function calculateOption1()
{
    if (document.getElementById)
    {
        var txtOpt1CarbonTns = document.getElementById('txtOpt1CarbonTns');
        
        // make the calculation based on the user's selections        
        var result = txtOpt1CarbonTns.value;
        
        // round to two decimal places
        var roundedResult = Math.round(result*100)/100;
        // show the cost of this offset
        var roundedCost = (Math.round((roundedResult * costPerTonne)*100)/100).toFixed(2);
        document.getElementById('ctl00_cphPrimary_hfOpt1Result').value = roundedResult;
        
        if(document.all)
        {        
            document.getElementById('divOpt1Result').innerHTML = roundedResult.toString();
            document.getElementById('divOpt1Cost').innerText = roundedCost;
        }
        else
        {
            document.getElementById('divOpt1Result').textContent = roundedResult.toString();
            document.getElementById('divOpt1Cost').textContent = roundedCost;
        }
        
    }
}