function calculate() 
{
	if (document.getElementById('dock_length').value<=0) 
		{
		alert("Dock length must be entered as a positive number")
		}
	else 
  		document.getElementById('boards').value = document.getElementById('dock_length').value * 2;
  		document.getElementById('clips').value = document.getElementById('dock_length').value * 8;
  		document.getElementById('clips100').value = Math.ceil(document.getElementById('clips').value/100)*100;
}