magento-conf-product-qty.php

<?php

$product=Mage::getModel("catalog/product");         
$prod=$product->load($_product->getId());
$conf = Mage::getModel('catalog/product_type_configurable')->setProduct($prod);
$col = $conf->getUsedProductCollection()->addAttributeToSelect('*')->addFilterByRequiredOptions();

$total_qty=0;
foreach($col as $sprod){
    $sprod=$product->load($sprod->getId());
    $qty = intval(Mage::getModel('cataloginventory/stock_item')->loadByProduct($sprod)->getQty());
    $total_qty+=$qty;
} 
echo $total_qty;