Código HTML:
<!DOCTYPE html> <html lang="es"> <head> <title></title> <script language="javascript" type="text/javascript"> function suma(obj) { total=parseInt(document.getElementById("numero").value); if(obj.checked){ total+=parseInt(obj.value); }else{ total-=parseInt(obj.value); } document.getElementById("numero").innerHTML=text(total); } </script> </head> <body> <input type="checkbox" name="chck1" value="1" onChange="suma(this)"> <input type="checkbox" name="chck2" value="1" onChange="suma(this)"> <input type="checkbox" name="chck3" value="1" onChange="suma(this)"> <hr> <h1 id="numero"></h1> </body> </html>