of theft. If you smell oil or see a spill, follow our oil tank leak guide and contact an OFTEC-registered technician.
Put your reading to use
Enter your tank size and level in the Tankful tank planner. Tell it whether you read a gauge or made a rough guess, and it widens the range for a guess. It gives you a date to order by and how much to top up. Then compare oil supplier prices in your county for that amount.