The range of things we can donate is endless, therefore to gather a representative broad carbon saving we have examined the CO2 embedded within common products:

Books: On average book 10 kg CO2.
CD: 10 kg CO2.
Mobile Phones: Various sources (listed within footnotes) have cited CO2 emissions embedded within a mobile as around 12 kg, 20 kg and 60 kg per phone. We have decided to use the median value of 20 kg CO2.

Computers: A UN study ⁴ found that a desktop computer and CRT screen (weighing 24kg) uses ten times its own weight in fossil fuels during manufacture, whereas a car or fridge uses 1 or 2 times its weight in fossil fuels. Fossil fuels emit around 3 tonnes of CO2 per tonne of fuel, so carbon emissions due to PC = 240 x 3 = 720 kg CO2.

Cookers: 250 kg CO2 as cited in 'How to live a low carbon life' by Chris Goodall.

Clothes: Every kg of clothes diverted from landfill saves 8 kg of CO2.

Furniture:  Assuming a small piece of furniture weighs 10 kg, this saves 15 kg CO2

 
Creating an average carbon saving:
Small items save around 12.6 kg CO2 each on average, or 13 kg CO2 approx. A collection of small items may save around 60 kg CO2.

Larger items can around 485 kg CO2 on average.
 
Based on the fact that larger donations will be rarer, PCs for example are replaced on average every 2 years, it makes sense to weight the final emission values accordingly.

Assuming that for the average person, one in ten donations will be a large object (1:10 = large:small).
Large object (1/11 x 485) +
Smaller object (10/11 x 60)
= 44.0 + 54.5
=98.5 kg CO2 approx per donation.