Tempura udon is one of the lighter hot meals in Japan at 370 calories. The udon is 250g cooked, about 240 calories, and the dashi broth is nearly nothing at 25. The one thing with real fat in it is the shrimp tempura, which is about 95 to 110 calories for one piece because the batter soaks up oil. Marugame Seimen publishes 299 for its kake udon regular size and 110 for an ebi ten, and that 409 total is a normal lunch for about 500 yen.
The number climbs fast at a self-serve counter. Marugame's vegetable kakiage is 490 calories on its own, more than the bowl of udon it sits on. A chikuwa ten is about 140, a kashiwa (chicken) ten 143, and a half-boiled egg tempura around 110. Two pieces of tempura on a large udon and you are at 700 calories, which is a katsu curry territory bill for what feels like a light noodle lunch.
Udon is almost all carbohydrate. A 370 calorie bowl has about 62g of carbs and only 14g of protein, so it is fine for calories but it will not keep you full past 2pm on its own. Adding one kashiwa ten or a soft egg gives you 10 more grams of protein for about 100 to 140 calories, which is a better trade than a second shrimp.