If you make dosa or idli batter at home, you already know the ratio of rice and lentils (urad dal) that works for you. If you are still experimenting, this article will help. Typically, there are two ratios for using rice and lentils in dosa batter – 2:1 (two parts rice, 1 part lentils) and 3:1.
The ratio of 2:1 makes softer dosa that can be broken like bread and the ratio of 3:1 makes crispier dosa that crumbles. Restaurants prefer to use the 3:1 ratio because using more rice than lentils saves money for them, though some restaurants may still be using the 2:1 ratio.
For restaurants, there is another relevant factor. Masala dosa (dosa with potato filling) is good when it is crispy. That means a restaurant where customers order masala dosa more often, will usually prepare the batter in the 3:1 ratio. On the other hand, a restaurant that sells more plain or set dosa will likely use the 2:1 ratio of rice and lentils.
Related: Authentic Dosa Recipe Video
At home, if you use the same batter for making both dosa and idli, the ratio of 3:1 may be the ideal one for you. And if you wish to make both plain and masala dosa with the same batter, there is a workaround that allows you to do that. For masala dosa, spread the batter really thin and for plain dosa, not so thin.
Another clarification is called for here. A good South Indian restaurant will not use only one type of batter for making both dosa and idli. Their batter for dosa will contain a mix of urad dal and rice whereas the batter for idli will contain urad dal and cream of rice (also called idli sooji or idli rava or rice rava).
Using cream of rice in idli batter gives you really soft and fluffy idli that breaks easily and melts in the mouth. You can buy cream of rice from a shop that sells South Indian provisions. Of course, you can buy it online if not available locally.
If you are not yet ready to take the trouble of finding cream of rice to make idli at home, perhaps you may watch this video on making idli with cream of rice for inspiration.
Do not make dosa batter at home? I think you should. It is really easy and you will know what went into it. You will use clean and safe ingredients, ferment the batter just right, and will have total control over the results. Please watch this video for all that you need to know: Authentic Dosa Recipe Video