GoogleSheetsCustomFunctions.gs

var WeightConstants = {
  POUNDS_PER_KILO: 2.20462,
  POUNDS_PER_STONE:14
};

// Convert Kilos to pounds
function KgToPounds(kilos) {
  return kilos * WeightConstants.POUNDS_PER_KILO;
}

// Convert Pounds to StonePounds
function PoundsToFormattedStonePounds(pounds) {
  var stones = parseInt(pounds/WeightConstants.POUNDS_PER_STONE);
  var poundsRemaining = pounds - (stones * 14);
  return stones + "st " + poundsRemaining.round(1) + "lb ";
}

function KgToStonePounds(kilos) {
  var pounds = KgToPounds(kilos);
  return PoundsToStonePounds(pounds);
}

function StonePoundsToFormattedStonePounds(stones, fractionalPounds) {
  totalPounds = StonePoundsToPounds(stones, fractionalPounds);
   return PoundsToFormattedStonePounds(totalPounds);
}

// Convert stones/pounds as separate values to pounds
function StonePoundsToPounds(stones, fractionalPounds) {
  return (WeightConstants.POUNDS_PER_STONE * stones) + fractionalPounds;
}

function StonePoundToKilos(stones, fractionalPounds) {
 return  (StonePoundsToPounds(stones, fractionalPounds) / WeightConstants.POUNDS_PER_KILO).round(2);
}

//https://stackoverflow.com/questions/11832914/round-to-at-most-2-decimal-places-only-if-necessary
Number.prototype.round = function(places) {
  return +(Math.round(this + "e+" + places)  + "e-" + places);
}