Snippet to convert the dimension in FedEx plugin. WOOCOMMERCE FEDEX SHIPPING PLUGIN WITH PRINT LABEL - https://www.pluginhive.com/product/woocommerce-fedex-shipping-plugin-with-print-label/

functions.php

/**
 * Snippet to convert the dimension in FedEx plugin.
 * Created at : 14 Sep 2018
 * Updated at : 14 Sep 2018
 * Xadapter Plugins : https://www.pluginhive.com/plugins/
 * Gist Link : https://gist.github.com/varun-pluginhive/068bb9aa5b54e79973255b571d0ab629
 */
if( ! function_exists('ph_fedex_convert_dimension') ) {
	function ph_fedex_convert_dimension( $converted_dimension, $dimension, $to_unit, $from_unit ) {
		
		if( empty($from_unit) )	$from_unit = get_option( 'woocommerce_dimension_unit' );
		$from_unit 	= strtolower($from_unit);
		$to_unit 	= strtolower($to_unit);
		if( $from_unit != $to_unit ) {
			switch ( $from_unit ) {
				case 'in' :
					$dimension *= 2.54;
					break;
				case 'm' :
					$dimension *= 100;
					break;
				case 'mm' :
					$dimension *= 0.1;
					break;
				case 'yd' :
					$dimension *= 91.44;
					break;
				case 'ft' :
					$dimension *= 30.48;
					break;
			}
			switch( $to_unit ) {
				case 'in' :
					$converted_dimension = $dimension * 0.3937;
					break;
				case 'cm' :
					$converted_dimension = $dimension;
					break;
			}
		}
		return $converted_dimension;
	}
	add_filter( 'ph_fedex_get_converted_dimension', 'ph_fedex_convert_dimension', 10, 4 );
}