1 gram (g) = 1,000 milligrams (mg)
==> Set mgPerGram = 1,000

1 carat (ct) = 200 mg
= 200 / mgPerGram g = 0.2 g
==> Set gramsPerCarat = 0.2

1 kilogram (kg) = 1,000 g
= gramsPerKg * mgPerGram g
= 1,000,000 mg
==> Set gramsPerKg = 1,000

1 tonne (t) = 1,000 kg
= kgPerTonne * gramsPerKg g
= 1,000,000 g
= kgPerTonne * gramsPerKg * mgPerGram mg
= 1,000,000,000 mg
==> Set kgPerTonne = 1,000

1 megatonne (Mt) = 1,000,000 t
= 1,000,000 * kgPerTonne kg
= 1,000,000,000 kg

For the avoirdupois (US) system:

1 pound (lb) = 7000 grains (gr)
==> Set grainsPerPound = 7000

1 pound (lb) = 16 ounces (oz)
==> Set ouncesPerPound = 16

48 ounces (oz) = 875 pennyweights (dwt)
==> Set dwtPerOunce = 875 / 48

1 short ton (sh tn) (US) = 2000 lb
 = poundsPerShortTon / poundsPerLongTon l tn
= 1 / 1.12 l tn
==> Set poundsPerShortTon = 2,000

1 long ton (l tn) (UK) = 2240 lb
= poundsPerLongTon / poundsPerShortTon sh tn
= 1.12 sh tn
==> Set poundsPerLongTon = 2,240

Short ton has been abbreviated to sh tn, in preference to st (as used by Hustrulid and Kuchta in "Open Pit Mine Planing and Design"), to eliminate potential confusion with stone (14 pounds). JIRA issue MM-30981 refers.

Troy Weight

Troy Ounce (as used for precious metals):

1 troy ounce (oz t) = 480 grains (gr)
==> Set grainsPerTroyOunce = 480

1 troy pound (lb t) = 5760 grains (gr)
==> Set grainsPerTroyPound = 5760

1 troy pound (1b t) = 12 troy ounces (oz t)
==> Set troyOuncesPerTroyPound = 12

1 troy ounce (oz t) = 20 pennyweights (dwt)
==> Set dwtPerTroyOunce = 20

1 troy ounce (oz t) = 31.1034768 grams (g)
==> Set gramsPerTroyOunce = 31.1034768

1 g
= 1 / gramsPerDwt dwt
= 1 / 1.55517384 dwt
= ouncesPerPound / ( gramsPerKg * kgPerPound ) oz
= 16 / 453.59237 oz
= 1 / ( gramsPerKg * kgPerPound ) lb
= 1 / 453.59237 lb

1 kg
= gramsPerKg / gramsPerDwt dwt
= 1,000 / 1.55517384 dwt
= ouncesPerPound / kgPerPound oz
= 16 / 0.45359237 oz
= 1 / kgPerPound lb
= 1 / 0.45359237 lb
= 1 / ( poundsPerShortTon * kgPerPound ) sh tn
= 1 / 907.18474 sh tn
= 1 / ( poundsPerLongTon * kgPerPound ) l tn
= 1 / ( poundsPerLongTon / poundsPerShortTon * poundsPerShortTon * kgPerPound ) l tn
= 1 / (1.12 * 907.18474) l tn

1 t
= kgPerTonne * gramsPerKg / gramsPerDwt dwt
= 1,000,000 / 1.55517384 dwt
= kgPerTonne * ouncesPerPound / kgPerPound oz
= 16,000 / 0.45359237 oz
= kgPerTonne / kgPerPound lb
= 1,000 / 0.45359237 lb
= kgPerTonne / ( poundsPerShortTon * kgPerPound ) sh tn
= 1,000 / 907.18474 sh tn
= kgPerTonne / ( poundsPerLongTon * kgPerPound ) l tn
= kgPerTonne / ( poundsPerLongTon / poundsPerShortTon * poundsPerShortTon * kgPerPound ) l tn
= 1,000 / 1016.0469088 l tn