How do I check how much oil is in my tank?

Sight gauges

A sight gauge is a clear tube up the side of the tank. Oil in the tube sits at the same height as the oil inside, so you open its valve, read the level against the scale and close it again.

In England, the oil storage rules for business tanks and for home tanks over 3,500 litres say a sight gauge must be bracketed along its length so it cannot be knocked over, and fitted with a valve that closes automatically when not in use. Wales, Scotland and Northern Ireland have their own rules (see the rules for domestic oil tanks), but a bracketed gauge with a self-closing valve is sensible on any tank, because a broken tube can let oil out.

Float and dial gauges

Many plastic tanks have a mechanical gauge on top: a float inside rises and falls with the oil and moves a pointer or marker. They are simple but can stick. UKIFDA lists gauges not working among the warning signs to look out for, and OFTEC points out that a gauge may show oil when the tank is actually empty. If the reading has not moved in weeks, or the boiler stops while the gauge still shows oil, check another way.

Dipping the tank

OFTEC's advice, if it is safe to do so, is to remove the fill point cap and look inside, or dip the tank with a long pole such as a bamboo cane. This works on steel tanks and on plastic ones with a reachable fill point:

1. Use a clean, dry rod or cane long enough to reach the bottom.
2. Lower it straight down until it touches the bottom, then lift it out.
3. Measure from the end of the rod to the wet line. That is your oil depth.
4. Refit the cap firmly so rain cannot get in.

Do not climb on the tank or stand on anything unsteady to reach the cap. A smear of water-finding paste on the end of the rod also shows whether there is water at the bottom (see water in your oil tank).

Electronic and ultrasonic monitors

Electronic monitors fit into the top of the tank and measure the distance down to the oil, usually with ultrasound, then send the reading to a display in the house or an app. Many will warn you if the level drops suddenly or falls below a set point, which can be the first sign of theft or a leak. UKIFDA suggests considering a remote electronic gauge and notes that one can flag a sudden drop or a level below a quarter full.

Two tips: set the monitor up with your exact tank model so its sums are right, and make sure the sensor is fitted tightly. OFTEC warns that a loose sensor can let rain into the tank and cause heating problems that are expensive to fix.

Turning a depth into litres

Tanks are not simple boxes. A horizontal cylinder, for example, holds less oil per centimetre near the top and bottom than in the middle, so half the height is not always half the oil. The reliable way is the tank maker's depth chart (also called a calibration or dipping chart):

• Find the make, model and capacity on the label or plate on the tank.
• Look up that model's chart on the maker's website, or ask your supplier or service technician.
• Read off the litres for your measured depth.

For a plain rectangular tank only, you can work it out: inside length times inside width times oil depth, all in centimetres, divided by 1,000 gives litres.

How often to check

Check little and often, and more often in winter. OFTEC says regular checks help you spot any unusual drop in fuel, which could point to a problem such as a leak. UKIFDA advises ordering before the level gets critically low and keeping at least 500 litres in the tank where possible (see when to reorder heating oil). When it is time to top up, compare prices for your postcode.