How to calculate how many gallons of heating oil is left

Updated April 17, 2017

If you heat your home with oil, you have one more thing to worry about than gas and electric consumers. If you don't keep tabs on how much fuel you have left, you might run out of oil on the coldest day of the year. But it might not be clear how much oil is left. Banging on the tank won't help. A full tank sounds pretty much like an empty one. Oil customers will need to make a little more effort to determine how many gallons of heating oil are left.

Inspect your tank for a gauge. Most indoor, above ground tanks have a gauge, which looks like a little clear plastic cylinder marked from "E" for empty at the bottom to "F" for full at the top. Calibrations will also indicate 1/4, 1/2 and 3/4 full. Simply read the gauge and multiply by the total capacity of your tank. If the tank has a 400 gallon capacity, and the gauge reads at 1/4 then you have 100 gallons left.

Make sure your gauge is working. If you suspect it isn't, carefully unscrew the plastic cover by hand counter-clockwise to remove it. Gently press the top of the gauge indicator rod downwards, then release it. If it moves back to where it was before, everything is probably working fine.

Replace your gauge if it is broken But it might take a while to get a technician out to your home to replace it. In the meantime, you can determine how much is left with a little more effort.

Make a dipstick. Start with a thin length of wood a little bit longer than the depth of your tank. Mark it off by cutting a notch at the point near the top that corresponds to a full tank. Then, using a ruler or tape measure, mark it off at regular intervals. A notch at the points 1/4, 1/2, and 3/4 of the way is probably all you need.

Clean your dipstick carefully with a clean rag before you dip it into the oil tank to avoid contaminating your oil. Dip it in and pull it out. You can see what fraction of full your tank is at, and then just multiply as in Step One in Section One. This will give a good estimation of the total amount left, unless your tank is a cylindrical tank laid on its side, in which case the math is slightly more complicated.

Do the math. Rectangular tanks and cylindrical tanks mounted on their end are easy. Just multiply the fraction by the total capacity. If you have a cylindrical tank on its side, it's a little more complicated because the amount of fuel per unit of depth changes. Use this chart to approximate.

Dipstick reading / Amount of fuel remaining

E __ Empty

¼ __ .2 (1/5)

½ __ .5 (1/2)

¾ __ .8 (4/5)

F __ Full

Things You'll Need

  • Long, thin piece of wood
  • Saw or knife
  • Clean rag
  • Ruler or measuring tape
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About the Author

Andrew Breslin has been writing professionally since 1994. His articles and op-ed pieces have appeared in the "South Florida Sun Sentinel," "St Paul Pioneer Press," "Detroit Free Press," "Charlotte Observer," “Good Medicine,” and others. He studied molecular biology at Westchester University and frequently writes about science and mathematics.