How to calculate therms usage from oil usage?

Question

The Nest thermostat states I heated my home for 155 hours in November. I estimate thats approx 100 gallons of oil used. It got me wondering if I converted to natural gas, how much would that be?

Unfortunately, I'm unable to find a calculator to go from gallons of oil used to therms. Or hours of heat to therms.

I'm not looking for someone to do the conversion for me. Just interested in the formula to calculate this.

Not sure if spec matter, but here they are:

My oil burner model is KV83 (I'm told it's middle quality burner so I may change it one day) It's supplied by a Riello burner oil gun. -pump pressure 145 psi -air set 3.5 -nozzle 0.90 x 80 degrees B -headset 1.0

Answer 1

Figures, just found that 1 gallon of oil equals 1.35 therms. Another consideration is the efficiency of the unit. From a recent energy audit in know its efficiency is 79%.

This means my 100 gallon estimate is 135 therms. Now to try and guess what the gas bill would look like, based on their charge model is another story.