function calculate() 
{
    if (document.getElementById('distance_from_house').value<=0 ||
        document.getElementById('distance_across').value<=0 ||
        document.getElementById('deck_height').value<=0)
        {
        alert("Distance from house, distance across, and deck height values must be entered as positive numbers")
        }
    else  
        document.getElementById('deck_square_feet').value = document.getElementById('distance_from_house').value * document.getElementById('distance_across').value;
            document.getElementById('boards').value = document.getElementById('deck_square_feet').value/2;
            document.getElementById('boards5').value = Math.ceil(document.getElementById('boards').value * 1.05);
            document.getElementById('stair_treds').value = (Math.ceil((document.getElementById('deck_height').value * 12)/7))*2;
        document.getElementById('trim').value = parseInt(document.getElementById('distance_from_house').value*2)+parseInt(document.getElementById('distance_across').value);
        document.getElementById('trim12').value = Math.ceil(document.getElementById('trim').value/12)*12;
        document.getElementById('riser_trim').value = Math.ceil(document.getElementById('deck_height').value*12/7)*4;
        document.getElementById('riser_trim12').value = Math.ceil(document.getElementById('riser_trim').value/12)*12;
        document.getElementById('clips').value = document.getElementById('deck_square_feet').value*2;
        document.getElementById('clips100').value = Math.ceil(document.getElementById('clips').value/100)*100;
}