Margin requirements are calculated as follows:

(100,000 units * Number of standard lots)/Leverage * conversion rate to your base currency

For example, for a USD account with 500:1 leverage, if you place a EUR/USD buy order of 0.1 lots (10,000 EUR) @ 1.3632, the calculation would be as follows:

{(100,000 *1.3632)*(0.1)}/500 = $27.26

100,000EUR * 1.3632 = $136,320 (margin required for a standard lot when leverage is 1:1)

$136,320 * 0.1 = $13,632 (margin required for 0.1 lots when leverage is 1:1)

$13,632/500 = $27.26 (margin required for 0.1 lots when leverage is 500:1)