<script> function Calculate(obj) { //All calculations in CM loop_dia = obj.loop_dia.value; line_diameter = obj.line_diameter.value; //Stretch factor stretch = 1.2; if (line_diameter <= .25) stretch = 1.1875; //Calculate var idia = 1.865 * line_diameter; var length = Math.PI * loop_dia * (1 + stretch) + 26 * line_diameter; var mark = Math.PI * loop_dia * stretch / 2; var length_extended = loop_dia * Math.PI / 2; //Assign and round obj.idia.value = idia.toFixed(1) +"cm"; obj.length.value = length.toFixed(1) +"cm"; obj.mark.value = mark.toFixed(1) +"cm"; obj.length_extended.value = length_extended.toFixed(1) +"cm"; } </script> <form> <div class="form-group"> <label for="line_diameter">Line diameter</label> <select class="form-control form-control-lg" name="line_diameter" onChange="Calculate(this.form)"> <option value=".2">2mm</option> <option value=".25">2.5mm</option> <option value=".3">3mm</option> <option value=".4" selected>4mm</option> <option value=".5">5mm</option> <option value=".6">6mm</option> <option value=".8">8mm</option> <option value="1.0">10mm</option> <option value="1.2">12mm</option> </select> <label for="loop_dia">Loop diameter (in centimeter)</label> <input type="number" min="0" step=".1" class="form-control form-control-lg" id="loop_dia" aria-describedby="emailHelp" placeholder="Enter loop diameter" onChange="Calculate(this.form)"> <button onClick="Calculate(this.form)" class="btn btn-primary">Calculate</button> </div> <hr> <div class="form-group"> <label for="idia">Loop size (Diameter of double braid)</label> <input type="text" readonly class="form-control-plaintext" id="idia"> <label for="length">Total rope needed</label> <input type="text" readonly class="form-control-plaintext" id="length"> <label for="mark">From center mark</label> <input type="text" readonly class="form-control-plaintext" id="mark"> <label for="length_extended">Tip to tip</label> <input type="text" readonly class="form-control-plaintext" id="length_extended"> </div> </form>