The relationship between cubic centimetres (cc) and horsepower (hp) is complex and depends on many factors. First of all, it's important to note that cc is a measure of engine size, while hp is a measure of how much power it produces. Generally, a larger engine will produce more power, but the ratio isn't uniform across all engines. It depends on how well-tuned the engine is, as well as the type of fuel it uses. Most modern passenger cars, however, do fall into a small range of ratios.
Divide the cc of the engine by 15. For example, the 1800cc of a 1.8-litre 16V Ford Escort's engine divided by 15 yields 120hp.
Divide the cc of the engine by 17. For example, the Escort's 1800cc divided by 17 yields approximately 106.
Recognise the horsepower falls somewhere in the range established by the previous steps. In fact, the Escort's horsepower is 115. Its cc to hp ratio is 15.6522.