Measure the square footage of the entire dwelling or office served by the panelboard, making sure to get its outside dimensions as well.
Collect information on every appliance and lighting system you plan to connect (or already have connected) to the panelboard. Obtain the volt/ampere load for all washers, dryers, air conditioners, heaters and small appliances. Most appliances will have this information printed on a panel; if yours do not, contact the manufacturer or search online for the name and model number to find the volt/ampere load.
Print out a load-calculation form (see Resources). This will provide formulas for collecting the loads of the various appliances and the area of the environment into a general load figure for the panelboard. These formulas vary depending on the type of dwelling or office you're calculating for.
Fill out the form and perform the math it indicates to calculate your panelboard's load.