Load calculations scare a lot of guys, but they’re not that complicated once you work through one. NEC Article 220 gives you two methods: the Standard Method (220.40-220.60) and the Optional Method (220.82-220.85). For most residential work, the Optional Method will give you a smaller service size, which matters when you’re trying to justify a 200A service instead of pushing a customer to 400A.
Here’s the Optional Method in short form: Take the square footage of the house and multiply by 3 VA per square foot for general lighting and small appliance circuits. Add 10,000 VA for the first 10 kVA of all other loads at 100%, then add the rest at 40%. HVAC gets special treatment - you take the larger of heating or cooling at 100%, not both. Add up everything, divide by your voltage (240V), and you’ve got your amperage.
Where people get tripped up: EV chargers, hot tubs, and induction ranges. These are “other loads” that go into the 40% bucket after the first 10 kVA, but if the customer has all three plus electric heat, you might still end up at 400A anyway. Run the numbers before you quote.