The simple system I use for calculating encumbrance is based on the system used in James Raggi’s LotFP. I have taken his system and added one more component: I subtract a character's STR Mod from the total number of encumbrance points to calculate a character's total encumbrance (ENC). Use the following to calculate ENC:
* Note that an item may refer to many items of a similar type. e.g. Carrying 24 arrows only counts as carrying 1 item even though there are 24 arrows in the quiver that is being carried. Furthermore 100 coins or gems carry the same "weight" as 1 item. Finally, carrying cases, containers, purses, etc. that serve only to hold other items are not counted; they are non-encumbering. Backpacks and large sacks do not come in under this exception as they may have uses beyond functioning as mere containers.
* Also note that dwarves in addition to the -1 modifier above may chose to subtract their CON Mod rather than their STR Mod from their ENC when calculating their Total ENC. These aberrations from the norm combine to represent dwarves' endurance and ability to heft a load over long distances without tiring.
Once the character's ENC has been calculated, align the result with the appropriate line on the following table, yielding check and save modifiers, initiative modifiers, and modified movement rates. I use a 3x-ish type system for initiative and movement (when tactical), so the the table has modifiers that work for those types of systems:
I have tried to integrate all of effects that encumbrance might have into this, I think, relativity simple system. It has met success in play so far.