TLDR (aka the summary):
While many DMs will be fine using the main supply number (Supply = strength score), other DMs may find it too "gamist" that supply is completely separate from carrying capacity. While you may not want to pull out the calculator for carrying capacity, you still want supply and capacity to be intertwined and for one to affect the other in a reasonable way. If so, here is a way to have your cake and eat it too.
The system below gives you a new (but still simple) means of calculating and adjusting supply, that is directly calculated from carrying capacity. Further, you can make adjustments to supply based on significant changes to carrying capacity, without having to pull out the calculator or spreadsheet.
Basic Supply
The basic math. I looked at an adventurer carrying an explorer's pack (which is ~40 lbs), some solid medium armor (another 40 lbs), and a couple of weapons (~5 lbs). So 85 lbs of normal equipment, I am not accounting for a variety of other things that a person might carry, but I am also not accounting for a person in lighter armor. Aka this is a medium range "ballpark", assuming a person is not maxed out on carrying capacity, but carrying a generally average load.
Now if we look at supply by weight, its roughly ~11 lbs per day (that's a lb of food and two waterskins of water).
So taking normal carrying capacity (strength score * 15), subtracting the 85 lbs of equipment, and then dividing by 11, gives me an approximate number of supply a person could carry. Those numbers are:
Small/Medium Creature: Supply = 5 + 3 * str modifier
Large Creature (or Medium creature that counts as large): Supply = 20 + 5 * str modifier
Example: A 14 str Human could hold a "normal amount of equipment" and 5+3*2 = 11 days of supply. A 20 str orc could hold 20 + 5 * 5 = 45 days of supply.
Max Supply
If you have a character that is doing nothing but carrying supply, you can use these formulas:
Small/Medium Creature: Max Supply (no other equipment carried) = 13 + 3 * str modifier
Large Creature: Supply = Max Supply (no other equipment carried) = 27 + 6 * str modifier
Adjustments on the Fly (+/- 4 supply rule)
Now since a bulky item is about 40 lbs, for convenience you could just say the person loses about 4 supply in capacity. Likewise, if the character is wearing very light armor (which drops the weight by about ~35ish lbs), than you could increase their capacity by 4 supply. So as a general rule:
+/- 4 supply for every "major" adjustment to carrying capacity.
You can use this rule to quickly adjust a character's carrying capacity when big changes in weight occur. The party has found a sack filled with heavy coins, which the fighter will take on. If the sack is notably heavy, as the DM you could just rule that the fighter's supply capacity is reduced by 4. Likewise, if the fighter decides to throw off their heavy armor for a bit, you could add 4 to their supply numbers if you wish.
This is a way to have big changes in carrying capacity affect supply without digging into the math too deeply.
Conclusion
In general, these formulas mean party members will carry less supply than the base rules, but in return you get more "accurate" numbers and it makes supply attrition a bigger factor in your journeys. The flip side, having large creatures (or abilities that you let you carry as large) have a major effect on supply numbers, for good or ill.
Using these formulas, you can quickly estimate how much supply your players should have along with normal loads, and can make snap adjustments if the players decide to hold an above or below average amount of weight.
Consider using these formulas to run supply based on "normal carrying capacity".
Small/Medium Creature: Supply = 5 + 3 * str modifier, Max Supply (no other equipment carried) = 13 + 3 * str modifier
Large Creature: Supply = 20 + 5 * str modifier, Max Supply (no other equipment carried) = 27 + 6 * str modifier
+4 supply if lightly armored (no greater than max supply)
- 4 supply per bulky or heavy load added
Small/Medium Creature: Supply = 5 + 3 * str modifier, Max Supply (no other equipment carried) = 13 + 3 * str modifier
Large Creature: Supply = 20 + 5 * str modifier, Max Supply (no other equipment carried) = 27 + 6 * str modifier
+4 supply if lightly armored (no greater than max supply)
- 4 supply per bulky or heavy load added
While many DMs will be fine using the main supply number (Supply = strength score), other DMs may find it too "gamist" that supply is completely separate from carrying capacity. While you may not want to pull out the calculator for carrying capacity, you still want supply and capacity to be intertwined and for one to affect the other in a reasonable way. If so, here is a way to have your cake and eat it too.
The system below gives you a new (but still simple) means of calculating and adjusting supply, that is directly calculated from carrying capacity. Further, you can make adjustments to supply based on significant changes to carrying capacity, without having to pull out the calculator or spreadsheet.
Basic Supply
The basic math. I looked at an adventurer carrying an explorer's pack (which is ~40 lbs), some solid medium armor (another 40 lbs), and a couple of weapons (~5 lbs). So 85 lbs of normal equipment, I am not accounting for a variety of other things that a person might carry, but I am also not accounting for a person in lighter armor. Aka this is a medium range "ballpark", assuming a person is not maxed out on carrying capacity, but carrying a generally average load.
Now if we look at supply by weight, its roughly ~11 lbs per day (that's a lb of food and two waterskins of water).
So taking normal carrying capacity (strength score * 15), subtracting the 85 lbs of equipment, and then dividing by 11, gives me an approximate number of supply a person could carry. Those numbers are:
Small/Medium Creature: Supply = 5 + 3 * str modifier
Large Creature (or Medium creature that counts as large): Supply = 20 + 5 * str modifier
Example: A 14 str Human could hold a "normal amount of equipment" and 5+3*2 = 11 days of supply. A 20 str orc could hold 20 + 5 * 5 = 45 days of supply.
Max Supply
If you have a character that is doing nothing but carrying supply, you can use these formulas:
Small/Medium Creature: Max Supply (no other equipment carried) = 13 + 3 * str modifier
Large Creature: Supply = Max Supply (no other equipment carried) = 27 + 6 * str modifier
Adjustments on the Fly (+/- 4 supply rule)
Now since a bulky item is about 40 lbs, for convenience you could just say the person loses about 4 supply in capacity. Likewise, if the character is wearing very light armor (which drops the weight by about ~35ish lbs), than you could increase their capacity by 4 supply. So as a general rule:
+/- 4 supply for every "major" adjustment to carrying capacity.
You can use this rule to quickly adjust a character's carrying capacity when big changes in weight occur. The party has found a sack filled with heavy coins, which the fighter will take on. If the sack is notably heavy, as the DM you could just rule that the fighter's supply capacity is reduced by 4. Likewise, if the fighter decides to throw off their heavy armor for a bit, you could add 4 to their supply numbers if you wish.
This is a way to have big changes in carrying capacity affect supply without digging into the math too deeply.
Conclusion
In general, these formulas mean party members will carry less supply than the base rules, but in return you get more "accurate" numbers and it makes supply attrition a bigger factor in your journeys. The flip side, having large creatures (or abilities that you let you carry as large) have a major effect on supply numbers, for good or ill.
Using these formulas, you can quickly estimate how much supply your players should have along with normal loads, and can make snap adjustments if the players decide to hold an above or below average amount of weight.
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