How this calculator works
An idling engine burns fuel while doing no useful transport work. Natural Resources Canada uses a working estimate of about 0.6 litres per hour for every litre of gasoline engine displacement, so a 2.0 litre engine burns roughly 1.2 litres an hour at idle. Diesel engines use less at idle, roughly 0.2 litres per hour per litre of displacement in this model. The estimate is a planning figure, not a measurement: idle speed, accessories, cold temperature, air conditioning and whether the engine is in gear all change the real rate. Use the measured mode with an OBD scan tool or a timed fuel consumption test for a figure specific to your vehicle.
Formula
Petrol idle rate (L/h) = 0.6 x engine size (L). Diesel idle rate (L/h) = 0.2 x engine size (L). Fuel burned = idle rate x idle hours. Cost = litres x price per litre. CO2 = litres x 2.289 kg/L for petrol or 2.663 kg/L for diesel.
Worked example
A 2.0 litre petrol engine idling for 1.5 hours burns 1.2 x 1.5 = 1.8 litres. At $1.65 a litre that is $2.97, and the carbon dioxide produced is about 4.12 kg.
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Frequently asked questions
Is idling really worse than restarting?
For most modern engines, idling for more than about a minute uses more fuel than a warm restart. Restarting also saves wear from low-oil-pressure idle running. Follow the maker's guidance for extreme cold and for vehicles with particular emissions equipment.
Why does the model use engine size?
Engine displacement is a convenient proxy for the air pumped through the engine at idle and therefore for the fuel burned. The 0.6 L/h per litre figure is a published working estimate, not a physical law.
Does air conditioning change the result?
Yes. The air conditioning compressor and alternator load the engine, so the idle fuel rate rises. A measured rate with the accessories running is the most accurate input.
What about diesel anti-idling rules?
Many jurisdictions restrict unnecessary idling for all vehicles and some have specific rules for heavy trucks. This page estimates fuel and carbon dioxide only; check local rules before relying on a number for compliance.