How downranking works
Every rank of a spell adds the same share of your bonus healing or spell damage: the spell’s coefficient. For a spell that lands at once, it is the cast time divided by 3.5 seconds (Greater Heal, 3 seconds: 85.7%). For one that works over time, it is the duration divided by 15 seconds (Renew, 15 seconds: 100%, spread over its ticks). Instants count as 1.5 seconds.
A lower rank has a smaller base value and costs less mana, but it gets the same bonus. So the more bonus healing you wear, the more a low rank heals per point of mana, and the closer its heal gets to the top rank’s. That is downranking: casting a lower rank to make the mana last, or to heal a small wound without wasting a large heal on it.
What a rank does is its base value at your level plus the coefficient times your bonus. The base value grows a little with your level, until about the level where the next rank is learned. Talents that add a percentage, like Spiritual Healing, multiply the whole heal; talents that cut the mana cost or the cast time change every rank alike, so they rarely change which rank is the most efficient.
Downranking in WoW Forever
The Forever client stores the full coefficient on every rank, including the ranks learned before level 20. Classic Era cuts those ranks: 3.75% less for every level under 20. Lesser Heal rank 1 (level 1) adds 12.3% of your bonus healing in Classic Era and 42.9% in the Forever client. Of the 391 ranks both clients have, 111 take more of your bonus in Forever, 75 of them ranks learned before level 20.
Forever also lowers the base value of many heals: 83 of 94 healing ranks heal less before bonus than in Classic Era. Greater Heal rank 1 heals 794 to 894, against 899 to 1,013 in Classic Era. While a rank learned before level 20 keeps its share, the larger share outweighs the lower base; a rank learned later keeps Classic’s share on a lower base, so it heals less. At level 20 with 60 bonus healing, Lesser Heal rank 1 heals 74 for 30 mana in Forever and 59 in Classic Era. At level 60 with 400, Greater Heal rank 1 heals 1,160 and 1,324.
In September, DoubleZug measured Healing Touch ranks 1 to 3 and Rejuvenation ranks 1 and 3 in the beta with a druid at levels 18 and 19, and every heal landed within 2 points of this calculator (table below). Healing Touch rank 1 took the full 42.9% of the bonus: Classic Era’s level 20 cut is gone. His notes say the same of Holy Light ranks 1 to 3 at level 20.
Forever cuts ranks far below your level. Blizzard explained it in its development notes of October 1 without numbers, and the client’s character sheet now says it: those ranks benefit less from your spell damage and healing, and trigger class abilities and talents less often. Blizzard’s example: a rank 1 Frostbolt cast at level 60 never triggers Frostbite. DoubleZug worked out the rate from tests at levels 20 to 30: a rank keeps its whole share of your bonus for 17 levels after you learn it, then loses 5% at each level. The calculator applies this rule to every Forever rank, and the downranking guide shows the measurements.
Downranking in Classic Era
Classic Era has no rule against downranking beyond the level 20 cut: a rank learned at level 20 or later adds the spell’s full coefficient, whatever your level. The cut is already applied in the coefficients the Classic client stores, so the calculator reads them as they are. Three spells have no coefficient in that client because the server applies it: Holy Light and Flash of Light (cast time divided by 3.5) and Power Word: Shield (10%). For those, the calculator uses the values Classic players measured, with the level 20 cut.
What each rank costs and does comes from the client’s spell tables, read through wago.tools. The full list of each class’s spells, rank by rank, is on its spellbook page: Priest, Druid, Paladin, Shaman, Mage, Warlock.