The principal ingredient that gives curry its “heat” is chilli.
And a curry becomes hotter when you use more chilli.
There are 3 main different ways how chilli gets into curries – as chopped fresh chilli, chilli powder or curry powder (all curry powders have chilli in them).
There are two great Curry Focus articles about what puts the heat into chillies and the Scoville scale of chilli heat that gives a good description of chillies and their heat.
You’ll see from these articles that the type of chilli also affects the heat level.
This is not an article that describes the merits of using fresh chilli, chilli powder or curry powder – that is entirely your choice.
The best plan is to choose one type of chilli, brand of chilli powder or curry powder (you often don’t know what quantities or types of chilli are in these powders but most producers will try to keep their product tastes and heat levels consistent).
If using fresh chillies, you have to choose whether to use the seeds as well as the flesh of the fresh chillies.
Once you’ve chosen your chilli (with or without seeds), chilli powder or curry powder then it’s all a matter of quantity.
If you make a particular curry using a single chilli and the curry is mild, then it’s a certainty that following the same recipe but using 2 chillies will result in a hotter curry – and using 3 chillies will be even hotter.
If you follow a curry recipe and the result is too hot for you, then using less chilli will make it milder.
In the end, it’s all a matter of personal taste and experimentation.
You do need to be sensible about the curry heat. If you like really hot curries but are cooking for somebody that only eats mild curries, you need to tone down the heat or cook 2 separate curries with different heat levels.
There’s a great article that you can read about how to cool down a curry that’s too hot.
If you’re throwing a curry dinner party, you can make several small curries with a variety of heat levels so that everyone can find something that they enjoy.
Here’s one last word of caution. If you’re guessing about how much chilli to put into a curry, then make sure that you err on the low side because nobody likes a curry that is too hot.
Enjoy your curries.