- 4 stone -- Plate mail.
- 2 stone -- Chain mail.
- 1 stone -- Leather, shield, polearm, halberd, pike, two-handed sword, morning star, flail, battle axe, staff, pole, standard rations, 1000 coins
- 1/3 stone -- Helmet, sword, spear, mace, handaxe, bow, arrows, water/wineskin, lantern, torches, rope, spikes, iron rations
Example calculation -- Typical dwarven fighter. What I do is note a stone value in pencil next to large items, or (*) for the 3-per-stone items, adding up from the bottom for the total. With a little practice, the whole thing can be done mentally at a glance.
- Plate mail (4)
- Battle axe (1)
- Shield (1)
- Helmet (*)
- Mace (*)
- 50' Rope (*)
- 12 Iron spikes (*)
- Iron Rations (*)
- Small sack
A secondary convenience is that these units are auto-magically scaled the same as a character's Strength score, i.e., a character's maximum "very heavy/armored load" (6" move rate) in stone is equal to their Strength. Much like ranged weapons, divide Strength in thirds for the other categories: up to 1/3 Strength for 12" move, 2/3 Strength for 9", full Strength for 6" (optionally allow "encumbered" movement, 3" rate, at up to twice Strength score).
In the example above with an 8-stone load, a character with 9 Strength would only move at 6" (over 2/3 Strength limit = 6 stone), while a character with 12 Strength would move at 9" (2/3 Strength limit = 8 stone).
A third advantage is in how the term "stone" carries with it a very nice, archaic, Imperial ring to your milieu. Even if one were so crass as to disagree with me on that score, I think the reasons above are more than compelling. (And, more generally, speaks to the advantages of human-based units of measurement.)