Kilometres per litre and litres per 100 km are reciprocals scaled by a hundred: l/100 km = 100 ÷ (km/l). Both are metric, both describe the same car, and they point in opposite directions — higher is better for one, lower for the other.
About the units
Kilometres per litre is an efficiency unit and the metric equivalent of miles per gallon. It is standard in Japan, India, parts of Latin America and much of Africa and Asia. Litres per 100 kilometres is a consumption unit and the European convention, used across the EU, Australia, New Zealand, Canada and South Africa. Neither is more correct, but they encourage different intuitions: the consumption form makes fuel cost scale linearly with the number, which is why it is preferred for regulation and carbon accounting.
The exact factor
The relationship is l/100 km = 100 ÷ (km/l), and the same formula converts back. So 20 km/l is 5 l/100 km, 12.5 km/l is 8 l/100 km, and 10 km/l is exactly 10 l/100 km — the crossover point where the two scales read the same number, which is a useful anchor. Below 10 km/l the consumption figure is the larger; above it, the efficiency figure is. Because the relationship is reciprocal, averaging fuel economy figures across a fleet gives different answers in the two units, and only the consumption form averages correctly for total fuel used.
Where you meet this conversion
Reading vehicle specifications from markets that use different conventions. A Japanese car brochure gives km/l, the same model's European brochure gives l/100 km, and the numbers look unrelated until converted. Motorcycle and scooter economy is often quoted in km/l even in Europe. Fleet management software usually stores one and displays the other. Hybrid and plug-in vehicles complicate matters further by adding electricity consumption in kWh per 100 km, which is a consumption unit by design for the same reason.