How is Margin calculated on MT4?

First, it is important to distinguish between Base Currency (aka Primary) and Term Currency (aka Secondary or Quote).

GOLDEN RULE

Margin is always denominated in the Base Ccy*

*If your account currency is in a different currency than the Base Currency, then it will be converted to your account currency.

The formula is as follows:

Lots x Contract Size x Margin%,

or, (Lots x Contract Size) / Leverage

You can find all Contract Sizes, by right-clicking on any Symbol through MT4's market watch and then 'Specification'

A window will pop up containing information about the selected symbol:

Example

1. If you want to Buy 1 lot on EURUSD with a margin percentage of 0.5%, then it is simply                    1 x 100,000 x 0.5%= 500 EUR
2. If I want to Buy 100,000 USDJPY with a margin percentage of 0.5%, then it is simply                          1 x 100,000 x 0.5% = 500 USD

The final amount though depends on your account’s currency.

If your account currency is in EUR, then in the 1st example you’ll need 500 EUR as a margin, while in the 2nd example, you'll have to convert the 500 USD into EUR.