Mee Hailam sits at around 550 calories for a standard chicken plate, which is honestly not bad for a hawker noodle dish. To be upfront, that number is a ballpark estimate because proper nutritional data for Mee Hailam is thin, and every hawker stall pours that gravy differently. The two main calorie drivers are the yellow noodles at 220 cal and the thick gravy at 135 cal. The chicken or seafood on top only contributes about 125 cal and it also gives you 28g protein, which is decent for keeping you full. Vegetable-only version drops to around 430 cal but the gravy is still doing work even without the meat, so do not assume it is a diet meal just because there is no chicken lah.
The maths changes most based on how heavy-handed the cook is with the gravy. That thick Hainanese sauce is made with oil, oyster sauce, sweet soy, and cornstarch, and a generous pour can easily add another 50 to 80 calories on top of the estimate. The noodle portion also varies, some stalls give you 150g of yellow noodles, others dump in 200g without thinking. If you are watching your intake, the single most effective thing you can do is ask for less gravy. The vegetables at only 30 calories for 70g are basically free food, so if the stall gives you extra choy sum or baby corn, take it.